Provide should also return last modified

auto/backup/uploader.go

@@ -30,8 +30,10 @@ type DataProvider interface {
 	// can be ignored.
 	Check(i int64) (int64, bool)
 
-	// Provide writes the data-for-upload to the file specified by path.
-	Provide(path string) error
+	// Provide writes the data-for-upload to the file specified by path. Because
+	// Provide may change the data in the DataProvider, it returns the current
+	// value of i, which should be passed to the next invocation of Check().
+	Provide(path string) (int64, error)
 }
 
 // stats captures stats for the Uploader service.
@@ -139,13 +141,14 @@ func (u *Uploader) upload(ctx context.Context) error {
 	}
 	defer os.Remove(filetoUpload)
 
-	lastI, changed := u.dataProvider.Check(u.lastI)
+	_, changed := u.dataProvider.Check(u.lastI)
 	if !changed {
 		stats.Add(numUploadsSkipped, 1)
 		return nil
 	}
 
-	if err := u.dataProvider.Provide(filetoUpload); err != nil {
+	lastI, err := u.dataProvider.Provide(filetoUpload)
+	if err != nil {
 		return err
 	}
 	if err := u.compressIfNeeded(filetoUpload); err != nil {

auto/backup/uploader_test.go

@@ -129,6 +129,14 @@ func Test_UploaderDoubleUpload(t *testing.T) {
 		}
 		return 2, false
 	}
+
+	provideLastFnCalled := int64(1)
+	dp.provideLastIFn = func() int64 {
+		// Keep it one ahead of checkFn.
+		defer func() { provideLastFnCalled++ }()
+		return provideLastFnCalled
+	}
+
 	uploader := NewUploader(sc, dp, time.Second, UploadNoCompress)
 	ctx, cancel := context.WithCancel(context.Background())
 
@@ -166,6 +174,14 @@ func Test_UploaderFailThenOK(t *testing.T) {
 	dp.checkFn = func(i int64) (int64, bool) {
 		return 1, true
 	}
+
+	provideLastFnCalled := int64(1)
+	dp.provideLastIFn = func() int64 {
+		// Keep it one ahead of checkFn.
+		defer func() { provideLastFnCalled++ }()
+		return provideLastFnCalled
+	}
+
 	uploader := NewUploader(sc, dp, time.Second, UploadNoCompress)
 	ctx, cancel := context.WithCancel(context.Background())
 
@@ -204,6 +220,12 @@ func Test_UploaderOKThenFail(t *testing.T) {
 	dp.checkFn = func(i int64) (int64, bool) {
 		return 1, true
 	}
+	provideLastFnCalled := int64(2)
+	dp.provideLastIFn = func() int64 {
+		// Keep it one ahead of checkFn.
+		defer func() { provideLastFnCalled++ }()
+		return provideLastFnCalled
+	}
 	uploader := NewUploader(sc, dp, time.Second, UploadNoCompress)
 	ctx, cancel := context.WithCancel(context.Background())
 
@@ -278,6 +300,12 @@ func Test_UploaderEnabledTrue(t *testing.T) {
 	dp.checkFn = func(i int64) (int64, bool) {
 		return 1, true
 	}
+	provideLastFnCalled := int64(2)
+	dp.provideLastIFn = func() int64 {
+		// Keep it one ahead of checkFn.
+		defer func() { provideLastFnCalled++ }()
+		return provideLastFnCalled
+	}
 	uploader := NewUploader(sc, dp, time.Second, UploadNoCompress)
 	ctx, cancel := context.WithCancel(context.Background())
 
@@ -327,9 +355,10 @@ func (mc *mockStorageClient) String() string {
 }
 
 type mockDataProvider struct {
-	data    string
-	err     error
-	checkFn func(i int64) (int64, bool)
+	data           string
+	err            error
+	checkFn        func(i int64) (int64, bool)
+	provideLastIFn func() int64
 }
 
 func (mp *mockDataProvider) Check(i int64) (int64, bool) {
@@ -339,9 +368,15 @@ func (mp *mockDataProvider) Check(i int64) (int64, bool) {
 	return 0, false
 }
 
-func (mp *mockDataProvider) Provide(path string) error {
+func (mp *mockDataProvider) Provide(path string) (int64, error) {
 	if mp.err != nil {
-		return mp.err
+		return 0, mp.err
 	}
-	return os.WriteFile(path, []byte(mp.data), 0644)
+
+	lastI := int64(0)
+	if mp.provideLastIFn != nil {
+		lastI = mp.provideLastIFn()
+	}
+
+	return lastI, os.WriteFile(path, []byte(mp.data), 0644)
 }

store/provider.go

@@ -53,7 +53,7 @@ func (p *Provider) Provide(path string) (retErr error) {
 
 	fd, err := os.Create(path)
 	if err != nil {
-		return err
+		return 0, err
 	}
 	defer fd.Close()
 
@@ -70,17 +70,21 @@ func (p *Provider) Provide(path string) (retErr error) {
 		time.Sleep(p.retryInterval)
 		nRetries++
 		if nRetries > p.nRetries {
-			return err
+			return 0, err
 		}
 	}
 
 	// Switch database to DELETE mode, to keep existing behaviour.
 	if err := fd.Close(); err != nil {
-		return err
+		return 0, err
 	}
 	if db.EnsureDeleteMode(path) != nil {
-		return err
+		return 0, err
+	}
+	lm, err := p.str.db.LastModified()
+	if err != nil {
+		return 0, err
 	}
 	stats.Add(numProvides, 1)
-	return nil
+	return lm.UnixNano(), nil
 }

store/provider_test.go

@@ -49,7 +49,7 @@ func Test_SingleNodeProvide(t *testing.T) {
 	tempFile := mustCreateTempFile()
 	defer os.Remove(tempFile)
 	provider := NewProvider(s0, false)
-	if err := provider.Provide(tempFile); err != nil {
+	if _, err := provider.Provide(tempFile); err != nil {
 		t.Fatalf("failed to provide SQLite data: %s", err.Error())
 	}
 
@@ -106,7 +106,7 @@ func Test_SingleNodeProvideNoData(t *testing.T) {
 	tmpFile := mustCreateTempFile()
 	defer os.Remove(tmpFile)
 	provider := NewProvider(s, false)
-	if err := provider.Provide(tmpFile); err != nil {
+	if _, err := provider.Provide(tmpFile); err != nil {
 		t.Fatalf("store failed to provide: %s", err.Error())
 	}
 }