fix(replay): a truncated index is a 500, not a partial 200

readAll reimplemented io.ReadAll and swallowed every error but fs.ErrInvalid,
returning the bytes it had managed to read with a nil error, so a half-written
index.html was indistinguishable from a whole one. io.ReadAll surfaces the
directory-handle case fs.ErrInvalid covered as well.

internal/replay/server.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"io/fs"
 	"net/http"
 	"path"
@@ -233,7 +234,7 @@ func serveIndex(responseWriter http.ResponseWriter, assets fs.FS) {
 		return
 	}
 	defer file.Close()
-	body, err := readAll(file)
+	body, err := io.ReadAll(file)
 	if err != nil {
 		http.Error(responseWriter, err.Error(), http.StatusInternalServerError)
 		return
@@ -242,25 +243,6 @@ func serveIndex(responseWriter http.ResponseWriter, assets fs.FS) {
 	_, _ = responseWriter.Write(body)
 }
 
-func readAll(file fs.File) ([]byte, error) {
-	const initialCapacity = 4 * 1024
-	buffer := make([]byte, 0, initialCapacity)
-	chunk := make([]byte, 4*1024)
-	for {
-		read, err := file.Read(chunk)
-		if read > 0 {
-			buffer = append(buffer, chunk[:read]...)
-		}
-		if err != nil {
-			if errors.Is(err, fs.ErrInvalid) {
-				return nil, err
-			}
-			break
-		}
-	}
-	return buffer, nil
-}
-
 func writeJSON(responseWriter http.ResponseWriter, status int, payload any) {
 	responseWriter.Header().Set("Content-Type", "application/json")
 	responseWriter.WriteHeader(status)

internal/replay/server_test.go

@@ -371,6 +371,55 @@ func TestAssets_FallbackToIndexHTML(t *testing.T) {
 	}
 }
 
+type truncatedIndexFS struct {
+	prefix []byte
+}
+
+func (assets truncatedIndexFS) Open(name string) (fs.File, error) {
+	if name != "index.html" {
+		return nil, fs.ErrNotExist
+	}
+	return &truncatedIndexFile{prefix: assets.prefix}, nil
+}
+
+type truncatedIndexFile struct {
+	prefix   []byte
+	consumed bool
+}
+
+func (file *truncatedIndexFile) Stat() (fs.FileInfo, error) { return nil, fs.ErrInvalid }
+
+func (file *truncatedIndexFile) Read(destination []byte) (int, error) {
+	if file.consumed {
+		return 0, io.ErrUnexpectedEOF
+	}
+	file.consumed = true
+	return copy(destination, file.prefix), nil
+}
+
+func (file *truncatedIndexFile) Close() error { return nil }
+
+func TestAssets_TruncatedIndexIsNotServedAsOK(t *testing.T) {
+	prefix := []byte(`<!doctype html><html><body><div id="root">`)
+	server, err := NewServer(ServerOptions{
+		RunsDirectory: t.TempDir(),
+		AssetsFS:      truncatedIndexFS{prefix: prefix},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	recorder := httptest.NewRecorder()
+	request := httptest.NewRequest(http.MethodGet, "/", nil)
+	server.Handler().ServeHTTP(recorder, request)
+
+	if recorder.Code != http.StatusInternalServerError {
+		t.Errorf("status = %d, want 500", recorder.Code)
+	}
+	if strings.Contains(recorder.Body.String(), string(prefix)) {
+		t.Errorf("served the partial index body: %q", recorder.Body.String())
+	}
+}
+
 func TestAssets_API404DoesNotFallThrough(t *testing.T) {
 	server, _ := newFixtureServer(t)
 	recorder := httptest.NewRecorder()