Validate downloads before installing workspace files

docs/internal/artifacts.md

@@ -183,7 +183,9 @@ Each read uses the generation and size metadata returned with its opened body.
 Actual bytes remain subject to a limit-plus-one read even if size metadata is
 absent or inaccurate. Immutable selections then retain their declared-size,
 checksum, generation, and identity checks. No current-state control object is
-downloaded through a temporary file.
+downloaded through a temporary file. Artifact and previous-session downloads
+validate their complete sibling temporary file against available committed
+identity before replacing a canonical local path.
 
 Core helpers:
 

docs/internal/command-restore.md

@@ -53,7 +53,8 @@ Execution order and safety:
 
 - non-manifest downloads happen before manifest install;
 - `manifest.json` installs last;
-- downloads use sibling temp files plus atomic rename;
+- downloads use sibling temp files, validate the complete object, then use an
+  atomic rename;
 - manifest replacement is validated before rename;
 - each committed object is verified against its declared checksum, size, and
   generation before installation;
@@ -73,9 +74,9 @@ Audio restore path:
 
 - uses `audio.MaterializeS3Audio`;
 - integrates spool and S3 audio cache paths;
-- reuses cached audio only when its no-follow regular file, content digest, and
+- reuses cached audio only when its opened regular file, content digest, and
   identity sidecar all match the selected remote object version; otherwise it
-  refreshes through the durable download path.
+  refreshes through the validated download path.
 
 ## Reporting Contract
 

docs/internal/storage.md

@@ -55,6 +55,11 @@ not own discovery, defaults, or configuration validation.
 - bounded reads never retain more than the caller's limit plus one byte and do
   not replace owner-specific size policy.
 - publish ordering semantics are owned by stage/app code, not storage adapters.
+- A caller that installs remote content into a canonical workspace path first
+  writes a sibling temporary file, validates the complete object against its
+  owner-specific contract, and only then replaces the destination. Temporary
+  files used solely for remote comparisons are removed by their callers and do
+  not represent installed workspace state.
 
 ## Implementation And Tests
 

docs/operations.md

@@ -498,6 +498,8 @@ Cache layout (durable S3 audio cache):
 Each cached audio file has an adjacent managed identity record. It binds the
 file to its remote object version and verified digest; deleting or altering the
 record simply causes Narratio to download and verify the object again.
+Remote audio and restored files are fully transferred into sibling temporary
+files and validated before they replace their canonical workspace paths.
 
 ### Workspace Permissions
 

docs/roadmap/implementation.md

@@ -304,7 +304,7 @@ loading.
 
 ## Stage 3 — Validate Downloads Before Canonical Installation
 
-**Status: Pending**
+**Status: Completed**
 
 ### Goal
 

internal/app/restore_execute.go

@@ -7,6 +7,7 @@ import (
 	"encoding/hex"
 	"fmt"
 	"io"
+	"os"
 	"path/filepath"
 	"strings"
 
@@ -101,10 +102,7 @@ func executeRestoreDownloadAction(
 		return executeRestoreAudioAction(ctx, cfg, safeLocalPath, action, store)
 	}
 
-	if err := fileops.EnsureWorkspaceDirectory(filepath.Dir(safeLocalPath)); err != nil {
-		return fmt.Errorf("create destination directory: %w", err)
-	}
-	temporary, err := fileops.DownloadToSiblingTemp(safeLocalPath, func(destination io.Writer) error {
+	temporaryPath, err := downloadRestoreTemp(safeLocalPath, func(destination io.Writer) error {
 		if len(action.VerifiedContent) > 0 {
 			_, err := io.Copy(destination, bytes.NewReader(action.VerifiedContent))
 			return err
@@ -114,13 +112,13 @@ func executeRestoreDownloadAction(
 	if err != nil {
 		return fmt.Errorf("download to temp file: %w", err)
 	}
-	defer func() { _ = temporary.Cleanup() }()
-	if err := verifyRestoredObject(ctx, store, action, temporary); err != nil {
+	defer func() { _ = os.Remove(temporaryPath) }()
+	if err := verifyRestoredObject(ctx, store, action, temporaryPath); err != nil {
 		return err
 	}
 
 	if isRestoreManifest(action.LocalRelativePath) {
-		file, err := temporary.Open()
+		file, err := os.Open(temporaryPath)
 		if err != nil {
 			return fmt.Errorf("open restored manifest: %w", err)
 		}
@@ -143,7 +141,7 @@ func executeRestoreDownloadAction(
 		return nil
 	}
 
-	if err := temporary.Install(filepath.Base(safeLocalPath), fileops.WorkspaceFileMode); err != nil {
+	if err := fileops.InstallSiblingTempFile(temporaryPath, safeLocalPath, fileops.WorkspaceFileMode); err != nil {
 		return fmt.Errorf("install file atomically: %w", err)
 	}
 
@@ -155,14 +153,42 @@ func isRestoreManifest(relativePath string) bool {
 	return clean == config.PathManifestFile || clean == config.PathPreviousDirSegment+"/"+config.PathManifestFile
 }
 
