fix(backend): rate-limit and cap report ingest, drop wildcard CORS (closes #20)

POST /api/v1/reports stays unauthenticated but is bounded. Each client
address, as the trusted-proxy logic resolves it, may send
REPORTS_PER_MINUTE reports a minute (default 60, all at once if it
likes), using golang.org/x/time/rate; past that it gets 429 with
Retry-After. Buckets that have refilled are dropped once a minute, so
idle addresses do not pile up. reportbuf refuses a report that would
take the report files past DATA_DIR_MAX_BYTES (default 1 GiB) with
ErrFull, answered with 507; the count starts from the files already in
DATA_DIR, and reports not yet written count at their uncompressed size.
CORS adds nothing unless CORS_ALLOWED_ORIGINS lists origins. A limit
that is not a positive number stops the server from starting.

Model: opus-5-5

backend/internal/reportbuf/export_test.go

@@ -0,0 +1,7 @@
+package reportbuf
+
+// Flush writes the buffered reports to a file now, as the periodic
+// flush does, so tests need not wait a minute for it.
+func (b *Buffer) Flush() error {
+	return b.flushLocked()
+}

backend/internal/reportbuf/reportbuf.go

@@ -6,11 +6,13 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io/fs"
 	"log/slog"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 	"time"
 
@@ -27,8 +29,16 @@ const (
 	defaultDataDir                 = "./data/reports"
 	dirPerms           fs.FileMode = 0o750
 	filePerms          fs.FileMode = 0o640
+
+	// Report files are named filePrefix + timestamp + fileSuffix.
+	filePrefix = "reports-"
+	fileSuffix = ".jsonl.zst"
 )
 
+// ErrFull is returned by Append when storing the report would
+// take the report files past the configured maximum size.
+var ErrFull = errors.New("report files at their size cap")
+
 // Params defines the dependencies for Buffer.
 type Params struct {
 	fx.In
@@ -44,8 +54,13 @@ type Buffer struct {
 	dataDir  string
 	done     chan struct{}
 	log      *slog.Logger
+	maxBytes int64
 	mu       sync.Mutex
 	stopOnce sync.Once
+	// usedBytes is what Append checks against maxBytes: the size
+	// of the report files in dataDir, plus the reports not yet
+	// written to one at their uncompressed size.
+	usedBytes int64
 }
 
 // New creates a Buffer and registers lifecycle hooks to
@@ -60,9 +75,10 @@ func New(
 	}
 
 	b := &Buffer{
-		dataDir: dir,
-		done:    make(chan struct{}),
-		log:     params.Logger.Get(),
+		dataDir:  dir,
+		done:     make(chan struct{}),
+		log:      params.Logger.Get(),
+		maxBytes: params.Config.DataDirMaxBytes,
 	}
 
 	lc.Append(fx.Hook{
@@ -72,6 +88,12 @@ func New(
 				return fmt.Errorf("create data dir: %w", err)
 			}
 
+			// Report files left by earlier runs count too.
+			b.usedBytes, err = reportFilesSize(b.dataDir)
+			if err != nil {
+				return err
+			}
+
 			go b.flushLoop()
 
 			return nil
@@ -97,15 +119,27 @@ func New(
 }
 
 // Append marshals v as a single JSON line and appends it to
-// the buffer. If the buffer reaches the size threshold, it is
-// drained and written to disk asynchronously.
+// the buffer. It stores nothing and returns ErrFull if the line
+// would take usedBytes past maxBytes. If the buffer reaches the
+// size threshold, it is drained and written to disk
+// asynchronously.
 func (b *Buffer) Append(v any) error {
 	line, err := json.Marshal(v)
 	if err != nil {
 		return fmt.Errorf("marshal report: %w", err)
 	}
 
+	lineBytes := int64(len(line)) + 1 // with its newline
+
 	b.mu.Lock()
+
+	if b.usedBytes+lineBytes > b.maxBytes {
+		b.mu.Unlock()
+
+		return ErrFull
+	}
+
+	b.usedBytes += lineBytes
 	b.buf.Write(line)
 	b.buf.WriteByte('\n')
 
@@ -178,8 +212,7 @@ func (b *Buffer) drainBuf() []byte {
 // in the data directory.
 func (b *Buffer) writeFile(data []byte) error {
 	ts := time.Now().UTC().Format("2006-01-02T15-04-05.000Z")
-	name := fmt.Sprintf("reports-%s.jsonl.zst", ts)
-	path := filepath.Join(b.dataDir, name)
+	path := filepath.Join(b.dataDir, filePrefix+ts+fileSuffix)
 
 	// path is built from the operator-supplied dataDir plus a
 	// generated timestamp, so it carries no external input.
@@ -214,10 +247,51 @@ func (b *Buffer) writeFile(data []byte) error {
 		return fmt.Errorf("close zstd encoder: %w", err)
 	}
 
+	info, err := f.Stat()
+	if err != nil {
+		return fmt.Errorf("stat report file: %w", err)
+	}
+
 	err = f.Close()
 	if err != nil {
 		return fmt.Errorf("close report file: %w", err)
 	}
 
