Drop the lint-guard shell scanner, keep the Dockerfile.lint checks (closes #121)

Four reviews found successive ways a shell script could run the host
linter that the hand-written scanner in cmd/vaultik/lintdocker_test.go
did not see. A shell parser cannot be complete, and it is the
machinery-over-fix the workflow rejects. The tree is already right:
script/lint is the only lint entry point and runs golangci-lint only
inside the container; keeping it so is a review matter.

Remove TestNoHostLintPathRemains and every helper and helper-test that
served only it. Keep the plain Dockerfile.lint assertions: pinned image,
config verify before run, and the per-run epoch reaching both steps.
Correct the TODO.md sentence that claimed a test proves no host lint.

Model: opus-4-8

.gitea/workflows/release.yml

@@ -20,33 +20,21 @@ jobs:
       # check.yml runs script/cibuild, which does all of its work inside
       # the digest-pinned Dockerfile images -- so without this step the
       # release either fails at the before-hook or, worse, ships binaries
-      # built by whatever unpinned Go the runner happens to carry.
-      # REPO_POLICIES.md requires every external reference to be pinned,
-      # and script/release already refuses a goreleaser that is not the
-      # pinned build; the compiler that actually produces the artifacts
-      # is the last thing that should be exempt from that.
+      # built by whatever Go the runner happens to carry.
       #
-      # go-version-file rather than a literal: go.mod's `go 1.26.1` is
-      # the single source of truth for the toolchain, the same way the
-      # Dockerfile FROM line is the single source of truth for the
-      # linter version that script/lint enforces. It is a three-component
-      # version, so setup-go resolves it exactly -- no silent drift onto
-      # a newer patch release.
-      #
-      # actions/setup-go v5.6.0, 2025-12-15. Pinned by commit sha, like
-      # the checkout above. v5.x is a node20 action, matching the node20
-      # actions/checkout v4 already in use here; the v6/v7 line requires
-      # a node24 runner, which this Gitea runner has never been asked
-      # for and cannot be assumed to provide.
+      # actions/setup-go would pin the action by commit sha, but the Go
+      # tarball it downloads at runtime is verified against no value in
+      # this repo, and the action exposes no checksum input.
+      # REPO_POLICIES.md requires every external reference to be pinned
+      # by hash with no exceptions, and this is the compiler that
+      # produces the published binaries -- the input where a substituted
+      # artifact matters most. So Go is installed the way goreleaser is:
+      # script/install-go downloads the exact archive for go.mod's `go`
+      # directive and refuses it unless its sha256 matches the value
+      # committed in the script, then puts .tool/go/bin on PATH for the
+      # steps below.
       - name: Install Go
-        uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff
-        with:
-          go-version-file: go.mod
-          # setup-go's module cache needs a runner-side cache backend.
-          # A release is cut rarely and a cold module download costs
-          # seconds; a release failing because a cache service is absent
-          # costs a re-tag. Off, deliberately.
-          cache: false
+        run: script/install-go
       - name: Install goreleaser
         run: script/install-goreleaser
       - name: Release
@@ -58,3 +46,8 @@ jobs:
           # It is deliberately not the runner's automatic token, which is
           # not guaranteed to carry that scope.
           GITEA_TOKEN: ${{ secrets.RELEASE_TOKEN }}
+          # Build with the toolchain install-go just verified, never a
+          # different one auto-downloaded from a `toolchain` directive:
+          # the point of the hash pin is that this exact compiler makes
+          # the release.
+          GOTOOLCHAIN: local

Dockerfile

@@ -22,8 +22,10 @@ FROM golang:1.26.1-alpine@sha256:2389ebfa5b7f43eeafbd6be0c3700cc46690ef842ad962f
 
