Adopt the shared .golangci.yml and fix the code to it (closes #6)

Vendor .golangci.yml byte-identical from sneak/prompts at cc440118 and
move the Dockerfile lint phase to golangci-lint v2.14.0 by the digest
REPO_POLICIES.md names. Fix the code to that config with flags, help
text, output files, SQL and the order of steps unchanged; long
functions are split into named steps.

Judgement call: the extraction transaction is now rolled back on every
early return; the old deferred rollback missed most failures and could
dereference a nil transaction.
Thirteen //nolint directives (gosec, unconvert, mnd, unqueryvet), each
with its reason.

Model: opus-5-5

.golangci.yml

@@ -0,0 +1,99 @@
+version: "2"
+
+# Config schema uses the golangci-lint v2 layout (settings live under
+# linters.settings, not top-level linters-settings) so that the
+# thresholds below are actually applied by golangci-lint >= v2.
+
+run:
+  timeout: 5m
+  modules-download-mode: readonly
+
+linters:
+  default: all
+  enable:
+    # Successor to the deprecated gomodguard. Named explicitly, rather than
+    # left to `default: all`, because it carries the module policy below.
+    - gomodguard_v2
+  disable:
+    # Genuinely incompatible with project patterns
+    - exhaustruct      # Requires all struct fields
+    - exhaustruct_v5   # Requires all struct fields (successor to exhaustruct)
+    - godot            # Requires comments to end with periods
+    - wrapcheck        # Too verbose for internal packages
+    - varnamelen       # Short names like db, id are idiomatic Go
+    # Deprecated: the warning is attached to the old name, so it is
+    # silenced by disabling that name, not by enabling the successor.
+    - wsl              # Deprecated, replaced by wsl_v5
+    - gomodguard       # Deprecated, replaced by gomodguard_v2
+  settings:
+    lll:
+      line-length: 88
+    funlen:
+      lines: 80
+      statements: 50
+    cyclop:
+      max-complexity: 15
+    dupl:
+      threshold: 100
+    depguard:
+      # Test-support code must not be compiled into the shipped binary. A
+      # test-support package exists to hand a test privileges the program
+      # itself must never have, so a file that is not a test must not import
+      # one. Test files, and the files inside a package whose directory name
+      # ends in `test`, are where that code belongs, and are exempt.
+      #
+      # The deny list below is the one part of this file a repository is
+      # expected to extend, and the only part it may. depguard matches an
+      # import path against a list of prefixes, so it cannot be told "any path
+      # whose last segment ends in test"; a repository's own test-support
+      # packages have to be named here one at a time, by full import path,
+      # under a module path that differs from repository to repository. Add
+      # them; change nothing else.
+      rules:
+        test-support:
+          list-mode: lax
+          files:
+            - "$all"
+            - "!$test"
+            - "!**/*test/**"
+          deny:
+            - pkg: net/http/httptest
+              desc: >-
+                Test-support code belongs in test files and in packages whose
+                directory name ends in test, not in the shipped binary.
+    # Only decisions already recorded in the Go package defaults are
+    # listed here. Every entry matches the module path exactly.
+    gomodguard_v2:
+      blocked:
+        - module: github.com/rs/zerolog
+          recommendations:
+            - log/slog
+          reason: "Structured logging is stdlib log/slog."
+        # One entry per pre-fork module path, because the later releases
+        # are separate paths. A prefix match would be shorter but would
+        # also reach github.com/go-redis/redismock, the test double for
+        # the successor these entries recommend.
+        - module: github.com/go-redis/redis
+          recommendations:
+            - github.com/redis/go-redis/v9
+          reason: "Pre-fork module; use the maintained go-redis v9."
+        - module: github.com/go-redis/redis/v7
+          recommendations:
+            - github.com/redis/go-redis/v9
+          reason: "Pre-fork module; use the maintained go-redis v9."
+        - module: github.com/go-redis/redis/v8
+          recommendations:
+            - github.com/redis/go-redis/v9
+          reason: "Pre-fork module; use the maintained go-redis v9."
+        - module: github.com/sergi/go-diff
+          recommendations:
+            - github.com/aymanbagabas/go-udiff
+          reason: "No unified diff output; use go-udiff."
+        - module: github.com/hexops/gotextdiff
+          recommendations:
+            - github.com/aymanbagabas/go-udiff
+          reason: "Unmaintained fork; use go-udiff."
+
+issues:
+  max-issues-per-linter: 0
+  max-same-issues: 0

Dockerfile

@@ -1,8 +1,8 @@
 # Lint phase. The linter is invoked directly rather than through `make
 # lint` or `script/lint`, which are themselves a docker build and would
 # recurse into a daemon that does not exist in a build step.
-# golangci/golangci-lint:v2.12.2-alpine, 2026-06-28
+# golangci/golangci-lint:v2.14.0 (Debian-based), 2026-10-06
-FROM golangci/golangci-lint:v2.12.2-alpine@sha256:91b27804074a0bacea298707f016911e60cf0cdbc6c7bf5ccacb5f0606d18d60 AS lint
+FROM golangci/golangci-lint@sha256:ad862ba6b3798cbe0fd9fd7408d498fd74fbd2623a92406b2fd3898faf0bf98f AS lint
 
 WORKDIR /src
 

README.md

@@ -35,9 +35,9 @@ issues are [tracked there](https://git.eeqj.de/sneak/bsdaily/issues).
 
 Changes must always be formatted with `make fmt` (`go fmt` for Go, prettier for
 Markdown), syntactically valid, and must pass the linting defined in the
-repository (presently the `golangci-lint` defaults), which can be run with a
+repository (the shared `.golangci.yml` from `sneak/prompts`), which can be run
-`make lint`. The `main` branch is protected and all changes must be made via
+with a `make lint`. The `main` branch is protected and all changes must be made
-[pull requests](https://git.eeqj.de/sneak/bsdaily/pulls) and pass CI to be
+via [pull requests](https://git.eeqj.de/sneak/bsdaily/pulls) and pass CI to be
 merged.
 
 See [`REPO_POLICIES.md`](REPO_POLICIES.md) for detailed coding standards,

TODO.md

@@ -14,12 +14,14 @@ pre-1.0
 
 # Next Step
 
-Add the canonical `.golangci.yml`, move the lint phase to golangci-lint v2.14.0
+Expand tests beyond the compilation smoke test: unit tests for the extraction,
-in the same commit, and fix the findings it surfaces
+verification, and atomic-publish paths.
-(https://git.eeqj.de/sneak/bsdaily/issues/6).
 
 # Completed Steps
 
+- 2026-10-06: Added the canonical `.golangci.yml`, moved the lint phase to
+  golangci-lint v2.14.0, and fixed the code to pass it
+  (https://git.eeqj.de/sneak/bsdaily/issues/6).
 - 2026-10-06: Formatted Markdown with prettier: `script/fmt` writes and
   `script/fmt-check` checks every Markdown file; prettier pinned in
   `package.json` and `yarn.lock`, installed by `script/bootstrap`, which
@@ -46,6 +48,4 @@ in the same commit, and fix the findings it surfaces
 
 # Future Steps
 
-- Expand tests beyond the compilation smoke test: unit tests for the extraction,
-  verification, and atomic-publish paths.
 - Cut a first SemVer release once compliance and test coverage land.

cmd/bsdaily/main.go

@@ -1,6 +1,9 @@
+// Package main is the bsdaily command. It extracts one day, or a range
+// of days, from the latest daily snapshot.
 package main
 
 import (
+	"errors"
 	"fmt"
 	"log/slog"
 	"os"
@@ -10,75 +13,112 @@ import (
 	"github.com/spf13/cobra"
 )
 
+var (
+	errDateExclusive  = errors.New("--date and --from/--to are mutually exclusive")
+	errFromRequiresTo = errors.New("--from requires --to")
+	errToRequiresFrom = errors.New("--to requires --from")
+	errFromAfterTo    = errors.New("is after --to")
+)
+
 func main() {
 	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{
 		Level: slog.LevelInfo,
 	}))
 	slog.SetDefault(logger)
 