-func verifyRestoredObject(ctx context.Context, store storage.ObjectStore, action RestoreAction, temporary *fileops.DownloadedTempFile) error {
+func downloadRestoreTemp(destination string, download func(io.Writer) error) (string, error) {
+	if strings.TrimSpace(destination) == "" {
+		return "", fmt.Errorf("restore destination path is required")
+	}
+	if download == nil {
+		return "", fmt.Errorf("restore temporary writer is required")
+	}
+	directory := filepath.Dir(destination)
+	if err := fileops.EnsureWorkspaceDirectory(directory); err != nil {
+		return "", fmt.Errorf("create destination directory: %w", err)
+	}
+	temporary, err := os.CreateTemp(directory, "."+filepath.Base(destination)+".tmp-*")
+	if err != nil {
+		return "", fmt.Errorf("create sibling temporary file: %w", err)
+	}
+	temporaryPath := temporary.Name()
+	if err := download(temporary); err != nil {
+		_ = temporary.Close()
+		_ = os.Remove(temporaryPath)
+		return "", err
+	}
+	if err := temporary.Close(); err != nil {
+		_ = os.Remove(temporaryPath)
+		return "", fmt.Errorf("close sibling temporary file: %w", err)
+	}
+	return temporaryPath, nil
+}
+
+func verifyRestoredObject(ctx context.Context, store storage.ObjectStore, action RestoreAction, temporaryPath string) error {
 	if strings.TrimSpace(action.SHA256) == "" && strings.TrimSpace(action.Generation) == "" {
 		return nil
 	}
 	if strings.TrimSpace(action.SHA256) == "" || strings.TrimSpace(action.Generation) == "" {
 		return fmt.Errorf("committed object identity for %q is incomplete", action.RemoteKey)
 	}
-	file, err := temporary.Open()
+	file, err := os.Open(temporaryPath)
 	if err != nil {
 		return fmt.Errorf("open downloaded object for verification: %w", err)
 	}

internal/audio/cache_identity.go

@@ -114,22 +114,25 @@ func materializeVerifiedCachedAudio(ctx context.Context, req S3MaterializeReques
 	if err := fileops.EnsureWorkspaceDirectory(filepath.Dir(req.DestPath)); err != nil {
 		return "", false, fmt.Errorf("create audio destination directory: %w", err)
 	}
-	temporary, err := fileops.DownloadToSiblingTemp(req.DestPath, func(destination io.Writer) error {
+	temporaryPath, err := writeAudioTemp(req.DestPath, func(destination io.Writer) error {
 		_, err := io.Copy(destination, source)
 		return err
 	})
 	if err != nil {
 		return "", false, fmt.Errorf("copy verified cached audio: %w", err)
 	}
-	defer func() { _ = temporary.Cleanup() }()
-	checksum, size, err := temporaryChecksum(temporary)
+	defer func() { _ = os.Remove(temporaryPath) }()
+	checksum, size, err := checksumAudioTemp(temporaryPath)
 	if err != nil {
 		return "", false, err
 	}
+	if err := validateLocalAudio(temporaryPath, identity.Size); err != nil {
+		return "", false, nil
+	}
 	if size != identity.Size || checksum != identity.SHA256 {
 		return "", false, nil
 	}
-	if err := temporary.Install(filepath.Base(req.DestPath), fileops.WorkspaceFileMode); err != nil {
+	if err := fileops.InstallSiblingTempFile(temporaryPath, req.DestPath, fileops.WorkspaceFileMode); err != nil {
 		return "", false, fmt.Errorf("install verified cached audio: %w", err)
 	}
 	return checksum, true, nil
@@ -147,8 +150,8 @@ func cacheRelativePath(cacheRoot, cachePath string) (string, error) {
 	return normalized, nil
 }
 
-func temporaryChecksum(temporary *fileops.DownloadedTempFile) (string, int64, error) {
-	file, err := temporary.Open()
+func checksumAudioTemp(path string) (string, int64, error) {
+	file, err := os.Open(path)
 	if err != nil {
 		return "", 0, fmt.Errorf("open cached audio copy: %w", err)
 	}

internal/audio/s3_audio.go

@@ -70,12 +70,9 @@ func MaterializeS3Audio(ctx context.Context, req S3MaterializeRequest) (S3Materi
 	if spoolPath == "" {
 		return S3MaterializeResult{}, fmt.Errorf("spool path is required for s3 audio download")
 	}
-	if err := downloadObjectAtomic(ctx, req.Store, key, spoolPath); err != nil {
+	if err := downloadObjectAtomic(ctx, req.Store, key, spoolPath, req.Object.Size); err != nil {
 		return S3MaterializeResult{}, fmt.Errorf("download s3 audio object %q: %w", key, err)
 	}
-	if err := validateLocalAudio(spoolPath, req.Object.Size); err != nil {
-		return S3MaterializeResult{}, fmt.Errorf("validate downloaded audio %q: %w", spoolPath, err)
-	}
 
 	checksum, err := fileops.CopyFileAtomicWithChecksum(spoolPath, req.DestPath, fileops.WorkspaceFileMode)
 	if err != nil {
@@ -118,15 +115,47 @@ func validateLocalAudio(path string, expectedSize int64) error {
 	return nil
 }
 