+	// The reports counted at their uncompressed size while they
+	// waited; now they count as the file. After a failed write they
+	// stay counted as they were, which errs toward refusing reports
+	// early rather than letting the files pass the cap.
+	b.mu.Lock()
+	b.usedBytes += info.Size() - int64(len(data))
+	b.mu.Unlock()
+
 	return nil
 }
+
+// reportFilesSize returns the total size of the report files in
+// dir.
+func reportFilesSize(dir string) (int64, error) {
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return 0, fmt.Errorf("read data dir: %w", err)
+	}
+
+	var total int64
+
+	for _, entry := range entries {
+		name := entry.Name()
+		if !strings.HasPrefix(name, filePrefix) ||
+			!strings.HasSuffix(name, fileSuffix) {
+			continue
+		}
+
+		info, err := entry.Info()
+		if err != nil {
+			return 0, fmt.Errorf("stat report file: %w", err)
+		}
+
+		total += info.Size()
+	}
+
+	return total, nil
+}

backend/internal/reportbuf/reportbuf_test.go

@@ -1,9 +1,12 @@
 package reportbuf_test
 
 import (
+	"encoding/json"
 	"errors"
 	"io/fs"
 	"os"
+	"path/filepath"
+	"strconv"
 	"strings"
 	"testing"
 
@@ -94,6 +97,131 @@ func TestFailedFinalFlushFailsStop(t *testing.T) {
 	}
 }
 
+// startBuffer starts a Buffer through fx, as main does, with the
+// DATA_DIR and DATA_DIR_MAX_BYTES the calling test has set.
+func startBuffer(t *testing.T) *reportbuf.Buffer {
+	t.Helper()
+
+	var buf *reportbuf.Buffer
+
+	app := fxtest.New(t,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+			reportbuf.New,
+		),
+		fx.Populate(&buf),
+	)
+
+	app.RequireStart()
+	t.Cleanup(app.RequireStop)
+
+	return buf
+}
+
+// lineBytes is what one report takes in the buffer: its JSON and a
+// newline.
+func lineBytes(t *testing.T, report any) int {
+	t.Helper()
+
+	line, err := json.Marshal(report)
+	if err != nil {
+		t.Fatalf("marshal report: %v", err)
+	}
+
+	return len(line) + 1
+}
+
+func TestAppendPastCapIsRefused(t *testing.T) {
+	report := map[string]string{"id": "cap"}
+
+	t.Setenv("DATA_DIR", t.TempDir())
+	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(lineBytes(t, report)))
+
+	buf := startBuffer(t)
+
+	err := buf.Append(report)
+	if err != nil {
+		t.Fatalf("report that fills the cap exactly: %v", err)
+	}
+
+	err = buf.Append(report)
+	if !errors.Is(err, reportbuf.ErrFull) {
+		t.Fatalf("report past the cap: error = %v, want ErrFull", err)
+	}
+}
+
+// TestCapCountsReportFilesAlreadyInDataDir starts on a data
+// directory holding a report file from an earlier run, and a file
+// that is not a report, which must not count.
+func TestCapCountsReportFilesAlreadyInDataDir(t *testing.T) {
+	const earlierBytes = 100
+
+	report := map[string]string{"id": "cap"}
+	dir := t.TempDir()
+
+	writeBytes(t, filepath.Join(dir, "reports-2026-01-01T00-00-00.000Z.jsonl.zst"),
+		earlierBytes)
+	writeBytes(t, filepath.Join(dir, "notes.txt"), 10*earlierBytes)
+
+	t.Setenv("DATA_DIR", dir)
+	t.Setenv("DATA_DIR_MAX_BYTES",
+		strconv.Itoa(earlierBytes+lineBytes(t, report)))
+
+	buf := startBuffer(t)
+
+	err := buf.Append(report)
+	if err != nil {
+		t.Fatalf("report that fills the cap exactly: %v", err)
+	}
+
+	err = buf.Append(report)
+	if !errors.Is(err, reportbuf.ErrFull) {
+		t.Fatalf("report past the cap: error = %v, want ErrFull", err)
+	}
+}
+
+// TestWrittenReportsCountAtFileSize checks that once reports are
+// written, they count as their compressed file, not their
+// uncompressed size, which frees room under the cap.
+func TestWrittenReportsCountAtFileSize(t *testing.T) {
+	// Repetitive, so its file is far smaller than its JSON.
+	report := map[string]string{"id": strings.Repeat("a", 1000)}
+	size := lineBytes(t, report)
+
+	t.Setenv("DATA_DIR", t.TempDir())
+	// Room for the report twice over only if the first one counts
+	// at its file's size by the time the second arrives.
+	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(2*size-1))
+
+	buf := startBuffer(t)
+
+	err := buf.Append(report)
+	if err != nil {
+		t.Fatalf("first report: %v", err)
+	}
+
+	err = buf.Flush()
+	if err != nil {
+		t.Fatalf("flush: %v", err)
+	}
+
+	err = buf.Append(report)
+	if err != nil {
+		t.Fatalf("second report, after the first was written: %v", err)
+	}
+}
+
+func writeBytes(t *testing.T, path string, n int) {
+	t.Helper()
+
+	err := os.WriteFile(path, make([]byte, n), 0o600)
+	if err != nil {
+		t.Fatalf("write %s: %v", path, err)
+	}
+}
+
 func hasReportFile(t *testing.T, dir string) bool {
 	t.Helper()