-	var dateFlag string
+	var dateFlag, fromFlag, toFlag string
-	var fromFlag string
-	var toFlag string
 
 	rootCmd := &cobra.Command{
 		Use:          "bsdaily",
 		Short:        "Extract a single day's data from the latest daily snapshot",
 		SilenceUsage: true,
-		RunE: func(cmd *cobra.Command, args []string) error {
+		RunE: func(_ *cobra.Command, _ []string) error {
-			hasDate := dateFlag != ""
+			targetDates, err := parseTargetDates(dateFlag, fromFlag, toFlag)
-			hasFrom := fromFlag != ""
+			if err != nil {
-			hasTo := toFlag != ""
-
-			// Validate mutual exclusivity
-			if hasDate && (hasFrom || hasTo) {
-				return fmt.Errorf("--date and --from/--to are mutually exclusive")
-			}
-			if hasFrom != hasTo {
-				if hasFrom {
-					return fmt.Errorf("--from requires --to")
-				}
-				return fmt.Errorf("--to requires --from")
-			}
-
-			var targetDates []time.Time
-
-			if hasDate {
-				t, err := time.Parse("2006-01-02", dateFlag)
-				if err != nil {
-					return fmt.Errorf("invalid --date %q (expected YYYY-MM-DD): %w", dateFlag, err)
-				}
-				targetDates = []time.Time{t}
-			} else if hasFrom {
-				from, err := time.Parse("2006-01-02", fromFlag)
-				if err != nil {
-					return fmt.Errorf("invalid --from %q (expected YYYY-MM-DD): %w", fromFlag, err)
-				}
-				to, err := time.Parse("2006-01-02", toFlag)
-				if err != nil {
-					return fmt.Errorf("invalid --to %q (expected YYYY-MM-DD): %w", toFlag, err)
-				}
-				if from.After(to) {
-					return fmt.Errorf("--from %s is after --to %s", fromFlag, toFlag)
-				}
-				for d := from; !d.After(to); d = d.AddDate(0, 0, 1) {
-					targetDates = append(targetDates, d)
-				}
-			}
-			// else: targetDates remains nil → Run() defaults to snapshot date minus one
-
-			if err := bsdaily.Run(targetDates); err != nil {
 				return err
 			}
+
+			err = bsdaily.Run(targetDates)
+			if err != nil {
+				return err
+			}
+
 			slog.Info("completed successfully")
+
 			return nil
 		},
 	}
 
-	rootCmd.Flags().StringVarP(&dateFlag, "date", "d", "", "target date to extract (YYYY-MM-DD); defaults to snapshot date minus one day")
+	rootCmd.Flags().StringVarP(&dateFlag, "date", "d", "",
-	rootCmd.Flags().StringVar(&fromFlag, "from", "", "start of date range to extract (YYYY-MM-DD, inclusive); use with --to")
+		"target date to extract (YYYY-MM-DD); "+
-	rootCmd.Flags().StringVar(&toFlag, "to", "", "end of date range to extract (YYYY-MM-DD, inclusive); use with --from")
+			"defaults to snapshot date minus one day")
+	rootCmd.Flags().StringVar(&fromFlag, "from", "",
+		"start of date range to extract (YYYY-MM-DD, inclusive); use with --to")
+	rootCmd.Flags().StringVar(&toFlag, "to", "",
+		"end of date range to extract (YYYY-MM-DD, inclusive); use with --from")
 
-	if err := rootCmd.Execute(); err != nil {
+	err := rootCmd.Execute()
+	if err != nil {
 		os.Exit(1)
 	}
 }
+
+// parseTargetDates turns the --date, --from and --to flags into the days
+// to extract. It returns nil when none of them is set, which Run takes
+// to mean the snapshot date minus one day.
+func parseTargetDates(dateFlag, fromFlag, toFlag string) ([]time.Time, error) {
+	hasDate := dateFlag != ""
+	hasFrom := fromFlag != ""
+	hasTo := toFlag != ""
+
+	// Validate mutual exclusivity
+	if hasDate && (hasFrom || hasTo) {
+		return nil, errDateExclusive
+	}
+
+	if hasFrom != hasTo {
+		if hasFrom {
+			return nil, errFromRequiresTo
+		}
+
+		return nil, errToRequiresFrom
+	}
+
+	if hasDate {
+		t, err := time.Parse("2006-01-02", dateFlag)
+		if err != nil {
+			return nil, fmt.Errorf(
+				"invalid --date %q (expected YYYY-MM-DD): %w", dateFlag, err)
+		}
+
+		return []time.Time{t}, nil
+	}
+
+	if !hasFrom {
+		return nil, nil
+	}
+
+	from, err := time.Parse("2006-01-02", fromFlag)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"invalid --from %q (expected YYYY-MM-DD): %w", fromFlag, err)
+	}
+
+	to, err := time.Parse("2006-01-02", toFlag)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"invalid --to %q (expected YYYY-MM-DD): %w", toFlag, err)
+	}
+
+	if from.After(to) {
+		return nil, fmt.Errorf("--from %s %w %s", fromFlag, errFromAfterTo, toFlag)
+	}
+
+	var targetDates []time.Time
+
+	for d := from; !d.After(to); d = d.AddDate(0, 0, 1) {
+		targetDates = append(targetDates, d)
+	}
+
+	return targetDates, nil
+}

internal/bsdaily/bsdaily_test.go

@@ -1,21 +1,32 @@
-package bsdaily
+package bsdaily_test
 
-import "testing"
+import (
+	"testing"
+
+	"git.eeqj.de/sneak/bsdaily/internal/bsdaily"
+)
 
 // TestCompiles is a minimal smoke test that references the package's exported
 // surface so that `go test` fails if the package stops compiling. It does not
 // touch the filesystem or any of the hard-coded production paths.
 func TestCompiles(t *testing.T) {
-	if DBFilename == "" || WALFilename == "" || SHMFilename == "" {
+	t.Parallel()
+
+	if bsdaily.DBFilename == "" || bsdaily.WALFilename == "" ||
+		bsdaily.SHMFilename == "" {
 		t.Fatal("expected database filename constants to be set")
 	}
-	if SnapshotBase == "" || TmpBase == "" || DailiesBase == "" {
+
+	if bsdaily.SnapshotBase == "" || bsdaily.TmpBase == "" ||
+		bsdaily.DailiesBase == "" {
 		t.Fatal("expected base path constants to be set")
 	}
-	if MinTmpFreeBytes == 0 || MinDailiesFreeBytes == 0 {
+
+	if bsdaily.MinTmpFreeBytes == 0 || bsdaily.MinDailiesFreeBytes == 0 {
 		t.Fatal("expected free-space thresholds to be set")
 	}
-	if ErrNoPosts == nil {
+
+	if bsdaily.ErrNoPosts == nil {
 		t.Fatal("expected ErrNoPosts sentinel to be set")
 	}
 }

internal/bsdaily/config.go

@@ -1,7 +1,11 @@
+// Package bsdaily extracts single days of firehose data from the latest
+// daily ZFS snapshot into zstd-compressed SQL dumps.
 package bsdaily
 
 import "regexp"
 
+// Paths, file names and tuning for a run. They are set for one
+// production host; see the README.
 const (
 	SnapshotBase = "/srv/berlin.sneak.fs.blueskyarchive/.zfs/snapshot"
 	TmpBase      = "/srv/storage/tmp"
@@ -27,4 +31,5 @@ const (
 	verificationHeadLines = 20
 )
 
-var snapshotPattern = regexp.MustCompile(`^zfs-auto-snap_daily-(\d{4}-\d{2}-\d{2})-\d{4}$`)
+var snapshotPattern = regexp.MustCompile(
+	`^zfs-auto-snap_daily-(\d{4}-\d{2}-\d{2})-\d{4}$`)

internal/bsdaily/copy.go

@@ -1,6 +1,7 @@
 package bsdaily
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"log/slog"
@@ -9,20 +10,30 @@ import (
 )
 
 const (
-	copyBufferSize = 256 * 1024 * 1024 // 256MB buffer for large file copies from fast storage
+	// 256MB buffer for large file copies from fast storage
+	copyBufferSize = 256 * 1024 * 1024
+	oneMB          = 1024 * 1024
 	oneGB          = 1024 * 1024 * 1024
 )
 
+var errShortCopy = errors.New("short copy")
+
+// CopyFile copies src to dst through a large buffer, pre-allocating dst
+// and syncing it to disk before returning.
 func CopyFile(src, dst string) (err error) {
 	startTime := time.Now()
+
 	slog.Info("copying file", "src", src, "dst", dst)
 