-func downloadObjectAtomic(ctx context.Context, store storage.ObjectStore, key, destPath string) error {
-	if strings.TrimSpace(destPath) == "" {
-		return fmt.Errorf("destination path is required")
-	}
-	dir := filepath.Dir(destPath)
-	if err := fileops.EnsureWorkspaceDirectory(dir); err != nil {
-		return fmt.Errorf("create destination directory: %w", err)
-	}
-	return fileops.DownloadAndInstall(destPath, fileops.WorkspaceFileMode, func(destination io.Writer) error {
+func downloadObjectAtomic(ctx context.Context, store storage.ObjectStore, key, destPath string, expectedSize int64) error {
+	temporaryPath, err := writeAudioTemp(destPath, func(destination io.Writer) error {
 		return storage.DownloadTo(ctx, store, key, destination)
 	})
+	if err != nil {
+		return err
+	}
+	defer func() { _ = os.Remove(temporaryPath) }()
+	if err := validateLocalAudio(temporaryPath, expectedSize); err != nil {
+		return fmt.Errorf("validate downloaded audio: %w", err)
+	}
+	if err := fileops.InstallSiblingTempFile(temporaryPath, destPath, fileops.WorkspaceFileMode); err != nil {
+		return err
+	}
+	return nil
+}
+
+func writeAudioTemp(destination string, write func(io.Writer) error) (string, error) {
+	if strings.TrimSpace(destination) == "" {
+		return "", fmt.Errorf("destination path is required")
+	}
+	if write == nil {
+		return "", fmt.Errorf("temporary audio writer is required")
+	}
+	dir := filepath.Dir(destination)
+	if err := fileops.EnsureWorkspaceDirectory(dir); err != nil {
+		return "", fmt.Errorf("create destination directory: %w", err)
+	}
+	temporary, err := os.CreateTemp(dir, "."+filepath.Base(destination)+".tmp-*")
+	if err != nil {
+		return "", fmt.Errorf("create sibling temporary file: %w", err)
+	}
+	temporaryPath := temporary.Name()
+	if err := write(temporary); err != nil {
+		_ = temporary.Close()
+		_ = os.Remove(temporaryPath)
+		return "", fmt.Errorf("write sibling temporary file: %w", err)
+	}
+	if err := temporary.Close(); err != nil {
+		_ = os.Remove(temporaryPath)
+		return "", fmt.Errorf("close sibling temporary file: %w", err)
+	}
+	return temporaryPath, nil
 }

internal/audio/s3_audio_test.go

@@ -288,6 +288,30 @@ func TestMaterializeS3AudioDownloadFailureLeavesDestinationMissing(t *testing.T)
 	assertMissing(t, req.DestPath)
 }
 
+func TestMaterializeS3AudioRejectsSizeMismatchBeforeReplacingSpool(t *testing.T) {
+	root := t.TempDir()
+	fake := &storage.FakeBackend{}
+	key := "dnd/campaigns/forsaken/sessions/2026-04-19/audio/alice.flac"
+	fake.SeedObject(storage.FakeObject{Key: key, Data: []byte("audio")})
+	req := testMaterializeRequest(t, root, fake, key, int64(len("audio")+1))
+	writeAudioTestFile(t, req.SpoolPath, "existing-audio")
+
+	_, err := MaterializeS3Audio(context.Background(), req)
+	if err == nil || !strings.Contains(err.Error(), "size") {
+		t.Fatalf("error = %v, want size validation failure", err)
+	}
+	assertFileEquals(t, req.SpoolPath, "existing-audio")
+	entries, err := os.ReadDir(filepath.Dir(req.SpoolPath))
+	if err != nil {
+		t.Fatalf("read spool directory: %v", err)
+	}
+	for _, entry := range entries {
+		if strings.HasPrefix(entry.Name(), ".alice.flac.tmp-") {
+			t.Fatalf("unexpected temporary file residue: %q", entry.Name())
+		}
+	}
+}
+
 func testMaterializeRequest(t *testing.T, root string, fake *storage.FakeBackend, key string, size int64) S3MaterializeRequest {
 	t.Helper()
 	return S3MaterializeRequest{

