build: preserve IPA output on extraction failure

internal/build/coordinator.go

@@ -341,16 +341,25 @@ func extractFile(f *zip.File, destPath string) (int64, error) {
 	}
 	defer func() { _ = rc.Close() }()
 
-	out, err := os.Create(destPath)
+	// Publish the IPA only after the entire entry (including its ZIP checksum)
+	// has been read and the output closed successfully. A failed extraction
+	// must not leave a broken archive for later commands to select.
+	out, err := os.CreateTemp(filepath.Dir(destPath), ".builder-*.ipa.part")
 	if err != nil {
 		return 0, fmt.Errorf("failed to create output file: %w", err)
 	}
-	defer func() { _ = out.Close() }()
+	defer func() { _ = out.Close(); _ = os.Remove(out.Name()) }()
 
 	size, err := io.Copy(out, rc)
 	if err != nil {
 		return 0, fmt.Errorf("failed to write IPA: %w", err)
 	}
 
+	if err := out.Close(); err != nil {
+		return 0, fmt.Errorf("failed to close IPA: %w", err)
+	}
+	if err := os.Rename(out.Name(), destPath); err != nil {
+		return 0, fmt.Errorf("failed to save IPA: %w", err)
+	}
 	return size, nil
 }

internal/build/coordinator_test.go

@@ -0,0 +1,120 @@
+package build
+
+import (
+	"archive/zip"
+	"bytes"
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func ipaArtifact(t *testing.T, corrupt bool) ([]byte, []byte) {
+	t.Helper()
+	payload := []byte("IPA bytes from the remote build")
+	var buf bytes.Buffer
+	zw := zip.NewWriter(&buf)
+	w, err := zw.CreateHeader(&zip.FileHeader{Name: "App.ipa", Method: zip.Store})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := w.Write(payload); err != nil {
+		t.Fatal(err)
+	}
+	if err := zw.Close(); err != nil {
+		t.Fatal(err)
+	}
+	data := buf.Bytes()
+	if corrupt {
+		r, err := zip.NewReader(bytes.NewReader(data), int64(len(data)))
+		if err != nil {
+			t.Fatal(err)
+		}
+		offset, err := r.File[0].DataOffset()
+		if err != nil {
+			t.Fatal(err)
+		}
+		data[offset] ^= 0xff // Keep the ZIP readable but fail the entry's CRC check.
+	}
+	return data, payload
+}
+
+func TestExtractIPAFromZipChecksumFailure(t *testing.T) {
+	for _, existing := range []bool{false, true} {
+		name := "new destination"
+		if existing {
+			name = "existing destination"
+		}
+		t.Run(name, func(t *testing.T) {
+			dir := t.TempDir()
+			dest := filepath.Join(dir, "App.ipa")
+			original := []byte("previous successful build")
+			if existing {
+				if err := os.WriteFile(dest, original, 0600); err != nil {
+					t.Fatal(err)
+				}
+			}
+			data, _ := ipaArtifact(t, true)
+			if _, err := extractIPAFromZip(data, dest); !errors.Is(err, zip.ErrChecksum) {
+				t.Fatalf("expected checksum failure, got %v", err)
+			}
+			got, err := os.ReadFile(dest)
+			if existing {
+				if err != nil || !bytes.Equal(got, original) {
+					t.Fatalf("failed extraction replaced the previous IPA: %q, %v", got, err)
+				}
+			} else if !errors.Is(err, os.ErrNotExist) {
+				t.Fatalf("failed extraction left an IPA at the destination: %q, %v", got, err)
+			}
+			entries, err := os.ReadDir(dir)
+			if err != nil {
+				t.Fatal(err)
+			}
+			want := 0
+			if existing {
+				want = 1
+			}
+			if len(entries) != want {
+				t.Fatalf("temporary files left after failed extraction: %v", entries)
+			}
+		})
+	}
+}
+
+func TestExtractIPAFromZipSuccess(t *testing.T) {
+	data, payload := ipaArtifact(t, false)
+	dir := t.TempDir()
+	dest := filepath.Join(dir, "App.ipa")
+	if err := os.WriteFile(dest, []byte("old build"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	size, err := extractIPAFromZip(data, dest)
+	if err != nil || size != int64(len(payload)) {
+		t.Fatalf("extraction: size %d, error %v", size, err)
+	}
+	got, err := os.ReadFile(dest)
+	if err != nil || !bytes.Equal(got, payload) {
+		t.Fatalf("extracted bytes: %q, %v", got, err)
+	}
+	entries, err := os.ReadDir(dir)
+	if err != nil || len(entries) != 1 {
+		t.Fatalf("unexpected output files: %v, %v", entries, err)
+	}
+}
+
+func TestExtractIPAFromZipRenameFailure(t *testing.T) {
+	data, _ := ipaArtifact(t, false)
+	dir := t.TempDir()
+	dest := filepath.Join(dir, "App.ipa")
+	// An existing directory cannot be replaced by the completed temporary file.
+	if err := os.Mkdir(dest, 0700); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := extractIPAFromZip(data, dest); err == nil {
+		t.Fatal("expected saving to a directory to fail")
+	}
+	entries, err := os.ReadDir(dir)
+	if err != nil || len(entries) != 1 || entries[0].Name() != "App.ipa" || !entries[0].IsDir() {
+		t.Fatalf("destination changed or temporary file leaked: %v, %v", entries, err)
+	}
+}