+	//nolint:gosec // src is a path this package built
 	srcFile, err := os.Open(src)
 	if err != nil {
 		return fmt.Errorf("opening source %s: %w", src, err)
 	}
+
 	defer func() {
-		if cerr := srcFile.Close(); cerr != nil {
+		cerr := srcFile.Close()
+		if cerr != nil {
 			slog.Warn("failed to close source file", "src", src, "error", cerr)
 		}
 	}()
@@ -37,40 +48,49 @@ func CopyFile(src, dst string) (err error) {
 		applyFileAdvice(srcFile, srcInfo.Size())
 	}
 
+	//nolint:gosec // dst is a path this package built
 	dstFile, err := os.Create(dst)
 	if err != nil {
 		return fmt.Errorf("creating destination %s: %w", dst, err)
 	}
+
 	defer func() {
-		if cerr := dstFile.Close(); cerr != nil && err == nil {
+		cerr := dstFile.Close()
+		if cerr != nil && err == nil {
 			err = fmt.Errorf("closing destination %s: %w", dst, cerr)
 		}
 	}()
 
 	// Pre-allocate space for the destination file to avoid fragmentation
-	if err := dstFile.Truncate(srcInfo.Size()); err != nil {
+	truncErr := dstFile.Truncate(srcInfo.Size())
-		slog.Warn("failed to pre-allocate destination file", "error", err)
+	if truncErr != nil {
+		slog.Warn("failed to pre-allocate destination file", "error", truncErr)
 	}
 
 	// Use a much larger buffer for NVMe-speed copies
 	buf := make([]byte, copyBufferSize)
+
 	written, err := io.CopyBuffer(dstFile, srcFile, buf)
 	if err != nil {
 		return fmt.Errorf("copying data: %w", err)
 	}
 
 	if written != srcInfo.Size() {
-		return fmt.Errorf("short copy: wrote %d bytes, expected %d", written, srcInfo.Size())
+		return fmt.Errorf("%w: wrote %d bytes, expected %d",
+			errShortCopy, written, srcInfo.Size())
 	}
 
-	if err := dstFile.Sync(); err != nil {
+	err = dstFile.Sync()
+	if err != nil {
 		return fmt.Errorf("syncing destination %s: %w", dst, err)
 	}
 
 	elapsed := time.Since(startTime)
-	throughputMBps := float64(written) / elapsed.Seconds() / (1024 * 1024)
+	throughputMBps := float64(written) / elapsed.Seconds() / oneMB
+
 	slog.Info("file copied", "dst", dst, "bytes", written,
 		"elapsed", elapsed.Round(time.Millisecond),
 		"throughput_mbps", fmt.Sprintf("%.1f", throughputMBps))
+
 	return nil
 }

internal/bsdaily/copy_linux.go

@@ -9,8 +9,7 @@ import (
 
 func applyFileAdvice(file *os.File, size int64) {
 	fd := int(file.Fd())
-	// POSIX_FADV_SEQUENTIAL = 2
+	_ = unix.Fadvise(fd, 0, size, unix.FADV_SEQUENTIAL)
-	_ = unix.Fadvise(fd, 0, size, 2)
+	// Prefetch the file into the page cache
-	// POSIX_FADV_WILLNEED = 3 - prefetch file into cache
+	_ = unix.Fadvise(fd, 0, size, unix.FADV_WILLNEED)
-	_ = unix.Fadvise(fd, 0, size, 3)
 }

internal/bsdaily/disk.go

@@ -1,26 +1,39 @@
 package bsdaily
 
 import (
+	"errors"
 	"fmt"
 	"log/slog"
 
 	"golang.org/x/sys/unix"
 )
 
+var errInsufficientSpace = errors.New("insufficient disk space")
+
+// CheckFreeSpace returns an error when the filesystem holding path has
+// fewer than minBytes bytes available. label names the location in the
+// log line and the error.
 func CheckFreeSpace(path string, minBytes uint64, label string) error {
 	var stat unix.Statfs_t
-	if err := unix.Statfs(path, &stat); err != nil {
+
+	err := unix.Statfs(path, &stat)
+	if err != nil {
 		return fmt.Errorf("statfs %s (%s): %w", path, label, err)
 	}
+
+	//nolint:gosec,unconvert // Bsize is never negative; Bavail is signed on FreeBSD
 	free := uint64(stat.Bavail) * uint64(stat.Bsize)
 	freeGB := float64(free) / float64(bytesPerGB)
 	minGB := float64(minBytes) / float64(bytesPerGB)
+
 	slog.Info("disk space check", "label", label, "path", path,
 		"free_gb", fmt.Sprintf("%.1f", freeGB),
 		"required_gb", fmt.Sprintf("%.1f", minGB))
+
 	if free < minBytes {
-		return fmt.Errorf("insufficient disk space on %s (%s): %.1f GB free, need %.1f GB",
+		return fmt.Errorf("%w on %s (%s): %.1f GB free, need %.1f GB",
-			path, label, freeGB, minGB)
+			errInsufficientSpace, path, label, freeGB, minGB)
 	}
+
 	return nil
 }

internal/bsdaily/dump.go

@@ -1,6 +1,8 @@
 package bsdaily
 
 import (
+	"context"
+	"errors"
 	"fmt"
 	"log/slog"
 	"os"
@@ -9,61 +11,44 @@ import (
 	"strings"
 )
 
+var errEmptyOutput = errors.New("compressed output is empty")
+
+// DumpAndCompress writes a `sqlite3 .dump` of the database at dbPath,
+// compressed by zstdmt, to outputPath.
 func DumpAndCompress(dbPath, outputPath string) (err error) {
 	for _, tool := range []string{"sqlite3", "zstdmt"} {
-		if _, err := exec.LookPath(tool); err != nil {
+		_, err = exec.LookPath(tool)
+		if err != nil {
 			return fmt.Errorf("required tool %q not found in PATH: %w", tool, err)
 		}
 	}
 
-	if err := CheckFreeSpace(filepath.Dir(outputPath), MinDailiesFreeBytes, "dailiesBase (pre-dump)"); err != nil {
+	err = CheckFreeSpace(filepath.Dir(outputPath), MinDailiesFreeBytes,
+		"dailiesBase (pre-dump)")
+	if err != nil {
 		return err
 	}
 
+	//nolint:gosec // outputPath is a path this package built
 	outFile, err := os.Create(outputPath)
 	if err != nil {
 		return fmt.Errorf("creating output file: %w", err)
 	}
+
 	defer func() {
-		if cerr := outFile.Close(); cerr != nil && err == nil {
+		cerr := outFile.Close()
+		if cerr != nil && err == nil {
 			err = fmt.Errorf("closing output: %w", cerr)
 		}
 	}()
 
-	// Dump all tables but use INSERT OR IGNORE for mergeable imports
+	err = runDumpPipeline(context.Background(), dbPath, outFile)
-	// This preserves all data while allowing multiple dumps to be merged
-	// Users should import with: zstdcat *.sql.zst | sed 's/INSERT INTO/INSERT OR IGNORE INTO/g' | sqlite3 merged.db
-	dumpCmd := exec.Command("sqlite3", dbPath, ".dump")
-	zstdCmd := exec.Command("zstdmt", fmt.Sprintf("-%d", zstdCompressionLevel))
-
-	pipe, err := dumpCmd.StdoutPipe()
 	if err != nil {
-		return fmt.Errorf("creating dump stdout pipe: %w", err)
+		return err
-	}
-	zstdCmd.Stdin = pipe
-	zstdCmd.Stdout = outFile
-
-	var dumpStderr, zstdStderr strings.Builder
-	dumpCmd.Stderr = &dumpStderr
-	zstdCmd.Stderr = &zstdStderr
-
-	slog.Info("starting sqlite3 dump and zstdmt compression")
-
-	if err := zstdCmd.Start(); err != nil {
-		return fmt.Errorf("starting zstdmt: %w", err)
-	}
-	if err := dumpCmd.Start(); err != nil {
-		return fmt.Errorf("starting sqlite3 dump: %w", err)
 	}
 