internal/fileops/confined.go

@@ -5,7 +5,6 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
-	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -325,232 +324,3 @@ func validateOpenedConfinedFile(parent *os.Root, name string, file *os.File) (*o
 	}
 	return file, nil
 }
-
-// DownloadedTempFile is a completed, destination-confined temporary file. It
-// keeps its parent directory open until the caller installs or cleans it up.
-type DownloadedTempFile struct {
-	parent *os.Root
-	name   string
-}
-
-// DownloadToSiblingTemp writes a remote object into a sibling temporary file.
-// The caller supplies transport behavior through download and retains object
-// identity and validation policy. The destination parent must already exist.
-func DownloadToSiblingTemp(destination string, download func(io.Writer) error) (*DownloadedTempFile, error) {
-	if download == nil {
-		return nil, fmt.Errorf("download function is required")
-	}
-	parent, name, err := openConfinedParent(destination, false, 0)
-	if err != nil {
-		return nil, err
-	}
-	file, tempName, err := createSiblingTemp(parent, name)
-	if err != nil {
-		_ = parent.Close()
-		return nil, fmt.Errorf("create sibling temporary file: %w", err)
-	}
-	if err := download(file); err != nil {
-		_ = file.Close()
-		_ = parent.Remove(tempName)
-		_ = parent.Close()
-		return nil, err
-	}
-	if err := file.Sync(); err != nil {
-		_ = file.Close()
-		_ = parent.Remove(tempName)
-		_ = parent.Close()
-		return nil, fmt.Errorf("sync downloaded temporary file: %w", err)
-	}
-	if err := file.Close(); err != nil {
-		_ = parent.Remove(tempName)
-		_ = parent.Close()
-		return nil, fmt.Errorf("close downloaded temporary file: %w", err)
-	}
-	return &DownloadedTempFile{parent: parent, name: tempName}, nil
-}
-
-// Open returns a read handle for validation before installation.
-func (f *DownloadedTempFile) Open() (*os.File, error) {
-	if f == nil || f.parent == nil || f.name == "" {
-		return nil, fmt.Errorf("downloaded temporary file is unavailable")
-	}
-	return f.parent.Open(f.name)
-}
-
-// Install atomically replaces destinationName in the opened parent directory.
-func (f *DownloadedTempFile) Install(destinationName string, mode os.FileMode) error {
-	if f == nil || f.parent == nil || f.name == "" {
-		return fmt.Errorf("downloaded temporary file is unavailable")
-	}
-	if filepath.Base(destinationName) != destinationName || destinationName == "." || destinationName == "" {
-		return fmt.Errorf("destination file name is required")
-	}
-	file, err := f.parent.Open(f.name)
-	if err != nil {
-		return fmt.Errorf("open downloaded temporary file: %w", err)
-	}
-	if err := file.Chmod(mode); err != nil {
-		_ = file.Close()
-		return fmt.Errorf("set downloaded temporary file permissions: %w", err)
-	}
-	if err := file.Sync(); err != nil {
-		_ = file.Close()
-		return fmt.Errorf("sync downloaded temporary file: %w", err)
-	}
-	if err := file.Close(); err != nil {
-		return fmt.Errorf("close downloaded temporary file: %w", err)
-	}
-	if err := f.parent.Rename(f.name, destinationName); err != nil {
-		return fmt.Errorf("install downloaded temporary file: %w", err)
-	}
-	f.name = ""
-	if err := syncOpenedDirectory(f.parent); err != nil {
-		return fmt.Errorf("sync destination directory after downloaded-file replacement: %w", err)
-	}
-	return f.closeParent()
-}
-
-// Cleanup removes an uninstalled temporary file. It is safe to call repeatedly.
-func (f *DownloadedTempFile) Cleanup() error {
-	if f == nil || f.parent == nil {
-		return nil
-	}
-	var err error
-	if f.name != "" {
-		err = f.parent.Remove(f.name)
-		if errors.Is(err, os.ErrNotExist) {
-			err = nil
-		}
-		f.name = ""
-	}
-	closeErr := f.closeParent()
-	if err != nil && closeErr != nil {
-		return errors.Join(err, closeErr)
-	}
-	if err != nil {
-		return err
-	}
-	return closeErr
-}
-
-func (f *DownloadedTempFile) closeParent() error {
-	if f.parent == nil {
-		return nil
-	}
-	err := f.parent.Close()
-	f.parent = nil
-	return err
-}
-
-// DownloadAndInstall obtains remote bytes through a confined sibling temporary
-// file and installs them with the caller-selected final mode.
-func DownloadAndInstall(destination string, mode os.FileMode, download func(io.Writer) error) error {
-	temporary, err := DownloadToSiblingTemp(destination, download)
-	if err != nil {
-		return err
-	}
-	defer func() { _ = temporary.Cleanup() }()
-	return temporary.Install(filepath.Base(destination), mode)
-}
-
-func replaceFileFromReaderConfined(dst string, source io.Reader, options ReplaceFileOptions) (resultErr error) {
-	parent, name, err := openConfinedParent(dst, false, 0)
-	if err != nil {
-		return err
-	}
-	defer func() { _ = parent.Close() }()
-	temporary, tempName, err := createSiblingTemp(parent, name)
-	if err != nil {
-		return fmt.Errorf("create temp file: %w", err)
-	}
-	removeTemp := true
-	defer func() {
-		if removeTemp {
-			if err := parent.Remove(tempName); err != nil && !errors.Is(err, os.ErrNotExist) {
-				resultErr = errors.Join(resultErr, fmt.Errorf("remove temporary file: %w", err))
-			}
-		}
-	}()
-
-	if _, err := io.Copy(temporary, source); err != nil {
-		return closeTemporaryFileAfterError(temporary, fmt.Errorf("write temp file: %w", err))
-	}
-	if err := temporary.Chmod(options.Mode); err != nil {
-		return closeTemporaryFileAfterError(temporary, fmt.Errorf("set temp file permissions: %w", err))
-	}
-	if err := temporary.Sync(); err != nil {
-		return closeTemporaryFileAfterError(temporary, fmt.Errorf("sync temp file: %w", err))
-	}
-	if err := temporary.Close(); err != nil {
-		return fmt.Errorf("close temp file: %w", err)
-	}
-	if options.BeforeRename != nil {
-		if err := options.BeforeRename(); err != nil {
-			return fmt.Errorf("check before file replacement: %w", err)
-		}
-	}
-	if err := parent.Rename(tempName, name); err != nil {
-		return fmt.Errorf("install temp file: %w", err)
-	}
-	removeTemp = false
-	if err := syncOpenedDirectory(parent); err != nil {
-		return fmt.Errorf("sync destination directory after replacement: %w", err)
-	}
-	return nil
-}
-
-func installDownloadedTempFileConfined(tmpPath, dst string, mode os.FileMode) error {
-	if filepath.Clean(filepath.Dir(tmpPath)) != filepath.Clean(filepath.Dir(dst)) {
-		source, err := os.Open(tmpPath)
-		if err != nil {
-			return fmt.Errorf("open downloaded temporary file: %w", err)
-		}
-		err = replaceFileFromReaderConfined(dst, source, ReplaceFileOptions{Mode: mode})
-		closeErr := source.Close()
-		if err != nil {
-			return fmt.Errorf("install downloaded file: %w", err)
-		}
-		if closeErr != nil {
-			return fmt.Errorf("close downloaded temporary file: %w", closeErr)
-		}
-		if err := os.Remove(tmpPath); err != nil {
-			return fmt.Errorf("remove installed temporary file: %w", err)
-		}
-		return nil
-	}
-	parent, destinationName, err := openConfinedParent(dst, false, 0)
-	if err != nil {
-		return err
-	}
-	defer func() { _ = parent.Close() }()
-	temporaryName := filepath.Base(tmpPath)
-	info, err := parent.Lstat(temporaryName)
-	if err != nil {
-		return fmt.Errorf("inspect downloaded temporary file: %w", err)
-	}
-	if info.Mode()&os.ModeSymlink != 0 || !info.Mode().IsRegular() {
-		return fmt.Errorf("downloaded temporary file is not a regular file")
-	}
-	file, err := parent.Open(temporaryName)
-	if err != nil {
-		return fmt.Errorf("open downloaded temporary file: %w", err)
-	}
-	if err := file.Chmod(mode); err != nil {
-		_ = file.Close()
-		return fmt.Errorf("set temp file permissions: %w", err)
-	}
-	if err := file.Sync(); err != nil {
-		_ = file.Close()
-		return fmt.Errorf("sync downloaded temp file: %w", err)
-	}
-	if err := file.Close(); err != nil {
-		return fmt.Errorf("close downloaded temp file: %w", err)
-	}
-	if err := parent.Rename(temporaryName, destinationName); err != nil {
-		return fmt.Errorf("install downloaded file: %w", err)
-	}
-	if err := syncOpenedDirectory(parent); err != nil {
-		return fmt.Errorf("sync destination directory after downloaded-file replacement: %w", err)
-	}
-	return nil
-}