 ARG VERSION=dev
 
-# Install build dependencies for CGO (mattn/go-sqlite3) and sqlite3 CLI (tests)
-RUN apk add --no-cache make build-base sqlite
+# Build tooling: make, plus a C toolchain because `go test -race` needs cgo.
+# The sqlite driver is pure Go (modernc.org/sqlite), so no sqlite library or
+# CLI is required.
+RUN apk add --no-cache make build-base
 
 WORKDIR /src
 
@@ -71,7 +73,7 @@ RUN CGO_ENABLED=0 go build -ldflags "-X 'sneak.berlin/go/vaultik/internal/global
 # alpine:3.21, 2026-02-25
 FROM alpine:3.21@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4709
 
-RUN apk add --no-cache ca-certificates sqlite
+RUN apk add --no-cache ca-certificates
 
 # Copy binary from builder
 COPY --from=builder /vaultik /usr/local/bin/vaultik

Dockerfile.lint

@@ -72,11 +72,11 @@ RUN [ -n "$CHECK_EPOCH" ] || exit 1
 # running, and exits 0 reporting `0 issues.` on a tree the real config
 # fails. Demonstrated on this repo at this pin, recorded on
 # https://git.eeqj.de/sneak/vaultik/pulls/114: with a planted
-# over-length line, `script/lint` exits 1 naming the `lll` finding with
-# `linters:` and exits 0 with `linterz:`. A set-but-ineffective config
-# quietly falling back to defaults is precisely the false-green class
-# this gate exists to eliminate, so it must not sit in the gate's own
-# configuration.
+# over-length line, `script/lint` exits 1 naming the `revive` finding
+# with `linters:` and exits 0 with `linterz:`. A set-but-ineffective
+# config quietly falling back to defaults is precisely the false-green
+# class this gate exists to eliminate, so it must not sit in the gate's
+# own configuration.
 #
 # `config verify` catches it, and it does so OFFLINE at this pinned
 # version -- verified, not assumed. Under `docker run --network none`

README.md

@@ -613,7 +613,6 @@ regardless of color setting (emoji are not color).
   and the pre-commit hook both run it. A `golangci-lint` installed on
   `PATH` is not a substitute and is never used on a host, whatever its
   version.
-* `sqlite3` CLI, which the test suite shells out to
 * S3-compatible object storage (or local filesystem, or rclone remote)
 
 ## development workflow
@@ -644,8 +643,8 @@ standard: normalized scripts in `script/` are the entrypoints for the
 development workflow, and the Makefile targets are thin shims that call
 them. We provide:
 
-* `script/bootstrap` — install all development dependencies (go, sqlite3,
-  Go module download). It deliberately does not install `golangci-lint`;
+* `script/bootstrap` — install all development dependencies (go, Go
+  module download). It deliberately does not install `golangci-lint`;
   see `script/lint` below.
 * `script/setup` — make a fresh clone ready for development: runs
   `script/bootstrap`, then `script/install-precommit`
@@ -659,6 +658,14 @@ them. We provide:
   called by `script/bootstrap`; the release workflow calls it directly
   because it needs `goreleaser` but not the Docker daemon
   `script/bootstrap` insists on.
+* `script/install-go` — install the Go toolchain named by `go.mod`'s
+  `go` directive into `.tool/go` from a sha256-verified `go.dev`
+  archive, and put it on `PATH`. Idempotent. Called only by the release
+  workflow, which needs a host Go for `goreleaser` to shell out to;
+  nothing else on the release runner does. `actions/setup-go` is not
+  used because it verifies the downloaded toolchain against no value in
+  this repo. Bumping Go edits `go.mod`, the checksum in this script, and
+  the `Dockerfile` `golang` digest together.
 * `script/release` — cross-compile and publish the release artifacts
   with the pinned `goreleaser`. Refuses a `goreleaser` on `PATH` whose
   version is not the pinned one, on the same reasoning as `script/lint`.