-	if err := dumpCmd.Wait(); err != nil {
+	err = outFile.Sync()
-		return fmt.Errorf("sqlite3 dump failed: %w; stderr: %s", err, dumpStderr.String())
+	if err != nil {
-	}
-	if err := zstdCmd.Wait(); err != nil {
-		return fmt.Errorf("zstdmt failed: %w; stderr: %s", err, zstdStderr.String())
-	}
-
-	if err := outFile.Sync(); err != nil {
 		return fmt.Errorf("syncing output: %w", err)
 	}
 
@@ -71,12 +56,66 @@ func DumpAndCompress(dbPath, outputPath string) (err error) {
 	if err != nil {
 		return fmt.Errorf("stat output: %w", err)
 	}
+
 	const bytesPerMB = 1024 * 1024
+
 	slog.Info("compressed output written", "path", outputPath,
 		"size_bytes", info.Size(), "size_mb", info.Size()/bytesPerMB)
 
 	if info.Size() == 0 {
-		return fmt.Errorf("compressed output is empty")
+		return errEmptyOutput
+	}
+
+	return nil
+}
+
+// runDumpPipeline runs `sqlite3 dbPath .dump | zstdmt` with the
+// compressed stream going to outFile, and waits for both to finish.
+func runDumpPipeline(ctx context.Context, dbPath string, outFile *os.File) error {
+	// The dump holds plain INSERT INTO statements. merge_daily_dumps.sh
+	// rewrites them to INSERT OR IGNORE INTO so that several dumps can be
+	// merged into one database.
+	//nolint:gosec // dbPath is a scratch file this package created
+	dumpCmd := exec.CommandContext(ctx, "sqlite3", dbPath, ".dump")
+	//nolint:gosec // the argument is built from a constant
+	zstdCmd := exec.CommandContext(ctx, "zstdmt",
+		fmt.Sprintf("-%d", zstdCompressionLevel))
+
+	pipe, err := dumpCmd.StdoutPipe()
+	if err != nil {
+		return fmt.Errorf("creating dump stdout pipe: %w", err)
+	}
+
+	zstdCmd.Stdin = pipe
+	zstdCmd.Stdout = outFile
+
+	var dumpStderr, zstdStderr strings.Builder
+
+	dumpCmd.Stderr = &dumpStderr
+	zstdCmd.Stderr = &zstdStderr
+
+	slog.Info("starting sqlite3 dump and zstdmt compression")
+
+	err = zstdCmd.Start()
+	if err != nil {
+		return fmt.Errorf("starting zstdmt: %w", err)
+	}
+
+	err = dumpCmd.Start()
+	if err != nil {
+		return fmt.Errorf("starting sqlite3 dump: %w", err)
+	}
+
+	err = dumpCmd.Wait()
+	if err != nil {
+		return fmt.Errorf("sqlite3 dump failed: %w; stderr: %s",
+			err, dumpStderr.String())
+	}
+
+	err = zstdCmd.Wait()
+	if err != nil {
+		return fmt.Errorf("zstdmt failed: %w; stderr: %s",
+			err, zstdStderr.String())
 	}
 
 	return nil

internal/bsdaily/extract.go

@@ -1,22 +1,29 @@
 package bsdaily
 
 import (
+	"context"
 	"database/sql"
 	"errors"
 	"fmt"
 	"log/slog"
 	"time"
 
+	// Registers the "sqlite" driver with database/sql.
 	_ "modernc.org/sqlite"
 )
 
+// ErrNoPosts is returned by ExtractDay when the source holds no posts for
+// the target day.
 var ErrNoPosts = errors.New("no posts found for target day")
 
+var errPostCountMismatch = errors.New("post count mismatch")
+
 // ExtractDay opens a new empty database at dstDBPath, attaches srcDBPath,
 // and copies only the target day's data into it. This is much faster than
 // pruning a full copy because it only reads/writes the small slice of data
 // being kept.
 func ExtractDay(srcDBPath, dstDBPath string, targetDay time.Time) error {
+	ctx := context.Background()
 	dayStart := targetDay.Format("2006-01-02") + "T00:00:00"
 	dayEnd := targetDay.AddDate(0, 0, 1).Format("2006-01-02") + "T00:00:00"
 
@@ -24,162 +31,282 @@ func ExtractDay(srcDBPath, dstDBPath string, targetDay time.Time) error {
 
 	// Maximum performance pragmas - we don't care about crash safety for temp files
 	// Use WAL mode for the source attachment to avoid locking issues
-	pragmas := fmt.Sprintf("?_pragma=journal_mode(WAL)&_pragma=synchronous(OFF)&_pragma=cache_size(%d)&_pragma=foreign_keys(OFF)&_pragma=temp_store(MEMORY)&_pragma=busy_timeout(5000)", sqliteCacheSizeKB)
+	pragmas := fmt.Sprintf("?_pragma=journal_mode(WAL)"+
+		"&_pragma=synchronous(OFF)&_pragma=cache_size(%d)"+
+		"&_pragma=foreign_keys(OFF)&_pragma=temp_store(MEMORY)"+
+		"&_pragma=busy_timeout(5000)", sqliteCacheSizeKB)
+
 	db, err := sql.Open("sqlite", dstDBPath+pragmas)
 	if err != nil {
 		return fmt.Errorf("opening destination database: %w", err)
 	}
+
 	defer func() {
-		if cerr := db.Close(); cerr != nil {
+		cerr := db.Close()
-			slog.Warn("failed to close destination database", "path", dstDBPath, "error", cerr)
+		if cerr != nil {
+			slog.Warn("failed to close destination database",
+				"path", dstDBPath, "error", cerr)
 		}
 	}()
 
 	// Attach source database
-	if _, err := db.Exec("ATTACH DATABASE ? AS src", srcDBPath); err != nil {
+	_, err = db.ExecContext(ctx, "ATTACH DATABASE ? AS src", srcDBPath)
+	if err != nil {
 		return fmt.Errorf("attaching source database: %w", err)
 	}
 
-	// Copy table DDL from source
+	err = createTables(ctx, db)
-	slog.Info("copying table DDL from source")
-	rows, err := db.Query("SELECT sql FROM src.sqlite_master WHERE type='table' AND name NOT LIKE 'sqlite_%' ORDER BY name")
 	if err != nil {
-		return fmt.Errorf("reading source schema: %w", err)
+		return err
-	}
-	defer func() {
-		if cerr := rows.Close(); cerr != nil {
-			slog.Warn("failed to close schema rows", "error", cerr)
-		}
-	}()
-
-	var ddlStatements []string
-	for rows.Next() {
-		var ddl string
-		if err := rows.Scan(&ddl); err != nil {
-			return fmt.Errorf("scanning DDL: %w", err)
-		}
-		ddlStatements = append(ddlStatements, ddl)
-	}
-	if err := rows.Err(); err != nil {
-		return fmt.Errorf("iterating DDL rows: %w", err)
 	}
 
-	for _, ddl := range ddlStatements {
+	postCount, err := insertDayRows(ctx, db, targetDay, dayStart, dayEnd)
-		if _, err := db.Exec(ddl); err != nil {
-			return fmt.Errorf("creating table: %w\nDDL: %s", err, ddl)
-		}
-	}
-
-	// Begin transaction for bulk inserts
-	tx, err := db.Begin()
 	if err != nil {
-		return fmt.Errorf("beginning transaction: %w", err)
+		return err
 	}
-	defer func() {
-		if err != nil {
-			if rerr := tx.Rollback(); rerr != nil && !errors.Is(rerr, sql.ErrTxDone) {
-				slog.Warn("failed to roll back transaction", "error", rerr)
-			}
-		}
-	}()
 