internal/fileops/fileops.go

@@ -12,8 +12,8 @@ import (
 	"strings"
 )
 
-// ErrDirectorySyncUnsupported remains for legacy confined migration paths.
-// The retained replacement entry points do not perform directory syncing.
+// ErrDirectorySyncUnsupported is used only by the retained directory-promotion
+// implementation. Single-file replacement does not perform directory syncing.
 var ErrDirectorySyncUnsupported = errors.New("directory sync is unsupported")
 
 // ReplaceFileOptions describes the caller-owned parts of a byte-file
@@ -131,147 +131,21 @@ func replaceFileFromReader(dst string, source io.Reader, options ReplaceFileOpti
 	return nil
 }
 
-// InstallDownloadedTempFile installs a fully downloaded sibling temporary file
-// at dst. On failure before the rename, the caller retains ownership of tmpPath
-// and is responsible for cleanup.
-func InstallDownloadedTempFile(tmpPath, dst string, perm os.FileMode) error {
+// InstallSiblingTempFile installs a fully written and validated sibling
+// temporary file at dst. The caller retains ownership of tmpPath if installation
+// fails and is responsible for removing it.
+func InstallSiblingTempFile(tmpPath, dst string, perm os.FileMode) error {
 	if strings.TrimSpace(tmpPath) == "" || strings.TrimSpace(dst) == "" {
 		return fmt.Errorf("temp and destination paths are required")
 	}
-	if err := EnsureWorkspaceDirectory(filepath.Dir(dst)); err != nil {
-		return fmt.Errorf("create destination directory: %w", err)
+	if filepath.Clean(filepath.Dir(tmpPath)) != filepath.Clean(filepath.Dir(dst)) {
+		return fmt.Errorf("temporary file must be a sibling of the destination")
 	}
-	return installDownloadedTempFileConfined(tmpPath, dst, perm)
-}
-
-type temporaryFile interface {
-	io.Writer
-	Name() string
-	Sync() error
-	Close() error
-}
-
-type atomicReplacementOperations struct {
-	createTemp    func(string, string) (temporaryFile, error)
-	chmod         func(string, os.FileMode) error
-	rename        func(string, string) error
-	remove        func(string) error
-	syncDirectory func(string) error
-	syncFile      func(string) error
-}
-
-var systemAtomicReplacementOperations = atomicReplacementOperations{
-	createTemp: func(dir, pattern string) (temporaryFile, error) {
-		return os.CreateTemp(dir, pattern)
-	},
-	chmod:         os.Chmod,
-	rename:        os.Rename,
-	remove:        os.Remove,
-	syncDirectory: syncDirectory,
-	syncFile:      syncFile,
-}
-
-func replaceFileAtomicWithOperations(
-	dst string,
-	data []byte,
-	options ReplaceFileOptions,
-	ops atomicReplacementOperations,
-) error {
-	return replaceFileFromReaderWithOperations(dst, bytes.NewReader(data), options, ops)
-}
-
-func replaceFileFromReaderWithOperations(
-	dst string,
-	source io.Reader,
-	options ReplaceFileOptions,
-	ops atomicReplacementOperations,
-) (resultErr error) {
-	if strings.TrimSpace(dst) == "" {
-		return fmt.Errorf("destination path is required")
-	}
-
-	dir := filepath.Dir(dst)
-	tmp, err := ops.createTemp(dir, "."+filepath.Base(dst)+".tmp-*")
-	if err != nil {
-		return fmt.Errorf("create temp file: %w", err)
-	}
-	tmpPath := tmp.Name()
-	removeTmp := true
-	defer func() {
-		if removeTmp {
-			if err := ops.remove(tmpPath); err != nil {
-				cleanupErr := fmt.Errorf("remove temporary file: %w", err)
-				if resultErr == nil {
-					resultErr = cleanupErr
-				} else {
-					resultErr = errors.Join(resultErr, cleanupErr)
-				}
-			}
-		}
-	}()
-
-	if _, err := io.Copy(tmp, source); err != nil {
-		return closeTemporaryFileAfterError(tmp, fmt.Errorf("write temp file: %w", err))
-	}
-	if err := ops.chmod(tmpPath, options.Mode); err != nil {
-		return closeTemporaryFileAfterError(tmp, fmt.Errorf("set temp file permissions: %w", err))
-	}
-	if err := tmp.Sync(); err != nil {
-		return closeTemporaryFileAfterError(tmp, fmt.Errorf("sync temp file: %w", err))
-	}
-	if err := tmp.Close(); err != nil {
-		return fmt.Errorf("close temp file: %w", err)
-	}
-	if options.BeforeRename != nil {
-		if err := options.BeforeRename(); err != nil {
-			return fmt.Errorf("check before file replacement: %w", err)
-		}
-	}
-	if err := ops.rename(tmpPath, dst); err != nil {
-		return fmt.Errorf("install temp file: %w", err)
-	}
-	removeTmp = false
-	if err := ops.syncDirectory(dir); err != nil {
-		return fmt.Errorf("sync destination directory after replacement: %w", err)
-	}
-	return nil
-}
-
-func closeTemporaryFileAfterError(tmp temporaryFile, operationErr error) error {
-	if err := tmp.Close(); err != nil {
-		return errors.Join(operationErr, fmt.Errorf("close temporary file: %w", err))
-	}
-	return operationErr
-}
-
-func installDownloadedTempFileWithOperations(
-	tmpPath, dst string,
-	perm os.FileMode,
-	ops atomicReplacementOperations,
-) error {
-	if err := ops.chmod(tmpPath, perm); err != nil {
+	if err := os.Chmod(tmpPath, perm); err != nil {
 		return fmt.Errorf("set temp file permissions: %w", err)
 	}
-	if err := ops.syncFile(tmpPath); err != nil {
-		return fmt.Errorf("sync downloaded temp file: %w", err)
-	}
-	if err := ops.rename(tmpPath, dst); err != nil {
+	if err := os.Rename(tmpPath, dst); err != nil {
 		return fmt.Errorf("install downloaded file: %w", err)
 	}
-	if err := ops.syncDirectory(filepath.Dir(dst)); err != nil {
-		return fmt.Errorf("sync destination directory after downloaded-file replacement: %w", err)
-	}
 	return nil
 }
-
-func syncFile(path string) error {
-	file, err := os.Open(path)
-	if err != nil {
-		return err
-	}
-	if err := file.Sync(); err != nil {
-		_ = file.Close()
-		return err
-	}
-	return file.Close()
-}

