Never write fetch's temp file into an existing file (closes #115)

downloadFile opened the temp file with os.Create, which opens and
empties a file already at that name. If that file was a hard link to a
file outside the destination directory, fetch overwrote the outside
file. It now removes whatever is at the temp name, which removes only
that name, and creates the temp file with O_EXCL, so the create fails if
anything is still or again there. A leftover temp file from an
interrupted run is still replaced. The new test puts a hard link at the
temp name and checks that fetch succeeds and the outside file is
unchanged. The command name is now the constant cmdFetch, like the other
command names, because lint requires it once a third test uses it.

Model: opus-5-5

TODO.md

@@ -24,6 +24,9 @@ only thing left of the `chore/align-repo-policies` branch is the list below.
 
 # Completed Steps
 
+- 2026-10-03: `fetch` removes whatever sits at a file's temp name and then
+  creates the temp file only if that name is free, so a hard link left there
+  cannot make it write into a file outside the destination directory (#115)
 - 2026-10-03: `fetch` refuses any manifest path that runs through a symlink
   already in the destination directory, checked before each of its writes
   (directories, temp file, rename), so such a symlink cannot send a write

internal/cli/entry_test.go

@@ -126,7 +126,7 @@ func TestHelpCommand(t *testing.T) {
 	stdout := testStdout(t, opts)
 	assert.Contains(t, stdout, cmdGenerate)
 	assert.Contains(t, stdout, cmdCheck)
-	assert.Contains(t, stdout, "fetch")
+	assert.Contains(t, stdout, cmdFetch)
 }
 
 func TestGenerateCommand(t *testing.T) {

internal/cli/fetch.go

@@ -36,6 +36,11 @@ const (
 	// traversal bit for group and other must stay set.
 	dirPerms os.FileMode = 0o755
 
+	// filePerms is the permission mode, before the umask, for downloaded
+	// files. It is the mode os.Create uses; like dirPerms, it keeps group
+	// and other read access.
+	filePerms os.FileMode = 0o666
+
 	// Bitrate unit thresholds in bits per second.
 	bpsPerGbps = 1e9
 	bpsPerMbps = 1e6
@@ -489,12 +494,20 @@ func downloadFile(
 		return err
 	}
 
-	// Create temp file.
+	// Remove whatever is at tmpPath, such as a leftover from an
+	// interrupted run, rather than write into it: it may be a hard link
+	// to a file outside the target directory, and removing a hard link
+	// removes only this name. If the removal fails, O_EXCL below makes
+	// the create fail.
+	_ = os.Remove(tmpPath)
+
+	// Create the temp file only if nothing is at tmpPath (O_EXCL).
 	//
 	// G304: tmpPath is a relative path that sanitizePath keeps inside the
 	// target directory as text, and checkNoSymlinks just found no symlink
 	// in it.
-	out, err := os.Create(tmpPath) //nolint:gosec // G304: see comment above
+	out, err := os.OpenFile( //nolint:gosec // G304: see comment above
+		tmpPath, os.O_RDWR|os.O_CREATE|os.O_EXCL, filePerms)
 	if err != nil {
 		return fmt.Errorf("failed to create temp file: %w", err)
 	}

internal/cli/fetch_test.go

@@ -478,7 +478,7 @@ func TestFetchRefusesSymlinks(t *testing.T) {
 			require.NoError(t, os.MkdirAll(filepath.Dir(tt.link), 0o750))
 			require.NoError(t, os.Symlink(filepath.Join(outside, tt.target), tt.link))
 
-			opts := testOpts([]string{testApp, "fetch", "-q", server.URL}, afero.NewOsFs())
+			opts := testOpts([]string{testApp, cmdFetch, "-q", server.URL}, afero.NewOsFs())
 			assert.Equal(t, 1, runCLI(opts))
 			assert.Contains(t, testStderr(t, opts), "failed to download "+tt.entry+
 				": symlink in path not allowed: "+tt.link)
@@ -489,3 +489,37 @@ func TestFetchRefusesSymlinks(t *testing.T) {
 		})
 	}
 }
+
+// TestFetchReplacesHardLinkAtTempName runs fetch into a destination
+// directory that holds, at the temp file's name, a hard link to a file
+// outside it. To fetch that is an ordinary leftover from an interrupted
+// earlier run: it must replace it and succeed, and the outside file must
+// not change.
+//
+//nolint:paralleltest // changes the process-global working directory
+func TestFetchReplacesHardLinkAtTempName(t *testing.T) {
+	content := []byte("fetched")
+	sourceFs := afero.NewMemMapFs()
+	require.NoError(t, afero.WriteFile(sourceFs, "/"+testFileTxt, content, 0o644))
+
+	server := httptest.NewServer(fetchTestHandler(
+		scanToManifest(t, sourceFs), map[string][]byte{testFileTxt: content}))
+	defer server.Close()
+
+	outsideFile := filepath.Join(t.TempDir(), "secret.txt")
+	require.NoError(t, os.WriteFile(outsideFile, []byte("outside"), 0o600))
+
+	chdirTemp(t)
+	require.NoError(t, os.Link(outsideFile, ".file.txt.tmp"))
+
+	opts := testOpts([]string{testApp, cmdFetch, "-q", server.URL}, afero.NewOsFs())
+	require.Equal(t, 0, runCLI(opts), testStderr(t, opts))
+
+	fetched, err := os.ReadFile(testFileTxt)
+	require.NoError(t, err)
+	assert.Equal(t, content, fetched)
+
+	outside, err := os.ReadFile(outsideFile) //nolint:gosec // test-controlled path
+	require.NoError(t, err)
+	assert.Equal(t, "outside", string(outside), "fetch wrote outside the destination")
+}

internal/cli/mfer.go

@@ -18,6 +18,7 @@ const (
 	cmdGenerate = "generate"
 	cmdCheck    = "check"
 	cmdExport   = "export"
+	cmdFetch    = "fetch"
 
 	flagProgress = "progress"
 
@@ -300,7 +301,7 @@ func (mfa *CLIApp) listCommand() *cli.Command {
 
 func (mfa *CLIApp) fetchCommand() *cli.Command {
 	return &cli.Command{
-		Name:  "fetch",
+		Name:  cmdFetch,
 		Usage: "fetch manifest and referenced files",
 		Action: func(c *cli.Context) error {
 			mfa.setVerbosity(c)