-	// Insert target day's data
+	err = createIndexes(ctx, db)
-	slog.Info("inserting posts for target day")
-	result, err := tx.Exec("INSERT INTO posts SELECT * FROM src.posts WHERE timestamp >= ? AND timestamp < ?", dayStart, dayEnd)
 	if err != nil {
-		return fmt.Errorf("inserting posts: %w", err)
+		return err
-	}
-	postCount, _ := result.RowsAffected()
-	slog.Info("inserted posts", "count", postCount)
-
-	if postCount == 0 {
-		return fmt.Errorf("%w %s - aborting to avoid producing empty output",
-			ErrNoPosts, targetDay.Format("2006-01-02"))
-	}
-
-	slog.Info("inserting junction and lookup tables")
-	if _, err := tx.Exec("INSERT INTO posts_hashtags SELECT * FROM src.posts_hashtags WHERE post_id IN (SELECT id FROM posts)"); err != nil {
-		return fmt.Errorf("inserting posts_hashtags: %w", err)
-	}
-
-	if _, err := tx.Exec("INSERT INTO posts_urls SELECT * FROM src.posts_urls WHERE post_id IN (SELECT id FROM posts)"); err != nil {
-		return fmt.Errorf("inserting posts_urls: %w", err)
-	}
-
-	if _, err := tx.Exec("INSERT INTO hashtags SELECT * FROM src.hashtags WHERE id IN (SELECT hashtag_id FROM posts_hashtags)"); err != nil {
-		return fmt.Errorf("inserting hashtags: %w", err)
-	}
-
-	if _, err := tx.Exec("INSERT INTO urls SELECT * FROM src.urls WHERE id IN (SELECT url_id FROM posts_urls)"); err != nil {
-		return fmt.Errorf("inserting urls: %w", err)
-	}
-
-	if _, err := tx.Exec("INSERT INTO users SELECT * FROM src.users WHERE did IN (SELECT user_did FROM posts)"); err != nil {
-		return fmt.Errorf("inserting users: %w", err)
-	}
-
-	// Check if media table exists in source and copy if present
-	var mediaTableExists int
-	if err := tx.QueryRow("SELECT COUNT(*) FROM src.sqlite_master WHERE type='table' AND name='media'").Scan(&mediaTableExists); err != nil {
-		slog.Warn("checking for media table", "error", err)
-	} else if mediaTableExists > 0 {
-		slog.Info("inserting media entries")
-		// Get post blob_cids for this day's posts
-		if _, err := tx.Exec("INSERT INTO media SELECT * FROM src.media WHERE content_hash IN (SELECT blob_cids FROM posts WHERE blob_cids IS NOT NULL)"); err != nil {
-			slog.Warn("inserting media (may not have matching entries)", "error", err)
-		}
-	}
-
-	// Commit the transaction before any further database operations
-	if err := tx.Commit(); err != nil {
-		return fmt.Errorf("committing transaction: %w", err)
-	}
-	tx = nil // Clear tx to ensure defer doesn't try to rollback
-
-	// Create indexes after bulk insert for speed
-	slog.Info("creating indexes")
-	idxRows, err := db.Query("SELECT sql FROM src.sqlite_master WHERE type='index' AND name NOT LIKE 'sqlite_%' AND sql IS NOT NULL ORDER BY name")
-	if err != nil {
-		return fmt.Errorf("reading source indexes: %w", err)
-	}
-	defer func() {
-		if cerr := idxRows.Close(); cerr != nil {
-			slog.Warn("failed to close index rows", "error", cerr)
-		}
-	}()
-
-	var idxStatements []string
-	for idxRows.Next() {
-		var idxSQL string
-		if err := idxRows.Scan(&idxSQL); err != nil {
-			return fmt.Errorf("scanning index DDL: %w", err)
-		}
-		idxStatements = append(idxStatements, idxSQL)
-	}
-	if err := idxRows.Err(); err != nil {
-		return fmt.Errorf("iterating index rows: %w", err)
-	}
-
-	for _, idxSQL := range idxStatements {
-		if _, err := db.Exec(idxSQL); err != nil {
-			return fmt.Errorf("creating index: %w\nDDL: %s", err, idxSQL)
-		}
 	}
 
 	// Detach source
-	if _, err := db.Exec("DETACH DATABASE src"); err != nil {
+	_, err = db.ExecContext(ctx, "DETACH DATABASE src")
+	if err != nil {
 		return fmt.Errorf("detaching source database: %w", err)
 	}
 
 	// Verify post count
 	var verifyCount int64
-	if err := db.QueryRow("SELECT COUNT(*) FROM posts").Scan(&verifyCount); err != nil {
+
+	err = db.QueryRowContext(ctx, "SELECT COUNT(*) FROM posts").Scan(&verifyCount)
+	if err != nil {
 		return fmt.Errorf("verifying post count: %w", err)
 	}
+
 	if verifyCount != postCount {
-		return fmt.Errorf("post count mismatch: inserted %d but found %d", postCount, verifyCount)
+		return fmt.Errorf("%w: inserted %d but found %d",
+			errPostCountMismatch, postCount, verifyCount)
 	}
+
 	slog.Info("extraction complete", "posts", verifyCount)
 