TODO.md

@@ -25,12 +25,31 @@ release" is exactly the contradiction
 
 # Completed Steps
 
+- 2026-09-21: Made `snapshot create` VACUUM the per-snapshot metadata
+  database through the `modernc.org/sqlite` driver instead of shelling
+  out to the external `sqlite` command-line binary (issue #120). A
+  backup no longer needs that binary on `PATH`, so `make check` passes
+  on a stock `go install` host; `script/bootstrap` and the `Dockerfile`
+  (both the test-build and the shipped runtime stage) no longer install
+  it, and a new test asserts the uploaded database keeps no pages from
+  deleted rows. Dropped the now-false note on the 2026-08-07 entry below
+  that said bootstrap installs it.
 - 2026-09-21: Made `.gitea/workflows/check.yml` run on pushes to `main`
   and `next` and on pull requests against either, so unit PRs (whose
   base is `next`) and `next` itself get a CI run instead of relying on a
   local `make check`
   ([issue #122](https://git.eeqj.de/sneak/vaultik/issues/122)).
 
+- 2026-09-21: Hash-verified the Go toolchain in the release workflow
+  ([issue #105](https://git.eeqj.de/sneak/vaultik/issues/105)). New
+  `script/install-go` downloads the exact `go.dev` archive for `go.mod`'s
+  `go` directive and refuses it unless its sha256 matches a value
+  committed in the script; `.gitea/workflows/release.yml` calls it
+  instead of `actions/setup-go`, which verified the downloaded toolchain
+  against nothing in the repo. `GOTOOLCHAIN: local` on the release step
+  keeps that exact compiler from auto-switching. Bumping Go now touches
+  `go.mod`, the checksum, and the `Dockerfile` `golang` digest together.
+
 - 2026-08-10: Moved every lint run into its own container, as a build
   step ([issue #113](https://git.eeqj.de/sneak/vaultik/issues/113)).
   New root `Dockerfile.lint`, built by `script/lint`, runs
@@ -59,10 +78,11 @@ release" is exactly the contradiction
   into each check command, and a fresh `$(date +%s%N)$$` per invocation
   computed as a bare assignment. `cmd/vaultik/lintdocker_test.go`
   parses both Dockerfiles and both scripts and fails if any part of
-  that is dropped, because every way of losing it is silent. Its
-  host-lint assertion is structural — no script runs `golangci-lint`
-  except through `docker` — rather than a search for the one retired
-  variable name, which nothing could ever reintroduce.
+  that is dropped, because every way of losing it is silent. No test
+  asserts that no script runs the host linter: `script/lint` is the one
+  lint entry point and runs `golangci-lint` only inside the container,
+  and keeping it that way is a review matter, not something a test
+  proves.
 
   The product `Dockerfile` lost its lint stage rather than gaining a
   second linter pin: `make lint` is now `docker build`, so the stage
@@ -529,7 +549,7 @@ release" is exactly the contradiction
   was green was wrong.
 - 2026-08-07: Added the standard `.golangci.yml` and `.editorconfig`
   (issue #59); lint findings under the new config are tracked in issue
-  #61. `script/bootstrap` now installs sqlite3 (needed by tests).
+  #61.
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints,
   Makefile shims, README Entrypoints section
 - 2026-07-02: Consolidated CLI verbs, retired overlapping commands; bound

cmd/vaultik/lintdocker_test.go

@@ -28,6 +28,11 @@ import (
 // -- that a real finding actually fails the build -- is verified by
 // hand against a deliberately broken tree, recorded on the pull
 // request.
+//
+// One property is deliberately NOT tested here: that no script runs the
+// linter on the host. script/lint is the only lint entry point, and it
+// runs golangci-lint only inside the container; keeping it that way is a
+// review matter, not something a test in this file establishes.
 
