Release unused file buffers for empty artifacts

internal/artifact/reader.go

@@ -158,7 +158,7 @@ func readFileArtifact(ctx context.Context, path string, open func(string) (artif
 			return nil, fmt.Errorf("failed to read file %s: %w", path, readErr)
 		}
 	}
-	if len(data) > 0 && cap(data)-len(data) > len(data) {
+	if cap(data)-len(data) > len(data) {
 		// A stale size hint should not make a small completed artifact retain a
 		// disproportionately large backing array.
 		compact := make([]byte, len(data))

internal/artifact/reader_benchmark_test.go

@@ -42,7 +42,20 @@ func BenchmarkArtifactFileRead(b *testing.B) {
 // BenchmarkRetainedArtifactFileRead measures live artifact bodies when an
 // opened file reports a size larger than the content that reaches EOF.
 func BenchmarkRetainedArtifactFileRead(b *testing.B) {
-	body := bytes.Repeat([]byte("x"), 256<<10)
+	for _, fixture := range []struct {
+		name string
+		body []byte
+	}{
+		{name: "small", body: bytes.Repeat([]byte("x"), 256<<10)},
+		{name: "empty", body: []byte{}},
+	} {
+		b.Run(fixture.name, func(b *testing.B) {
+			benchmarkRetainedArtifactFileRead(b, fixture.body)
+		})
+	}
+}
+
+func benchmarkRetainedArtifactFileRead(b *testing.B, body []byte) {
 	path := filepath.Join(b.TempDir(), "overreported.txt")
 	if err := os.WriteFile(path, body, 0o600); err != nil {
 		b.Fatal(err)

internal/artifact/reader_test.go

@@ -241,6 +241,7 @@ func TestFileReaderUsesOpenedSizeOnlyAsACapacityHint(t *testing.T) {
 		reportedSize int64
 	}{
 		{name: "overreported size", body: []byte("actual file content"), reportedSize: 128},
+		{name: "overreported size with empty content", body: []byte{}, reportedSize: 16 << 20},
 		{name: "underreported size", body: bytes.Repeat([]byte("x"), fileReadChunkSize+17), reportedSize: 1},
 		{name: "very large hint with small content", body: []byte("small actual content"), reportedSize: int64(^uint64(0) >> 1)},
 	}
@@ -280,6 +281,9 @@ func TestFileReaderUsesOpenedSizeOnlyAsACapacityHint(t *testing.T) {
 			if artifact.Size != int64(len(tc.body)) {
 				t.Fatalf("artifact size=%d, want %d", artifact.Size, len(tc.body))
 			}
+			if artifact.Name != filepath.Base(filePath) || artifact.URI != filePath || artifact.ContentType == "" {
+				t.Fatalf("unexpected artifact metadata: name=%q URI=%q content type=%q", artifact.Name, artifact.URI, artifact.ContentType)
+			}
 			wantHash := fmt.Sprintf("%x", sha256.Sum256(tc.body))
 			if artifact.Hash != wantHash {
 				t.Fatalf("artifact hash=%q, want %q", artifact.Hash, wantHash)