 	return nil
 }
+
+// createTables creates every table of the attached source database, empty,
+// in the destination database.
+func createTables(ctx context.Context, db *sql.DB) error {
+	// Copy table DDL from source
+	slog.Info("copying table DDL from source")
+
+	rows, err := db.QueryContext(ctx, "SELECT sql FROM src.sqlite_master "+
+		"WHERE type='table' AND name NOT LIKE 'sqlite_%' ORDER BY name")
+	if err != nil {
+		return fmt.Errorf("reading source schema: %w", err)
+	}
+
+	defer func() {
+		cerr := rows.Close()
+		if cerr != nil {
+			slog.Warn("failed to close schema rows", "error", cerr)
+		}
+	}()
+
+	var ddlStatements []string
+
+	for rows.Next() {
+		var ddl string
+
+		err = rows.Scan(&ddl)
+		if err != nil {
+			return fmt.Errorf("scanning DDL: %w", err)
+		}
+
+		ddlStatements = append(ddlStatements, ddl)
+	}
+
+	err = rows.Err()
+	if err != nil {
+		return fmt.Errorf("iterating DDL rows: %w", err)
+	}
+
+	for _, ddl := range ddlStatements {
+		_, err = db.ExecContext(ctx, ddl)
+		if err != nil {
+			return fmt.Errorf("creating table: %w\nDDL: %s", err, ddl)
+		}
+	}
+
+	return nil
+}
+
+// insertDayRows copies the target day's posts, and the rows they refer
+// to, from the source database in one transaction. It returns the number
+// of posts copied, or ErrNoPosts when there are none.
+func insertDayRows(ctx context.Context, db *sql.DB, targetDay time.Time,
+	dayStart, dayEnd string,
+) (int64, error) {
+	// Begin transaction for bulk inserts
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		return 0, fmt.Errorf("beginning transaction: %w", err)
+	}
+
+	// After a successful Commit this does nothing and returns ErrTxDone.
+	defer func() {
+		rerr := tx.Rollback()
+		if rerr != nil && !errors.Is(rerr, sql.ErrTxDone) {
+			slog.Warn("failed to roll back transaction", "error", rerr)
+		}
+	}()
+
+	// Insert target day's data
+	slog.Info("inserting posts for target day")
+
+	//nolint:unqueryvet // copies whole rows; the source defines the columns
+	result, err := tx.ExecContext(ctx, "INSERT INTO posts SELECT * FROM src.posts "+
+		"WHERE timestamp >= ? AND timestamp < ?", dayStart, dayEnd)
+	if err != nil {
+		return 0, fmt.Errorf("inserting posts: %w", err)
+	}
+
+	postCount, _ := result.RowsAffected()
+
+	slog.Info("inserted posts", "count", postCount)
+
+	if postCount == 0 {
+		return 0, fmt.Errorf("%w %s - aborting to avoid producing empty output",
+			ErrNoPosts, targetDay.Format("2006-01-02"))
+	}
+
+	slog.Info("inserting junction and lookup tables")
+
+	err = insertRelatedRows(ctx, tx)
+	if err != nil {
+		return 0, err
+	}
+
+	copyMediaRows(ctx, tx)
+
+	// Commit the transaction before any further database operations
+	err = tx.Commit()
+	if err != nil {
+		return 0, fmt.Errorf("committing transaction: %w", err)
+	}
+
+	return postCount, nil
+}
+
+// insertRelatedRows copies the hashtag and URL links of the posts already
+// inserted, the hashtags and URLs they link to, and the posting users.
+//
+//nolint:unqueryvet // copies whole rows; the source defines the columns
+func insertRelatedRows(ctx context.Context, tx *sql.Tx) error {
+	_, err := tx.ExecContext(ctx, "INSERT INTO posts_hashtags "+
+		"SELECT * FROM src.posts_hashtags WHERE post_id IN (SELECT id FROM posts)")
+	if err != nil {
+		return fmt.Errorf("inserting posts_hashtags: %w", err)
+	}
+
+	_, err = tx.ExecContext(ctx, "INSERT INTO posts_urls "+
+		"SELECT * FROM src.posts_urls WHERE post_id IN (SELECT id FROM posts)")
+	if err != nil {
+		return fmt.Errorf("inserting posts_urls: %w", err)
+	}
+
+	_, err = tx.ExecContext(ctx, "INSERT INTO hashtags "+
+		"SELECT * FROM src.hashtags "+
+		"WHERE id IN (SELECT hashtag_id FROM posts_hashtags)")
+	if err != nil {
+		return fmt.Errorf("inserting hashtags: %w", err)
+	}
+
+	_, err = tx.ExecContext(ctx, "INSERT INTO urls "+
+		"SELECT * FROM src.urls WHERE id IN (SELECT url_id FROM posts_urls)")
+	if err != nil {
+		return fmt.Errorf("inserting urls: %w", err)
+	}
+
+	_, err = tx.ExecContext(ctx, "INSERT INTO users "+
+		"SELECT * FROM src.users WHERE did IN (SELECT user_did FROM posts)")
+	if err != nil {
+		return fmt.Errorf("inserting users: %w", err)
+	}
+
+	return nil
+}
+
+// copyMediaRows copies the media rows of the posts already inserted, when
+// the source has a media table. Failures are logged, not returned.
+func copyMediaRows(ctx context.Context, tx *sql.Tx) {
+	// Check if media table exists in source and copy if present
+	var mediaTableExists int
+
+	err := tx.QueryRowContext(ctx, "SELECT COUNT(*) FROM src.sqlite_master "+
+		"WHERE type='table' AND name='media'").Scan(&mediaTableExists)
+	if err != nil {
+		slog.Warn("checking for media table", "error", err)
+	} else if mediaTableExists > 0 {
+		slog.Info("inserting media entries")
+
+		// Get post blob_cids for this day's posts
+		//nolint:unqueryvet // copies whole rows; the source defines the columns
+		_, err = tx.ExecContext(ctx, "INSERT INTO media SELECT * FROM src.media "+
+			"WHERE content_hash IN "+
+			"(SELECT blob_cids FROM posts WHERE blob_cids IS NOT NULL)")
+		if err != nil {
+			slog.Warn("inserting media (may not have matching entries)",
+				"error", err)
+		}
+	}
+}
+
+// createIndexes creates every index of the attached source database in
+// the destination database. Doing this after the bulk insert is faster
+// than inserting into indexed tables.
+func createIndexes(ctx context.Context, db *sql.DB) error {
+	// Create indexes after bulk insert for speed
+	slog.Info("creating indexes")
+
+	idxRows, err := db.QueryContext(ctx, "SELECT sql FROM src.sqlite_master "+
+		"WHERE type='index' AND name NOT LIKE 'sqlite_%' AND sql IS NOT NULL "+
+		"ORDER BY name")
+	if err != nil {
+		return fmt.Errorf("reading source indexes: %w", err)
+	}
+
+	defer func() {
+		cerr := idxRows.Close()
+		if cerr != nil {
+			slog.Warn("failed to close index rows", "error", cerr)
+		}
+	}()
+
+	var idxStatements []string
+
+	for idxRows.Next() {
+		var idxSQL string
+
+		err = idxRows.Scan(&idxSQL)
+		if err != nil {
+			return fmt.Errorf("scanning index DDL: %w", err)
+		}
+
+		idxStatements = append(idxStatements, idxSQL)
+	}
+
+	err = idxRows.Err()
+	if err != nil {
+		return fmt.Errorf("iterating index rows: %w", err)
+	}
+
+	for _, idxSQL := range idxStatements {
+		_, err = db.ExecContext(ctx, idxSQL)
+		if err != nil {
+			return fmt.Errorf("creating index: %w\nDDL: %s", err, idxSQL)
+		}
+	}
+
+	return nil
+}

internal/bsdaily/run.go

@@ -9,21 +9,29 @@ import (
 	"time"
 )
 
+var errEmptySource = errors.New("source file is empty")
+
 // cleanup removes a temporary file, logging a warning if removal fails so
 // that leaked scratch files are surfaced rather than silently ignored. A
 // missing file is not an error.
 func cleanup(path string) {
-	if err := os.Remove(path); err != nil && !os.IsNotExist(err) {
+	err := os.Remove(path)
+	if err != nil && !os.IsNotExist(err) {
 		slog.Warn("failed to remove temporary file", "path", path, "error", err)
 	}
 }
 
+// Run writes a compressed SQL dump of each day in targetDates, taken from
+// the latest daily snapshot, skipping days that already have one or have
+// no posts. With no dates it does the day before the snapshot date.
 func Run(targetDates []time.Time) error {
 	snapshotDir, snapshotDate, err := FindLatestDailySnapshot()
 	if err != nil {
 		return fmt.Errorf("finding latest snapshot: %w", err)
 	}
-	slog.Info("found latest daily snapshot", "dir", snapshotDir, "snapshot_date", snapshotDate.Format("2006-01-02"))
+
+	slog.Info("found latest daily snapshot", "dir", snapshotDir,
+		"snapshot_date", snapshotDate.Format("2006-01-02"))
 
 	if len(targetDates) == 0 {
 		targetDates = []time.Time{snapshotDate.AddDate(0, 0, -1)}
@@ -34,10 +42,13 @@ func Run(targetDates []time.Time) error {
 		"last", targetDates[len(targetDates)-1].Format("2006-01-02"))
 
 	// Check disk space
-	if err := CheckFreeSpace(TmpBase, MinTmpFreeBytes, "tmpBase"); err != nil {
+	err = CheckFreeSpace(TmpBase, MinTmpFreeBytes, "tmpBase")
+	if err != nil {
 		return err
 	}
-	if err := CheckFreeSpace(DailiesBase, MinDailiesFreeBytes, "dailiesBase"); err != nil {
+
+	err = CheckFreeSpace(DailiesBase, MinDailiesFreeBytes, "dailiesBase")
+	if err != nil {
 		return err
 	}
 
@@ -46,14 +57,53 @@ func Run(targetDates []time.Time) error {
 	if err != nil {
 		return fmt.Errorf("creating temp directory in %s: %w", TmpBase, err)
 	}
+
 	slog.Info("created temp directory", "path", tmpDir)
+
 	defer func() {
 		slog.Info("cleaning up temp directory", "path", tmpDir)
-		if err := os.RemoveAll(tmpDir); err != nil {
+
-			slog.Error("failed to remove temp directory", "path", tmpDir, "error", err)
+		rerr := os.RemoveAll(tmpDir)
+		if rerr != nil {
+			slog.Error("failed to remove temp directory",
+				"path", tmpDir, "error", rerr)
 		}
 	}()
 
+	dstDB, err := copySnapshotFiles(snapshotDir, tmpDir)
+	if err != nil {
+		return err
+	}
+
+	// Process each day completely before moving to the next. This ensures
+	// we don't have multiple SQLite operations competing for the same
+	// source database.
+	processed := 0
+	skipped := 0
+
+	for _, targetDay := range targetDates {
+		written, err := processDay(tmpDir, dstDB, targetDay)
+		if err != nil {
+			return err
+		}
+
+		if written {
+			processed++
+		} else {
+			skipped++
+		}
+	}
+
+	slog.Info("run summary", "processed", processed, "skipped", skipped,
+		"total", len(targetDates))
+
+	return nil
+}
+
+// copySnapshotFiles copies the database, its WAL and, if present, its SHM
+// file from snapshotDir into tmpDir, and returns the copied database's
+// path.
+func copySnapshotFiles(snapshotDir, tmpDir string) (string, error) {
 	// Copy database files from snapshot to temp
 	srcDB := filepath.Join(snapshotDir, DBFilename)
 	srcWAL := filepath.Join(snapshotDir, WALFilename)
@@ -65,100 +115,137 @@ func Run(targetDates []time.Time) error {
 	for _, f := range []string{srcDB, srcWAL} {
 		info, err := os.Stat(f)
 		if err != nil {
-			return fmt.Errorf("source file missing: %s: %w", f, err)
+			return "", fmt.Errorf("source file missing: %s: %w", f, err)
 		}
+
 		if info.Size() == 0 {
-			return fmt.Errorf("source file is empty: %s", f)
+			return "", fmt.Errorf("%w: %s", errEmptySource, f)
 		}
+
 		slog.Info("source file", "path", f, "size_bytes", info.Size())
 	}
 
