Refuse a file whose size changed between freshen's scan and hash (closes #207)

freshen took a changed or new file's size from the scan and its hash
from a later read, without comparing the bytes read with that size. A
file that grew or shrank in between got an entry no content can match,
so check failed it with nothing saying why. processEntry now fails with
a size mismatch naming the file, as gen does, so freshen exits 1 and
leaves the manifest as it was.

Judgement call: the error wraps the package's existing errSizeMismatch
from fetch; its comment now covers both uses.

Model: opus-5-5

internal/cli/freshen_test.go

@@ -243,6 +243,58 @@ func TestFreshenWithChanges(t *testing.T) {
 	}
 }
 
+// growingFileFs appends to one file each time it is opened, as a program
+// still writing to the file would between freshen's scan and its hash.
+type growingFileFs struct {
+	afero.Fs
+
+	path string
+}
+
+//nolint:ireturn // Open must return afero.File to satisfy afero.Fs.
+func (f growingFileFs) Open(name string) (afero.File, error) {
+	if name == f.path {
+		file, err := f.OpenFile(name, os.O_APPEND|os.O_WRONLY, 0)
+		if err != nil {
+			return nil, err
+		}
+
+		_, err = file.WriteString(" world")
+		_ = file.Close()
+
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return f.Fs.Open(name)
+}
+
+// TestFreshenFileChangedSinceScan has a new file grow after freshen's scan
+// recorded its size and before freshen hashes it: freshen must fail and
+// leave the manifest as it was.
+func TestFreshenFileChangedSinceScan(t *testing.T) {
+	t.Parallel()
+
+	osFs := afero.NewOsFs()
+	root, manifestPath := setupFreshenDir(t, osFs,
+		map[string]string{testFileTxt: "content"})
+	grown := filepath.Join(root, "log.txt")
+	writeTestFile(t, osFs, grown, "hello")
+
+	before, err := afero.ReadFile(osFs, manifestPath)
+	require.NoError(t, err)
+
+	opts := testOpts([]string{
+		testApp, cmdFreshen, "-q", testFlagBase, root, manifestPath,
+	}, growingFileFs{Fs: osFs, path: grown})
+	assert.Equal(t, 1, runCLI(opts), "stderr: %s", testStderr(t, opts))
+
+	after, err := afero.ReadFile(osFs, manifestPath)
+	require.NoError(t, err)
+	assert.Equal(t, before, after, "manifest changed")
+}
+
 // TestFreshenLeavesManifestOutOfListing freshens a manifest kept in a
 // subdirectory of the tree it lists: the manifest is not listed, while an
 // ordinary file of the same name at the top of the tree is. The manifest