internal/fileops/fileops_test.go

@@ -3,16 +3,12 @@ package fileops
 import (
 	"crypto/sha256"
 	"encoding/hex"
-	"errors"
-	"io"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
 )
 
-var errAtomicReplacementTest = errors.New("atomic replacement test failure")
-
 func TestWriteFileAtomicOverwritesAndLeavesNoTempFile(t *testing.T) {
 	root := t.TempDir()
 	dst := filepath.Join(root, "out", "value.txt")
@@ -116,16 +112,16 @@ func TestCopyFileAtomicCleansTempFileOnInstallFailure(t *testing.T) {
 	assertNoMatchingTempFiles(t, root, ".blocked.tmp-")
 }
 
-func TestInstallDownloadedTempFileSetsPermissions(t *testing.T) {
+func TestInstallSiblingTempFileSetsPermissions(t *testing.T) {
 	root := t.TempDir()
+	dst := filepath.Join(root, "payload.json")
 	tmpPath := filepath.Join(root, ".payload.tmp")
-	dst := filepath.Join(root, "out", "payload.json")
 	if err := os.WriteFile(tmpPath, []byte("{\"ok\":true}\n"), 0o600); err != nil {
 		t.Fatalf("WriteFile(temp) error = %v", err)
 	}
 
-	if err := InstallDownloadedTempFile(tmpPath, dst, 0o644); err != nil {
-		t.Fatalf("InstallDownloadedTempFile() error = %v", err)
+	if err := InstallSiblingTempFile(tmpPath, dst, 0o644); err != nil {
+		t.Fatalf("InstallSiblingTempFile() error = %v", err)
 	}
 	if _, err := os.Stat(tmpPath); !os.IsNotExist(err) {
 		t.Fatalf("temp file still exists: stat err = %v", err)
@@ -186,253 +182,6 @@ func TestWriteFileAtomicReplacesLeafSymlinkWithoutFollowingIt(t *testing.T) {
 	}
 }
 