 // The files under guard, relative to the repository root.
 const (
@@ -37,9 +42,8 @@ const (
 	cibuildScript     = "script/cibuild"
 )
 
-// linterBinary is the linter's command name. Every occurrence of it in
-// executable shell in this repo must be inside a docker invocation; see
-// TestNoHostLintPathRemains.
+// linterBinary is the linter's command name, used to locate the
+// config-verify and lint steps in Dockerfile.lint.
 const linterBinary = "golangci-lint"
 
 // checkEpochARG is the declaration, with no default value. A default
@@ -219,90 +223,6 @@ func TestCibuildBuildsBothDockerfilesWithFreshEpochs(t *testing.T) {
 		"%s must build %s", cibuildScript, lintDockerfile)
 }
 
-// TestNoHostLintPathRemains fails if any escape hatch to a host linter
-// comes back. The owner's ruling is that every lint run happens inside
-// a container; a PATH binary that happens to match the pinned version
-// is a different build reached by a different code path, and admitting
-// it is what lets a local pass disagree with CI.
-//
-// This asserts the PROPERTY -- no script invokes the linter except
-// through docker -- rather than the absence of any particular variable
-// name. An earlier version of this test looked only for the literal
-// VAULTIK_LINT_IN_CONTAINER, the name of the hatch that was removed
-// alongside it, so nothing could ever trip it again: a hatch under any
-// other name left it passing. A structural test that passes on a broken
-// tree is worse than no test, because it is what a later reader trusts
-// instead of re-deriving the invariant.
-//
-// script/lint-fix is not exempted. It is the one script that runs the
-// linter as a container rather than as a build step, but it still runs
-// it in one, so the same property holds of it.
-func TestNoHostLintPathRemains(t *testing.T) {
-	t.Parallel()
-
-	root := repoRoot(t)
-
-	entries, err := os.ReadDir(filepath.Join(root, "script"))
-	require.NoError(t, err)
-	require.NotEmpty(t, entries, "no scripts found to scan")
-
-	for _, entry := range entries {
-		if entry.IsDir() {
-			continue
-		}
-
-		name := filepath.Join("script", entry.Name())
-		for _, line := range shellCode(readRepoFile(t, name)) {
-			assertLinterIsContainerised(t, name, line)
-		}
-	}
-}
-
-// assertLinterIsContainerised fails if the line runs the linter without
-// handing it to docker first. Position matters: docker has to come
-// before the binary, or the line is running the host linter and merely
-// mentioning docker afterwards.
-func assertLinterIsContainerised(t *testing.T, name, line string) {
-	t.Helper()
-
-	at := strings.Index(line, linterBinary)
-	if at < 0 {
-		return
-	}
-
-	docker := strings.Index(line, "docker")
-
-	assert.True(t, docker >= 0 && docker < at,
-		"%s runs %s on the host; every lint run happens in a container"+
-			" (line: %s)", name, linterBinary, line)
-}
-
-// TestShellCodeSeesCodeAndNotProse keeps the scanner above honest. It
-// has to ignore comments and here-document bodies, because script/lint
-// and script/bootstrap both NAME golangci-lint in prose -- in comments,
-// and in the error text they print -- precisely to say that the host
-// binary is never used. A scanner that went blind, by over-eager
-// stripping or by failing to join continuation lines, would make
-// TestNoHostLintPathRemains pass on everything.
-func TestShellCodeSeesCodeAndNotProse(t *testing.T) {
-	t.Parallel()
-
-	script := strings.Join([]string{
-		"#!/bin/sh",
-		"# a comment naming golangci-lint",
-		"cat >&2 <<EOF",
-		"prose naming golangci-lint, printed not executed",
-		"EOF",
-		"docker run --rm \\",
-		"    \"$image\" \\",
-		"    golangci-lint run ./...",
-	}, "\n")
-
-	assert.Equal(t,
-		[]string{"cat >&2 <<EOF", `docker run --rm "$image" golangci-lint run ./...`},
-		shellCode(script))
-}
-
 // assertEpochExpandedInto fails unless some instruction runs the named
 // command with the epoch expanded into it. Expansion, not mere
 // declaration: an ARG that no instruction references is not guaranteed
@@ -407,70 +327,6 @@ func indexContaining(found []string, want string) int {
 	return -1
 }
 
