Limit how much fetch and check read for a manifest or a file (closes #168)

NewManifestFromReader reads at most one byte past MaxManifestSize, a new
constant of 258 MiB: the 256 MiB decompressed limit grown by zstd's worst
case of 1/256, plus 1 MiB for the signature, the signing key and the
other outer fields. It refuses a larger manifest. fetch, and check given
a URL, stop downloading a manifest at the same point, so the refusal
comes from the library. fetch stops reading a file one byte past its
listed size, so a longer body ends in the size mismatch at once instead
of filling the disk. docs/FORMAT.md states the limit and gives the
decompressed limit as 256 MiB, the size the code uses.

Model: opus-5-5

internal/cli/fetch_test.go

@@ -441,6 +441,80 @@ func TestFetchSizeMismatch(t *testing.T) {
 		"temp file should be cleaned up on size mismatch")
 }
 
+// TestFetchStopsReadingFilePastListedSize serves 1 MiB for a file the
+// manifest lists at 16 bytes. fetch must stop reading one byte past the
+// listed size, report the size mismatch and remove its temp file.
+//
+//nolint:paralleltest // changes the process-global working directory
+func TestFetchStopsReadingFilePastListedSize(t *testing.T) {
+	content := []byte("sixteen bytes...")
+
+	manifest, err := mfer.NewManifestFromReader(bytes.NewReader(
+		manifestOf(t, map[string][]byte{testFileTxt: content})))
+	require.NoError(t, err)
+
+	server := httptest.NewServer(
+		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			_, _ = w.Write(make([]byte, 1<<20))
+		}))
+	defer server.Close()
+
+	chdirTemp(t)
+
+	err = downloadFile(context.Background(), testClient(),
+		server.URL+"/"+testFileTxt, ".", testFileTxt, manifest.Files()[0], nil)
+	require.ErrorIs(t, err, errSizeMismatch)
+	assert.EqualError(t, err, "size mismatch: expected 16 bytes, got 17")
+	assert.NoFileExists(t, tempPathFor(testFileTxt))
+}
+
+// zeros is an io.Reader of zero bytes without end.
+type zeros struct{}
+
+func (zeros) Read(p []byte) (int, error) {
+	clear(p)
+
+	return len(p), nil
+}
+
+// TestManifestDownloadStopsPastLimit serves a manifest 32 MiB larger than
+// mfer.MaxManifestSize to fetch and to check. Each must stop reading it
+// once it passes the maximum, fail with an error naming the maximum and
+// leave no temp file. The connection's buffers hold less than 32 MiB, so a
+// client that stops reading leaves the server unable to send all of it.
+//
+//nolint:paralleltest // sets TMPDIR, where check writes the manifest
+func TestManifestDownloadStopsPastLimit(t *testing.T) {
+	size := mfer.MaxManifestSize + 32<<20
+	limit := strconv.FormatInt(mfer.MaxManifestSize, 10)
+
+	tmpDir := t.TempDir()
+	t.Setenv("TMPDIR", tmpDir)
+
+	for _, command := range []string{cmdFetch, cmdCheck} {
+		var sent atomic.Int64
+
+		server := httptest.NewServer(
+			http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+				n, _ := io.CopyN(w, zeros{}, size)
+				sent.Store(n)
+			}))
+
+		opts := testOpts([]string{testApp, command, "-q", server.URL + "/index.mf"},
+			afero.NewOsFs())
+		assert.Equal(t, 1, runCLI(opts), command)
+		assert.Contains(t, testStderr(t, opts),
+			"maximum allowed size of "+limit+" bytes", command)
+
+		server.Close()
+		assert.Less(t, sent.Load(), size, "%s read the whole manifest", command)
+	}
+
+	leftover, err := os.ReadDir(tmpDir)
+	require.NoError(t, err)
+	assert.Empty(t, leftover)
+}
+
 //nolint:paralleltest // changes the process-global working directory
 func TestFetchProgress(t *testing.T) {
 	// Create source filesystem with a larger test file