fix(sqlite): stage snapshot in temp file instead of /dev/stdout

The Alpine sqlite3 binary refuses to open /dev/stdout when running as
a non-root uid in a container ('Error: cannot open "/dev/stdout"'),
which breaks every backup attempt. Switch the dump path to stage the
snapshot in a temp file, then stream that file through gzip into the
pipeline. Adds streamSideEffect to the test mockRunner so the existing
gzip-wrap test can simulate the sqlite3 process writing to its
destination path.

apps/agent/internal/pipeline/pg_dump_test.go

@@ -17,7 +17,11 @@ type mockRunner struct {
 	outputResp map[string][]byte // key = first arg (e.g. "--version")
 	streamResp []byte
 	streamErr  error
-	calls      []mockCall
+	// streamSideEffect, when non-nil, runs before streamResp is written
+	// to the supplied writer. Lets sqlite-style drivers that write to a
+	// real file-system path simulate the file write.
+	streamSideEffect func(args []string) error
+	calls            []mockCall
 }
 
 type mockCall struct {
@@ -41,6 +45,11 @@ func (m *mockRunner) RunStream(_ context.Context, _ string, args []string, env [
 	if m.streamErr != nil {
 		return m.streamErr
 	}
+	if m.streamSideEffect != nil {
+		if err := m.streamSideEffect(args); err != nil {
+			return err
+		}
+	}
 	if len(m.streamResp) > 0 {
 		_, _ = out.Write(m.streamResp)
 	}

apps/agent/internal/pipeline/sqlite.go

@@ -98,17 +98,34 @@ func (s *sqliteDriver) Dump(ctx context.Context, target *backupv1.Target, out io
 		return DumpInfo{}, fmt.Errorf("pipeline: sqlite: cannot stat database %q: %w", path, err)
 	}
 
-	gz := gzip.NewWriter(out)
-	defer gz.Close()
+	// `.backup` requires a regular file as the destination — the Alpine
+	// sqlite3 binary refuses /dev/stdout for non-root processes. We
+	// stage the snapshot in a temp file and stream it to `out` after.
+	tmpDir := os.TempDir()
+	tmpFile, err := os.CreateTemp(tmpDir, "sqlite-backup-*.db")
+	if err != nil {
+		return DumpInfo{}, fmt.Errorf("pipeline: sqlite: create temp file: %w", err)
+	}
+	tmpPath := tmpFile.Name()
+	_ = tmpFile.Close()
+	defer func() { _ = os.Remove(tmpPath) }()
 
-	// sqlite3 expects a single positional argument (the database path)
-	// followed by a dot-command. `.backup` writes a consistent snapshot
-	// to the supplied filename; we pass /dev/stdout so the bytes flow
-	// through stdout into our gzip writer.
-	args := []string{path, ".backup '/dev/stdout'"}
-	if err := s.runner.RunStream(ctx, s.binary, args, nil, gz); err != nil {
+	args := []string{path, fmt.Sprintf(".backup '%s'", tmpPath)}
+	if err := s.runner.RunStream(ctx, s.binary, args, nil, io.Discard); err != nil {
 		return DumpInfo{}, fmt.Errorf("pipeline: sqlite3 .backup exec: %w", err)
 	}
+
+	src, err := os.Open(tmpPath)
+	if err != nil {
+		return DumpInfo{}, fmt.Errorf("pipeline: sqlite: open snapshot: %w", err)
+	}
+	defer src.Close()
+
+	gz := gzip.NewWriter(out)
+	if _, err := io.Copy(gz, src); err != nil {
+		_ = gz.Close()
+		return DumpInfo{}, fmt.Errorf("pipeline: sqlite: gzip copy: %w", err)
+	}
 	if err := gz.Close(); err != nil {
 		return DumpInfo{}, fmt.Errorf("pipeline: sqlite: close gzip: %w", err)
 	}

apps/agent/internal/pipeline/sqlite_test.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -91,7 +92,23 @@ func TestSqlite_Dump_WrapsOutputInGzip(t *testing.T) {
 	payload := bytes.Repeat([]byte{0x53, 0x51, 0x4c}, 16) // pretend SQLite header bytes
 	mock := &mockRunner{
 		outputResp: map[string][]byte{"--version": []byte("3.45.1 2024-01-30\n")},
-		streamResp: payload,
+		// The new Dump implementation stages the snapshot in a temp
+		// file, then re-reads it into the gzip pipe. Simulate the
+		// real sqlite3 process by parsing the destination path out
+		// of the .backup dot-command and writing the canned payload
+		// to that path.
+		streamSideEffect: func(args []string) error {
+			if len(args) < 2 {
+				return errors.New("expected at least 2 args")
+			}
+			dot := args[1]
+			const prefix = ".backup '"
+			if !strings.HasPrefix(dot, prefix) || !strings.HasSuffix(dot, "'") {
+				return errors.New("malformed .backup dot-command")
+			}
+			path := dot[len(prefix) : len(dot)-1]
+			return os.WriteFile(path, payload, 0o600)
+		},
 	}
 	d := &sqliteDriver{binary: "sqlite3", runner: mock, statFn: os.Stat}
 
@@ -111,11 +128,12 @@ func TestSqlite_Dump_WrapsOutputInGzip(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, payload, got)
 
-	// Confirm `.backup '/dev/stdout'` was invoked with the right path.
+	// Confirm the dump invoked `.backup '<tmpfile>'`.
 	require.NotEmpty(t, mock.calls)
 	streamCall := mock.calls[0]
 	require.Equal(t, tmp, streamCall.Args[0])
-	require.Equal(t, ".backup '/dev/stdout'", streamCall.Args[1])
+	require.True(t, strings.HasPrefix(streamCall.Args[1], ".backup '"))
+	require.True(t, strings.HasSuffix(streamCall.Args[1], "'"))
 }
 
 func TestSqlite_Dump_MissingPath(t *testing.T) {