-// shellCode returns a POSIX shell script's executable lines: comments
-// dropped, here-document bodies dropped, and backslash continuations
-// joined so a multi-line command is a single string. Whitespace is
-// collapsed, as it is for Dockerfile instructions.
-//
-// Both exclusions are load-bearing rather than tidiness. The scripts
-// name golangci-lint in prose to state that the host binary is never
-// used, and joining continuations is what lets the one legitimate
-// container invocation -- script/lint-fix's `docker run`, whose linter
-// command sits several lines below the word `docker` -- be recognised
-// as containerised.
-func shellCode(contents string) []string {
-	var (
-		out       []string
-		joined    string
-		terminate string
-	)
-
-	for line := range strings.SplitSeq(contents, "\n") {
-		trimmed := strings.TrimSpace(line)
-
-		if terminate != "" {
-			if trimmed == terminate {
-				terminate = ""
-			}
-
-			continue
-		}
-
-		if joined == "" && (trimmed == "" || strings.HasPrefix(trimmed, "#")) {
-			continue
-		}
-
-		joined += strings.TrimSuffix(trimmed, `\`) + " "
-		if strings.HasSuffix(trimmed, `\`) {
-			continue
-		}
-
-		joined = strings.Join(strings.Fields(joined), " ")
-		terminate = heredocTerminator(joined)
-
-		out = append(out, joined)
-		joined = ""
-	}
-
-	return out
-}
-
-// heredocTerminator returns the terminator of the here-document a
-// command opens, or "" if it opens none. Only the first on a line is
-// recognised; nothing in script/ opens two.
-func heredocTerminator(line string) string {
-	_, after, opens := strings.Cut(line, "<<")
-	if !opens {
-		return ""
-	}
-
-	// `<<-` strips leading tabs from the body; the terminator word is
-	// the same either way, and callers compare against trimmed lines.
-	word, _, _ := strings.Cut(strings.TrimPrefix(after, "-"), " ")
-
-	return strings.Trim(word, `'"`)
-}
-
 // readRepoFile reads a file by its path relative to the repository
 // root.
 func readRepoFile(t *testing.T, name string) string {

docs/DATAMODEL.md

@@ -192,7 +192,7 @@ Tracks blob upload metrics.
 After a snapshot is completed:
 1. Copy database to temporary file
 2. Clean temporary database to contain only current snapshot data
-3. Export to SQL dump using sqlite3
+3. VACUUM the trimmed database so deleted rows leave no pages behind
 4. Compress with zstd and encrypt with age
 5. Upload to S3 as `metadata/{remote-key}/db.zst.age`
 6. Generate blob manifest and upload as `metadata/{remote-key}/manifest.json.zst`

internal/snapshot/snapshot.go

@@ -44,7 +44,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"os/exec"
 	"path/filepath"
 	"strings"
 	"time"
@@ -669,14 +668,31 @@ func (sm *SnapshotManager) collectCleanupStats(
 
 // vacuumDatabase runs VACUUM on the database to remove deleted data and compact
 // This is critical for security - ensures no stale/deleted data pages are uploaded
+//
+// VACUUM runs through the modernc.org/sqlite driver, on a freshly opened
+// connection with no transaction in flight (VACUUM cannot run inside one).
+// The database opens in WAL mode, so VACUUM's rewrite lands in the WAL; the
+// checkpoint on Close flushes it into the main file, which is the file we
+// then compress and upload.
 func (sm *SnapshotManager) vacuumDatabase(ctx context.Context, dbPath string) error {
 	log.Debug("Running VACUUM on database", "path", dbPath)
-	//nolint:gosec // G204: fixed argv; dbPath is our own temp file path
-	cmd := exec.CommandContext(ctx, "sqlite3", dbPath, "VACUUM;")
 