-	if err := CopyFile(srcDB, dstDB); err != nil {
+	err := CopyFile(srcDB, dstDB)
-		return fmt.Errorf("copying database: %w", err)
+	if err != nil {
-	}
+		return "", fmt.Errorf("copying database: %w", err)
-	if err := CopyFile(srcWAL, dstWAL); err != nil {
-		return fmt.Errorf("copying WAL: %w", err)
-	}
-	if _, err := os.Stat(srcSHM); err == nil {
-		if err := CopyFile(srcSHM, dstSHM); err != nil {
-			return fmt.Errorf("copying SHM: %w", err)
-		}
 	}
 
-	// Process each day completely before moving to the next
+	err = CopyFile(srcWAL, dstWAL)
-	// This ensures we don't have multiple SQLite operations competing for the same source database
+	if err != nil {
-	processed := 0
+		return "", fmt.Errorf("copying WAL: %w", err)
-	skipped := 0
+	}
 
-	for _, targetDay := range targetDates {
+	_, err = os.Stat(srcSHM)
-		dayStr := targetDay.Format("2006-01-02")
+	if err == nil {
-		slog.Info("processing day", "date", dayStr)
+		err = CopyFile(srcSHM, dstSHM)
-
-		// Check if output already exists
-		outputDir := filepath.Join(DailiesBase, targetDay.Format("2006-01"))
-		outputFinal := filepath.Join(outputDir, dayStr+".sql.zst")
-		if _, err := os.Stat(outputFinal); err == nil {
-			slog.Info("output already exists, skipping", "path", outputFinal)
-			skipped++
-			continue
-		}
-
-		// Extract target day into a per-day database
-		extractedDB := filepath.Join(tmpDir, "extracted-"+dayStr+".db")
-		slog.Info("extracting target day", "src", dstDB, "dst", extractedDB)
-		if err := ExtractDay(dstDB, extractedDB, targetDay); err != nil {
-			if errors.Is(err, ErrNoPosts) {
-				slog.Warn("no posts found, skipping day", "date", dayStr)
-				cleanup(extractedDB)
-				skipped++
-				continue
-			}
-			return fmt.Errorf("extracting day %s: %w", dayStr, err)
-		}
-
-		// Dump to SQL and compress
-		if err := os.MkdirAll(outputDir, 0755); err != nil {
-			cleanup(extractedDB)
-			return fmt.Errorf("creating output directory %s: %w", outputDir, err)
-		}
-
-		outputTmp := filepath.Join(outputDir, "."+dayStr+".sql.zst.tmp")
-
-		slog.Info("dumping and compressing", "tmp_output", outputTmp)
-		if err := DumpAndCompress(extractedDB, outputTmp); err != nil {
-			cleanup(outputTmp)
-			cleanup(extractedDB)
-			return fmt.Errorf("dump and compress for %s: %w", dayStr, err)
-		}
-
-		slog.Info("verifying compressed output")
-		if err := VerifyOutput(outputTmp); err != nil {
-			cleanup(outputTmp)
-			cleanup(extractedDB)
-			return fmt.Errorf("verification failed for %s: %w", dayStr, err)
-		}
-
-		// Atomic rename to final path
-		slog.Info("renaming to final output", "from", outputTmp, "to", outputFinal)
-		if err := os.Rename(outputTmp, outputFinal); err != nil {
-			cleanup(outputTmp)
-			cleanup(extractedDB)
-			return fmt.Errorf("atomic rename for %s: %w", dayStr, err)
-		}
-
-		info, err := os.Stat(outputFinal)
 		if err != nil {
-			cleanup(extractedDB)
+			return "", fmt.Errorf("copying SHM: %w", err)
-			return fmt.Errorf("stat final output: %w", err)
 		}
-		slog.Info("day completed", "date", dayStr, "path", outputFinal, "size_bytes", info.Size())
-
-		// Remove extracted DB to reclaim space immediately
-		cleanup(extractedDB)
-		processed++
 	}
 
-	slog.Info("run summary", "processed", processed, "skipped", skipped, "total", len(targetDates))
+	return dstDB, nil
+}
+
+// processDay extracts one day from the copied database at dstDB and
+// publishes its compressed dump. It returns false, with no error, for a
+// day it skips: one whose output already exists or that has no posts.
+func processDay(tmpDir, dstDB string, targetDay time.Time) (bool, error) {
+	dayStr := targetDay.Format("2006-01-02")
+
+	slog.Info("processing day", "date", dayStr)
+
+	// Check if output already exists
+	outputDir := filepath.Join(DailiesBase, targetDay.Format("2006-01"))
+	outputFinal := filepath.Join(outputDir, dayStr+".sql.zst")
+
+	_, err := os.Stat(outputFinal)
+	if err == nil {
+		slog.Info("output already exists, skipping", "path", outputFinal)
+
+		return false, nil
+	}
+
+	// Extract target day into a per-day database
+	extractedDB := filepath.Join(tmpDir, "extracted-"+dayStr+".db")
+
+	slog.Info("extracting target day", "src", dstDB, "dst", extractedDB)
+
+	err = ExtractDay(dstDB, extractedDB, targetDay)
+	if err != nil {
+		if errors.Is(err, ErrNoPosts) {
+			slog.Warn("no posts found, skipping day", "date", dayStr)
+			cleanup(extractedDB)
+
+			return false, nil
+		}
+
+		return false, fmt.Errorf("extracting day %s: %w", dayStr, err)
+	}
+
+	// Dump to SQL and compress
+	//nolint:gosec,mnd // world-readable on purpose: the dailies tree is published
+	err = os.MkdirAll(outputDir, 0755)
+	if err != nil {
+		cleanup(extractedDB)
+
+		return false, fmt.Errorf("creating output directory %s: %w", outputDir, err)
+	}
+
+	outputTmp := filepath.Join(outputDir, "."+dayStr+".sql.zst.tmp")
+
+	slog.Info("dumping and compressing", "tmp_output", outputTmp)
+
+	err = DumpAndCompress(extractedDB, outputTmp)
+	if err != nil {
+		cleanup(outputTmp)
+		cleanup(extractedDB)
+
+		return false, fmt.Errorf("dump and compress for %s: %w", dayStr, err)
+	}
+
+	slog.Info("verifying compressed output")
+
+	err = VerifyOutput(outputTmp)
+	if err != nil {
+		cleanup(outputTmp)
+		cleanup(extractedDB)
+
+		return false, fmt.Errorf("verification failed for %s: %w", dayStr, err)
+	}
+
+	err = publishOutput(outputTmp, outputFinal, dayStr)
+
+	// Remove extracted DB to reclaim space immediately
+	cleanup(extractedDB)
+
+	if err != nil {
+		return false, err
+	}
+
+	return true, nil
+}
+
+// publishOutput renames the verified temporary output to its final path
+// and logs the finished day. The rename is atomic, so the final path never
+// holds a partial file.
+func publishOutput(outputTmp, outputFinal, dayStr string) error {
+	// Atomic rename to final path
+	slog.Info("renaming to final output", "from", outputTmp, "to", outputFinal)
+
+	err := os.Rename(outputTmp, outputFinal)
+	if err != nil {
+		cleanup(outputTmp)
+
+		return fmt.Errorf("atomic rename for %s: %w", dayStr, err)
+	}
+
+	info, err := os.Stat(outputFinal)
+	if err != nil {
+		return fmt.Errorf("stat final output: %w", err)
+	}
+
+	slog.Info("day completed", "date", dayStr, "path", outputFinal,
+		"size_bytes", info.Size())
 
 	return nil
 }

internal/bsdaily/snapshot.go

@@ -1,6 +1,7 @@
 package bsdaily
 
 import (
+	"errors"
 	"fmt"
 	"log/slog"
 	"os"
@@ -9,10 +10,16 @@ import (
 	"time"
 )
 
