Check copyFile destination Close() error (u9)

The deferred out.Close() silently dropped disk-full, quota, and I/O
errors surfaced when the OS flushes buffered writes. Capture the Close
error explicitly, remove the partial destination on failure, and add
copyFile success / missing-source unit tests.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

2 files changed, 50 insertions, 4 deletions

diff --git a/player-server/internal/service/write.go b/player-server/internal/service/write.go
index 3fe4d47..d76dd87 100644
--- a/player-server/internal/service/write.go
+++ b/player-server/internal/service/write.go
@@ -237,15 +237,23 @@ func copyFile(src, dst string) error {
 	}
 	defer in.Close()
 
+	// Do NOT defer out.Close(): for the destination file we must observe the
+	// Close error explicitly. When the OS flushes buffered writes during Close
+	// it can surface disk-full, quota, or I/O errors that a deferred close
+	// would silently swallow, leaving a truncated/corrupt destination behind.
 	out, err := os.Create(dst)
 	if err != nil {
 		return err
 	}
-	defer out.Close()
-
-	if _, err := io.Copy(out, in); err != nil {
+	_, copyErr := io.Copy(out, in)
+	closeErr := out.Close()
+	if copyErr != nil {
 		_ = os.Remove(dst)
-		return err
+		return copyErr
+	}
+	if closeErr != nil {
+		_ = os.Remove(dst)
+		return fmt.Errorf("close %q: %w", dst, closeErr)
 	}
 	return nil
 }
diff --git a/player-server/internal/service/write_test.go b/player-server/internal/service/write_test.go
index 122ca78..3cf276b 100644
--- a/player-server/internal/service/write_test.go
+++ b/player-server/internal/service/write_test.go
@@ -3,6 +3,8 @@ package service
 import (
 	"context"
 	"errors"
+	"os"
+	"path/filepath"
 	"testing"
 
 	"codeberg.org/snonux/player/internal/clock"
@@ -98,3 +100,39 @@ func TestWriteService_RestoreMedia(t *testing.T) {
 		})
 	}
 }
+
+// TestCopyFile_Success verifies that copyFile creates the destination with
+// the exact contents of the source on a clean run (and returns no error).
+func TestCopyFile_Success(t *testing.T) {
+	dir := t.TempDir()
+	src := filepath.Join(dir, "src.bin")
+	dst := filepath.Join(dir, "dst.bin")
+	payload := []byte("hello copyFile world")
+	if err := os.WriteFile(src, payload, 0o644); err != nil {
+		t.Fatalf("write src: %v", err)
+	}
+	if err := copyFile(src, dst); err != nil {
+		t.Fatalf("copyFile: %v", err)
+	}
+	got, err := os.ReadFile(dst)
+	if err != nil {
+		t.Fatalf("read dst: %v", err)
+	}
+	if string(got) != string(payload) {
+		t.Fatalf("dst contents mismatch: got %q want %q", got, payload)
+	}
+}
+
+// TestCopyFile_MissingSource verifies the source-open error path propagates
+// cleanly (no destination should be created or leaked).
+func TestCopyFile_MissingSource(t *testing.T) {
+	dir := t.TempDir()
+	src := filepath.Join(dir, "does-not-exist.bin")
+	dst := filepath.Join(dir, "dst.bin")
+	if err := copyFile(src, dst); err == nil {
+		t.Fatal("expected error for missing source, got nil")
+	}
+	if _, err := os.Stat(dst); !os.IsNotExist(err) {
+		t.Fatalf("expected dst to not exist, stat err=%v", err)
+	}
+}