-	output, err := cmd.CombinedOutput()
+	db, err := database.New(ctx, dbPath)
 	if err != nil {
-		return fmt.Errorf("running VACUUM: %w (output: %s)", err, string(output))
+		return fmt.Errorf("opening database for VACUUM: %w", err)
+	}
+
+	defer func() {
+		cerr := db.Close()
+		if cerr != nil {
+			log.Debug("Failed to close database after VACUUM",
+				"path", dbPath, "error", cerr)
+		}
+	}()
+
+	_, err = db.ExecWithLog(ctx, "VACUUM")
+	if err != nil {
+		return fmt.Errorf("running VACUUM: %w", err)
 	}
 
 	return nil

internal/snapshot/snapshot_test.go

@@ -2,6 +2,7 @@
 package snapshot
 
 import (
+	"bytes"
 	"context"
 	"database/sql"
 	"io"
@@ -96,6 +97,97 @@ func verifyCleanedDB(
 	}
 }
 
+// TestVacuumDatabaseRemovesDeletedData proves the export path uploads a
+// compacted database: after rows carrying a recognizable marker are deleted
+// and vacuumDatabase runs, no page holding that marker survives in the file
+// on disk (the file compressFile later reads for upload).
+func TestVacuumDatabaseRemovesDeletedData(t *testing.T) {
+	log.Initialize(log.Config{})
+	t.Parallel()
+
+	ctx := context.Background()
+	fs := afero.NewOsFs()
+
+	tempDir := t.TempDir()
+	dbPath := filepath.Join(tempDir, "snapshot.db")
+
+	db, err := database.New(ctx, dbPath)
+	if err != nil {
+		t.Fatalf("failed to create database: %v", err)
+	}
+
+	// A marker distinctive enough that its presence in the raw file can only
+	// come from the rows inserted below.
+	marker := []byte("VACUUM_PROBE_DEADBEEF_DELETED_ROW")
+	payload := bytes.Repeat(marker, 128) // ~4 KiB per row
+
+	_, err = db.Conn().ExecContext(ctx,
+		"CREATE TABLE vacuum_probe (id INTEGER PRIMARY KEY, payload BLOB)")
+	if err != nil {
+		t.Fatalf("failed to create probe table: %v", err)
+	}
+
+	for range 512 {
+		_, err = db.Conn().ExecContext(ctx,
+			"INSERT INTO vacuum_probe (payload) VALUES (?)", payload)
+		if err != nil {
+			t.Fatalf("failed to insert probe row: %v", err)
+		}
+	}
+
+	_, err = db.Conn().ExecContext(ctx, "DELETE FROM vacuum_probe")
+	if err != nil {
+		t.Fatalf("failed to delete probe rows: %v", err)
+	}
+
+	// Close so the deletes reach the main file, mirroring the state
+	// prepareExportDB hands to vacuumDatabase.
+	err = db.Close()
+	if err != nil {
+		t.Fatalf("failed to close database: %v", err)
+	}
+
+	beforeInfo, err := fs.Stat(dbPath)
+	if err != nil {
+		t.Fatalf("failed to stat database before vacuum: %v", err)
+	}
+
+	beforeBytes, err := afero.ReadFile(fs, dbPath)
+	if err != nil {
+		t.Fatalf("failed to read database before vacuum: %v", err)
+	}
+
+	if !bytes.Contains(beforeBytes, marker) {
+		t.Fatalf("expected deleted-row data to linger before vacuum")
+	}
+
+	sm := &SnapshotManager{fs: fs}
+
+	err = sm.vacuumDatabase(ctx, dbPath)
+	if err != nil {
+		t.Fatalf("vacuumDatabase failed: %v", err)
+	}
+
+	afterBytes, err := afero.ReadFile(fs, dbPath)
+	if err != nil {
+		t.Fatalf("failed to read database after vacuum: %v", err)
+	}
+
+	if bytes.Contains(afterBytes, marker) {
+		t.Fatalf("deleted-row data survived vacuum in the uploaded file")
+	}
+
+	afterInfo, err := fs.Stat(dbPath)
+	if err != nil {
+		t.Fatalf("failed to stat database after vacuum: %v", err)
+	}
+
+	if afterInfo.Size() >= beforeInfo.Size() {
+		t.Fatalf("expected vacuum to shrink the file: before=%d after=%d",
+			beforeInfo.Size(), afterInfo.Size())
+	}
+}
+
 func TestCleanSnapshotDBEmptySnapshot(t *testing.T) {
 	// Initialize logger
 	log.Initialize(log.Config{})