-func FindLatestDailySnapshot() (dir string, snapshotDate time.Time, err error) {
+var errNoSnapshots = errors.New("no daily snapshots found")
+
+// FindLatestDailySnapshot returns the directory and date of the newest
+// daily snapshot in SnapshotBase. It fails when there is none or when the
+// newest one has no database file.
+func FindLatestDailySnapshot() (string, time.Time, error) {
 	entries, err := os.ReadDir(SnapshotBase)
 	if err != nil {
-		return "", time.Time{}, fmt.Errorf("reading snapshot directory %s: %w", SnapshotBase, err)
+		return "", time.Time{}, fmt.Errorf("reading snapshot directory %s: %w",
+			SnapshotBase, err)
 	}
 
 	type snapshot struct {
@@ -21,24 +28,30 @@ func FindLatestDailySnapshot() (dir string, snapshotDate time.Time, err error) {
 	}
 
 	var snapshots []snapshot
+
 	for _, e := range entries {
 		if !e.IsDir() {
 			continue
 		}
+
 		m := snapshotPattern.FindStringSubmatch(e.Name())
 		if m == nil {
 			continue
 		}
+
 		d, err := time.Parse("2006-01-02", m[1])
 		if err != nil {
-			slog.Warn("skipping snapshot with unparseable date", "name", e.Name(), "error", err)
+			slog.Warn("skipping snapshot with unparseable date",
+				"name", e.Name(), "error", err)
+
 			continue
 		}
+
 		snapshots = append(snapshots, snapshot{name: e.Name(), date: d})
 	}
 
 	if len(snapshots) == 0 {
-		return "", time.Time{}, fmt.Errorf("no daily snapshots found in %s", SnapshotBase)
+		return "", time.Time{}, fmt.Errorf("%w in %s", errNoSnapshots, SnapshotBase)
 	}
 
 	sort.Slice(snapshots, func(i, j int) bool {
@@ -46,11 +59,14 @@ func FindLatestDailySnapshot() (dir string, snapshotDate time.Time, err error) {
 	})
 
 	latest := snapshots[0]
-	dir = filepath.Join(SnapshotBase, latest.name)
+	dir := filepath.Join(SnapshotBase, latest.name)
 
 	dbPath := filepath.Join(dir, DBFilename)
-	if _, err := os.Stat(dbPath); err != nil {
+
-		return "", time.Time{}, fmt.Errorf("database not found in snapshot %s: %w", dir, err)
+	_, err = os.Stat(dbPath)
+	if err != nil {
+		return "", time.Time{}, fmt.Errorf("database not found in snapshot %s: %w",
+			dir, err)
 	}
 
 	return dir, latest.date, nil

internal/bsdaily/verify.go

@@ -1,6 +1,7 @@
 package bsdaily
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"log/slog"
@@ -9,6 +10,11 @@ import (
 	"strings"
 )
 
+var (
+	errEmptyDecompressed = errors.New("decompressed content is empty")
+	errNotSQL            = errors.New("decompressed content does not look like SQL")
+)
+
 // killCat terminates the zstdcat process, ignoring the benign case where it
 // has already exited (e.g. after receiving SIGPIPE when head closed the pipe)
 // and logging any other failure.
@@ -16,70 +22,126 @@ func killCat(cmd *exec.Cmd) {
 	if cmd.Process == nil {
 		return
 	}
-	if err := cmd.Process.Kill(); err != nil && !errors.Is(err, os.ErrProcessDone) {
+
+	err := cmd.Process.Kill()
+	if err != nil && !errors.Is(err, os.ErrProcessDone) {
 		slog.Warn("failed to kill zstdcat process", "error", err)
 	}
 }
 
+// VerifyOutput checks that the compressed file at path passes zstdmt's
+// integrity test and that its first lines look like SQL.
 func VerifyOutput(path string) error {
+	ctx := context.Background()
+
 	slog.Info("running zstdmt integrity check")
-	testCmd := exec.Command("zstdmt", "--test", path)
+
+	//nolint:gosec // path is an output file this package named
+	testCmd := exec.CommandContext(ctx, "zstdmt", "--test", path)
+
 	var testStderr strings.Builder
+
 	testCmd.Stderr = &testStderr
-	if err := testCmd.Run(); err != nil {
+
-		return fmt.Errorf("zstdmt --test failed: %w; stderr: %s", err, testStderr.String())
+	err := testCmd.Run()
+	if err != nil {
+		return fmt.Errorf("zstdmt --test failed: %w; stderr: %s",
+			err, testStderr.String())
 	}
+
 	slog.Info("zstdmt integrity check passed")
 
 	slog.Info("verifying SQL content")
-	catCmd := exec.Command("zstdcat", path)
+
-	headCmd := exec.Command("head", fmt.Sprintf("-%d", verificationHeadLines))
+	content, err := readDecompressedHead(ctx, path)
+	if err != nil {
+		return err
+	}
+
+	err = checkLooksLikeSQL(content)
+	if err != nil {
+		return err
+	}
+
+	slog.Info("SQL content verification passed")
+
+	return nil
+}
+
+// readDecompressedHead returns the first verificationHeadLines lines of
+// the decompressed file at path, read through `zstdcat path | head`.
+func readDecompressedHead(ctx context.Context, path string) (string, error) {
+	//nolint:gosec // path is an output file this package named
+	catCmd := exec.CommandContext(ctx, "zstdcat", path)
+	//nolint:gosec // the argument is built from a constant
+	headCmd := exec.CommandContext(ctx, "head",
+		fmt.Sprintf("-%d", verificationHeadLines))
 
 	pipe, err := catCmd.StdoutPipe()
 	if err != nil {
-		return fmt.Errorf("creating zstdcat pipe: %w", err)
+		return "", fmt.Errorf("creating zstdcat pipe: %w", err)
 	}
+
 	headCmd.Stdin = pipe
 
 	var headOut strings.Builder
+
 	headCmd.Stdout = &headOut
 
-	if err := catCmd.Start(); err != nil {
+	err = catCmd.Start()
-		return fmt.Errorf("starting zstdcat: %w", err)
+	if err != nil {
+		return "", fmt.Errorf("starting zstdcat: %w", err)
 	}
-	if err := headCmd.Start(); err != nil {
+
+	err = headCmd.Start()
+	if err != nil {
 		killCat(catCmd) // Clean up if head fails to start
-		return fmt.Errorf("starting head: %w", err)
+
+		return "", fmt.Errorf("starting head: %w", err)
 	}
 
 	// Wait for head first (it will exit when it has enough lines)
-	if err := headCmd.Wait(); err != nil {
+	err = headCmd.Wait()
+	if err != nil {
 		killCat(catCmd)
-		return fmt.Errorf("head command failed: %w", err)
+
+		return "", fmt.Errorf("head command failed: %w", err)
 	}
 
 	// Kill zstdcat since head closed the pipe (expected SIGPIPE)
 	killCat(catCmd)
+
 	_ = catCmd.Wait() // Reap the process
 
-	content := headOut.String()
+	return headOut.String(), nil
+}
+
+// checkLooksLikeSQL returns an error when content is empty or contains
+// none of the keywords expected near the start of a `sqlite3 .dump`.
+func checkLooksLikeSQL(content string) error {
 	if len(content) == 0 {
-		return fmt.Errorf("decompressed content is empty")
+		return errEmptyDecompressed
 	}
 
 	hasSQLMarker := false
-	for _, marker := range []string{"BEGIN TRANSACTION", "CREATE TABLE", "INSERT INTO", "PRAGMA"} {
+
+	for _, marker := range []string{
+		"BEGIN TRANSACTION", "CREATE TABLE", "INSERT INTO", "PRAGMA",
+	} {
 		if strings.Contains(content, marker) {
 			hasSQLMarker = true
+
 			break
 		}
 	}
+
 	const verificationSampleBytes = 200
+
 	if !hasSQLMarker {
-		return fmt.Errorf("decompressed content does not look like SQL; first %d bytes: %s",
+		return fmt.Errorf("%w; first %d bytes: %s", errNotSQL,
-			verificationSampleBytes, content[:min(verificationSampleBytes, len(content))])
+			verificationSampleBytes,
+			content[:min(verificationSampleBytes, len(content))])
 	}
 
-	slog.Info("SQL content verification passed")
 	return nil
 }