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
the name reappears in between. 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.

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/fetch.go

@@ -489,12 +489,24 @@ 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.
+	err = os.Remove(tmpPath)
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
+		return fmt.Errorf("failed to remove old temp file: %w", err)
+	}
+
+	// Create the temp file with os.Create's mode, but fail if anything
+	// has appeared at tmpPath since the removal (O_EXCL).
 	//
-	// G304: tmpPath is a relative path that sanitizePath keeps inside the
+	// G302: the mode keeps fetched files readable by group and other,
-	// target directory as text, and checkNoSymlinks just found no symlink
+	// like dirPerms. G304: tmpPath is a relative path that sanitizePath
-	// in it.
+	// keeps inside the target directory as text, and checkNoSymlinks just
-	out, err := os.Create(tmpPath) //nolint:gosec // G304: see comment above
+	// found no symlink in it.
+	out, err := os.OpenFile( //nolint:gosec // G302, G304: see comment above
+		tmpPath, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0o666)
 	if err != nil {
 		return fmt.Errorf("failed to create temp file: %w", err)
 	}

internal/cli/fetch_test.go

@@ -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, "fetch", "-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")
+}