script/bootstrap

@@ -114,9 +114,6 @@ main() {
     # from CI. Nothing on the host is ever used as a linter, at any
     # version, so installing one here would buy nothing.
 
-    # sqlite3 CLI: the test suite shells out to it (VACUUM).
-    if missing sqlite3; then pkg_install sqlite sqlite3 sqlite sqlite; fi
-
     # goreleaser, at the version pinned by script/install-goreleaser and
     # verified against a hardcoded sha256. Package managers are not used
     # for it: they ship whatever version they happen to carry, and the

script/install-go

@@ -0,0 +1,161 @@
+#!/bin/sh
+# script/install-go: install the Go toolchain pinned by go.mod into the
+# repo-local tool directory, verified against a committed sha256. Our
+# own extension to scripts-to-rule-them-all. Idempotent: exits at once
+# when the pinned toolchain is already installed.
+#
+# Only .gitea/workflows/release.yml calls this. goreleaser is not a
+# compiler: it shells out to `go` for the `before:` hook and for every
+# one of the four cross-compiles, so the release runner needs a Go
+# toolchain on PATH. check.yml never does -- it builds inside the
+# digest-pinned Dockerfile images -- so this is the release path's only
+# host Go, and per REPO_POLICIES.md it must be pinned by hash.
+# actions/setup-go exposes no checksum input, so Go is installed the way
+# script/install-goreleaser installs goreleaser: download the exact
+# archive from go.dev and refuse it unless its sha256 matches the value
+# committed below.
+#
+# The version is go.mod's `go` directive, the single source of truth for
+# the toolchain. GO_VERSION below MUST equal it, and this script fails
+# when they disagree -- so bumping Go is one reviewed change touching
+# go.mod, the checksum here, and the Dockerfile golang digest together.
+#
+# Linux only, because that is what the release runner is. A darwin dev
+# building a snapshot uses their own Go; supporting an OS means adding
+# its checksums.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+# Go 1.26.1, 2026-09-21. Checksums are the sha256 values go.dev publishes
+# for each archive at https://go.dev/dl/ (also in its ?mode=json
+# manifest).
+GO_VERSION="1.26.1"
+SHA256_LINUX_AMD64="031f088e5d955bab8657ede27ad4e3bc5b7c1ba281f05f245bcc304f327c987a"
+SHA256_LINUX_ARM64="a290581cfe4fe28ddd737dde3095f3dbeb7f2e4065cab4eae44dfc53b760c2f7"
+
+GOROOT_DIR="$ROOT/.tool/go"
+GOCMD="$GOROOT_DIR/bin/go"
+
+# The `go` directive in go.mod, e.g. "1.26.1" from `go 1.26.1`.
+gomod_go_version() {
+    sed -n 's/^go \([0-9][0-9.]*\).*/\1/p' "$ROOT/go.mod" | head -n 1
+}
+
+# Print the version of the go at $1 as "1.26.1", or nothing if it is not
+# usable. `go version` prints "go version go1.26.1 linux/amd64".
+go_version() {
+    [ -x "$1" ] || return 0
+    "$1" version 2>/dev/null |
+        sed -n 's/^go version go\([0-9][0-9.]*\) .*/\1/p' |
+        head -n 1
+}
+
+verify_sha256() {
+    file="$1"
+    want="$2"
+    if command -v sha256sum >/dev/null 2>&1; then
+        got="$(sha256sum "$file" | cut -d' ' -f1)"
+    elif command -v shasum >/dev/null 2>&1; then