-func TestDownloadAndInstallRejectsSymlinkedDestinationAncestor(t *testing.T) {
-	root := t.TempDir()
-	outside := t.TempDir()
-	if err := os.Symlink(outside, filepath.Join(root, "redirect")); err != nil {
-		t.Skipf("Symlink unavailable: %v", err)
-	}
-
-	err := DownloadAndInstall(filepath.Join(root, "redirect", "output.txt"), 0o640, func(io.Writer) error {
-		return nil
-	})
-	if err == nil {
-		t.Fatal("DownloadAndInstall() error = nil, want symlink ancestor rejection")
-	}
-	entries, err := os.ReadDir(outside)
-	if err != nil {
-		t.Fatalf("ReadDir(outside) error = %v", err)
-	}
-	if len(entries) != 0 {
-		t.Fatalf("outside destination received entries: %v", entries)
-	}
-}
-
-func TestReplaceFileAtomicOrdersDurableOperations(t *testing.T) {
-	events := make([]string, 0, 8)
-	tmp := &recordingTemporaryFile{name: "/work/.result.tmp-1", events: &events}
-	ops := recordingAtomicReplacementOperations(&events, tmp)
-
-	err := replaceFileAtomicWithOperations(
-		"/work/result",
-		[]byte("replacement"),
-		ReplaceFileOptions{
-			Mode: 0o640,
-			BeforeRename: func() error {
-				events = append(events, "check")
-				return nil
-			},
-		},
-		ops,
-	)
-	if err != nil {
-		t.Fatalf("replaceFileAtomicWithOperations() error = %v", err)
-	}
-
-	want := []string{"create", "write", "chmod:rw-r-----", "sync", "close", "check", "rename", "sync-directory"}
-	if !equalStrings(events, want) {
-		t.Fatalf("operation order = %v, want %v", events, want)
-	}
-}
-
-func TestReplaceFileAtomicFailuresCleanUninstalledTemporaryFile(t *testing.T) {
-	tests := []struct {
-		name       string
-		configure  func(*recordingTemporaryFile, *atomicReplacementOperations)
-		wantErr    error
-		wantRemove bool
-	}{
-		{
-			name: "short write",
-			configure: func(tmp *recordingTemporaryFile, _ *atomicReplacementOperations) {
-				tmp.shortWrite = true
-			},
-			wantErr:    io.ErrShortWrite,
-			wantRemove: true,
-		},
-		{
-			name: "write",
-			configure: func(tmp *recordingTemporaryFile, _ *atomicReplacementOperations) {
-				tmp.writeErr = errAtomicReplacementTest
-			},
-			wantErr:    errAtomicReplacementTest,
-			wantRemove: true,
-		},
-		{
-			name: "sync",
-			configure: func(tmp *recordingTemporaryFile, _ *atomicReplacementOperations) {
-				tmp.syncErr = errAtomicReplacementTest
-			},
-			wantErr:    errAtomicReplacementTest,
-			wantRemove: true,
-		},
-		{
-			name: "rename",
-			configure: func(_ *recordingTemporaryFile, ops *atomicReplacementOperations) {
-				ops.rename = func(string, string) error { return errAtomicReplacementTest }
-			},
-			wantErr:    errAtomicReplacementTest,
-			wantRemove: true,
-		},
-		{
-			name: "directory sync",
-			configure: func(_ *recordingTemporaryFile, ops *atomicReplacementOperations) {
-				ops.syncDirectory = func(string) error { return ErrDirectorySyncUnsupported }
-			},
-			wantErr:    ErrDirectorySyncUnsupported,
-			wantRemove: false,
-		},
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			events := make([]string, 0, 8)
-			tmp := &recordingTemporaryFile{name: "/work/.result.tmp-1", events: &events}
-			ops := recordingAtomicReplacementOperations(&events, tmp)
-			test.configure(tmp, &ops)
-
-			err := replaceFileAtomicWithOperations("/work/result", []byte("replacement"), ReplaceFileOptions{Mode: 0o640}, ops)
-			if !errors.Is(err, test.wantErr) {
-				t.Fatalf("replaceFileAtomicWithOperations() error = %v, want %v", err, test.wantErr)
-			}
-			gotRemove := containsString(events, "remove")
-			if gotRemove != test.wantRemove {
-				t.Fatalf("remove temporary file = %t, want %t; operations = %v", gotRemove, test.wantRemove, events)
-			}
-		})
-	}
-}
-
-func TestReplaceFileAtomicReportsCleanupFailureWithPrimaryFailure(t *testing.T) {
-	events := make([]string, 0, 5)
-	tmp := &recordingTemporaryFile{
-		name:     "/work/.result.tmp-1",
-		events:   &events,
-		writeErr: errAtomicReplacementTest,
-	}
-	ops := recordingAtomicReplacementOperations(&events, tmp)
-	cleanupErr := errors.New("temporary cleanup failed")
-	ops.remove = func(string) error {
-		events = append(events, "remove")
-		return cleanupErr
-	}
-
-	err := replaceFileAtomicWithOperations("/work/result", []byte("replacement"), ReplaceFileOptions{Mode: 0o640}, ops)
-	if !errors.Is(err, errAtomicReplacementTest) {
-		t.Fatalf("replacement error = %v, want write failure", err)
-	}
-	if !errors.Is(err, cleanupErr) {
-		t.Fatalf("replacement error = %v, want cleanup failure", err)
-	}
-}
-
-func TestInstallDownloadedTempFileOrdersDurableOperations(t *testing.T) {
-	events := make([]string, 0, 4)
-	ops := atomicReplacementOperations{
-		chmod: func(string, os.FileMode) error {
-			events = append(events, "chmod")
-			return nil
-		},
-		syncFile: func(string) error {
-			events = append(events, "sync-file")
-			return nil
-		},
-		rename: func(string, string) error {
-			events = append(events, "rename")
-			return nil
-		},
-		syncDirectory: func(string) error {
-			events = append(events, "sync-directory")
-			return nil
-		},
-	}
-
-	if err := installDownloadedTempFileWithOperations("/work/.download.tmp", "/work/result", 0o640, ops); err != nil {
-		t.Fatalf("installDownloadedTempFileWithOperations() error = %v", err)
-	}
-	want := []string{"chmod", "sync-file", "rename", "sync-directory"}
-	if !equalStrings(events, want) {
-		t.Fatalf("operation order = %v, want %v", events, want)
-	}
-}
-
-type recordingTemporaryFile struct {
-	name       string
-	events     *[]string
-	shortWrite bool
-	writeErr   error
-	syncErr    error
-}
-
-func (f *recordingTemporaryFile) Write(data []byte) (int, error) {
-	*f.events = append(*f.events, "write")
-	if f.writeErr != nil {
-		return 0, f.writeErr
-	}
-	if f.shortWrite {
-		return len(data) - 1, nil
-	}
-	return len(data), nil
-}
-
-func (f *recordingTemporaryFile) Name() string { return f.name }
-
-func (f *recordingTemporaryFile) Sync() error {
-	*f.events = append(*f.events, "sync")
-	return f.syncErr
-}
-
-func (f *recordingTemporaryFile) Close() error {
-	*f.events = append(*f.events, "close")
-	return nil
-}
-
-func recordingAtomicReplacementOperations(events *[]string, tmp temporaryFile) atomicReplacementOperations {
-	return atomicReplacementOperations{
-		createTemp: func(string, string) (temporaryFile, error) {
-			*events = append(*events, "create")
-			return tmp, nil
-		},
-		chmod: func(_ string, mode os.FileMode) error {
-			*events = append(*events, "chmod:"+mode.Perm().String()[1:])
-			return nil
-		},
-		rename: func(string, string) error {
-			*events = append(*events, "rename")
-			return nil
-		},
-		remove: func(string) error {
-			*events = append(*events, "remove")
-			return nil
-		},
-		syncDirectory: func(string) error {
-			*events = append(*events, "sync-directory")
-			return nil
-		},
-	}
-}
-
-func containsString(values []string, target string) bool {
-	for _, value := range values {
-		if value == target {
-			return true
-		}
-	}
-	return false
-}
-
-func equalStrings(got, want []string) bool {
-	if len(got) != len(want) {
-		return false
-	}
-	for i := range got {
-		if got[i] != want[i] {
-			return false
-		}
-	}
-	return true
-}
-
 func assertNoMatchingTempFiles(t *testing.T, dir, prefix string) {
 	t.Helper()
 	entries, err := os.ReadDir(dir)