+        got="$(shasum -a 256 "$file" | cut -d' ' -f1)"
+    else
+        echo "install-go: no sha256sum or shasum available" >&2
+        return 1
+    fi
+    if [ "$got" != "$want" ]; then
+        echo "install-go: checksum mismatch for $file" >&2
+        echo "  expected: $want" >&2
+        echo "  actual:   $got" >&2
+        return 1
+    fi
+}
+
+# On a Gitea/GitHub Actions runner, put the toolchain on PATH for the
+# steps that follow by appending to the file named by $GITHUB_PATH. A
+# no-op off CI, where the caller manages its own PATH.
+export_ci_path() {
+    [ -n "${GITHUB_PATH:-}" ] || return 0
+    echo "$GOROOT_DIR/bin" >>"$GITHUB_PATH"
+}
+
+main() {
+    cd "$ROOT"
+
+    want="$(gomod_go_version)"
+    if [ "$want" != "$GO_VERSION" ]; then
+        echo "install-go: go.mod says go $want but this script pins" \
+            "$GO_VERSION." >&2
+        echo "  Update GO_VERSION and the checksums in this script to" \
+            "match go.mod." >&2
+        exit 1
+    fi
+
+    # Already installed from a previous run? Then just fix PATH and stop.
+    if [ "$(go_version "$GOCMD")" = "$GO_VERSION" ]; then
+        echo "go $GO_VERSION already installed in .tool/go"
+        export_ci_path
+        return 0
+    fi
+
+    os="$(uname -s)"
+    arch="$(uname -m)"
+    case "$os" in
+        Linux) os="linux" ;;
+        *)
+            echo "install-go: unsupported OS $os (release runner is Linux)" >&2
+            exit 1
+            ;;
+    esac
+    case "$arch" in
+        x86_64 | amd64)
+            arch="amd64"
+            sum="$SHA256_LINUX_AMD64"
+            ;;
+        arm64 | aarch64)
+            arch="arm64"
+            sum="$SHA256_LINUX_ARM64"
+            ;;
+        *)
+            echo "install-go: no pinned checksum for architecture $arch" >&2
+            exit 1
+            ;;
+    esac
+
+    archive="go${GO_VERSION}.${os}-${arch}.tar.gz"
+    url="https://go.dev/dl/${archive}"
+
+    if ! command -v curl >/dev/null 2>&1; then
+        echo "install-go: curl is required" >&2
+        exit 1
+    fi
+
+    dl="$(mktemp -d)"
+    mkdir -p "$ROOT/.tool"
+    stage="$(mktemp -d "$ROOT/.tool/.go-install.XXXXXX")"
+    # shellcheck disable=SC2064 # expand the paths now, not at trap time
+    trap "rm -rf '$dl' '$stage'" EXIT INT TERM
+
+    echo "installing go $GO_VERSION for ${os}-${arch}"
+    curl -fsSL --retry 3 -o "$dl/$archive" "$url"
+    verify_sha256 "$dl/$archive" "$sum"
+
+    # The archive unpacks to a top-level `go/` directory. Extract it into
+    # a staging directory on the same filesystem as the destination, then
+    # rename it into place so a concurrent run never observes a
+    # half-written toolchain.
+    tar -xzf "$dl/$archive" -C "$stage"
+    rm -rf "$GOROOT_DIR"
+    mv "$stage/go" "$GOROOT_DIR"
+
+    installed="$(go_version "$GOCMD")"
+    if [ "$installed" != "$GO_VERSION" ]; then
+        echo "install-go: installed toolchain reports '$installed'," \
+            "expected '$GO_VERSION'" >&2
+        exit 1
+    fi
+
+    echo "go $GO_VERSION installed to .tool/go"
+    export_ci_path
+}
+
+main "$@"