internal/stage/prepare_previous.go

@@ -5,8 +5,10 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"os"
 	"path/filepath"
 	"sort"
+	"strings"
 
 	"gitea.maximumdirect.net/eric/narratio/internal/adapters/storage"
 	"gitea.maximumdirect.net/eric/narratio/internal/artifacts"
@@ -58,23 +60,23 @@ func hydratePreviousSessionArtifacts(
 			return nil, fmt.Errorf("create previous-session path directory for %q: %w", record.LocalPath, err)
 		}
 
-		if err := fileops.DownloadAndInstall(record.LocalPath, fileops.WorkspaceFileMode, func(destination io.Writer) error {
+		temporaryPath, err := writePreviousTemp(record.LocalPath, func(destination io.Writer) error {
 			if len(record.VerifiedContent) > 0 {
 				_, err := io.Copy(destination, bytes.NewReader(record.VerifiedContent))
 				return err
 			}
 			return storage.DownloadTo(ctx, env.ObjectStore, record.RemoteKey, destination)
-		}); err != nil {
-			return nil, err
-		}
-		if record.Kind == preparePreviousInputKindArtifact {
-			if err := requireNonEmptyFile(record.LocalPath, "previous-session artifact "+record.RequirementName); err != nil {
-				return nil, err
-			}
-		}
-		checksum, err := env.ArtifactStore.Checksum(record.LocalPath)
+		})
 		if err != nil {
-			return nil, fmt.Errorf("checksum previous-session object %q: %w", record.RemoteKey, err)
+			return nil, fmt.Errorf("write previous-session object %q: %w", record.RemoteKey, err)
+		}
+		defer func() { _ = os.Remove(temporaryPath) }()
+		checksum, err := validatePreviousTemp(record, temporaryPath)
+		if err != nil {
+			return nil, err
+		}
+		if err := fileops.InstallSiblingTempFile(temporaryPath, record.LocalPath, fileops.WorkspaceFileMode); err != nil {
+			return nil, fmt.Errorf("install previous-session object %q: %w", record.RemoteKey, err)
 		}
 
 		result.Inputs = append(result.Inputs, manifest.InputRecord{
@@ -100,3 +102,55 @@ func hydratePreviousSessionArtifacts(
 	})
 	return result, nil
 }
+
+func writePreviousTemp(destination string, write func(io.Writer) error) (string, error) {
+	if strings.TrimSpace(destination) == "" {
+		return "", fmt.Errorf("previous-session destination path is required")
+	}
+	if write == nil {
+		return "", fmt.Errorf("previous-session temporary writer is required")
+	}
+	directory := filepath.Dir(destination)
+	if err := fileops.EnsureWorkspaceDirectory(directory); err != nil {
+		return "", fmt.Errorf("create previous-session path directory: %w", err)
+	}
+	temporary, err := os.CreateTemp(directory, "."+filepath.Base(destination)+".tmp-*")
+	if err != nil {
+		return "", fmt.Errorf("create sibling temporary file: %w", err)
+	}
+	temporaryPath := temporary.Name()
+	if err := write(temporary); err != nil {
+		_ = temporary.Close()
+		_ = os.Remove(temporaryPath)
+		return "", err
+	}
+	if err := temporary.Close(); err != nil {
+		_ = os.Remove(temporaryPath)
+		return "", fmt.Errorf("close sibling temporary file: %w", err)
+	}
+	return temporaryPath, nil
+}
+
+func validatePreviousTemp(record previouscache.Record, temporaryPath string) (string, error) {
+	info, err := os.Stat(temporaryPath)
+	if err != nil {
+		return "", fmt.Errorf("inspect previous-session object %q: %w", record.RemoteKey, err)
+	}
+	hasDeclaredSize := record.Size > 0 || strings.TrimSpace(record.SHA256) != "" || strings.TrimSpace(record.Generation) != ""
+	if hasDeclaredSize && info.Size() != record.Size {
+		return "", fmt.Errorf("previous-session object %q size mismatch: got %d, want %d", record.RemoteKey, info.Size(), record.Size)
+	}
+	if record.Kind == preparePreviousInputKindArtifact {
+		if err := requireNonEmptyFile(temporaryPath, "previous-session artifact "+record.RequirementName); err != nil {
+			return "", err
+		}
+	}
+	checksum, err := artifacts.SHA256File(temporaryPath)
+	if err != nil {
+		return "", fmt.Errorf("checksum previous-session object %q: %w", record.RemoteKey, err)
+	}
+	if expected := strings.TrimSpace(record.SHA256); expected != "" && !strings.EqualFold(checksum, expected) {
+		return "", fmt.Errorf("previous-session object %q checksum mismatch", record.RemoteKey)
+	}
+	return checksum, nil
+}