Reduce metric writes and preserve scheduler history

internal/store/store.go

@@ -1,6 +1,7 @@
 package store
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -70,16 +71,48 @@ func (s *Store) SaveRuns(runs []model.Run) error {
 	return writeJSONAtomic(s.runsPath(), runs, 0600)
 }
 
-func (s *Store) LoadBackupMetrics() ([]model.BackupMetric, error) {
+func (s *Store) LoadBackupMetrics() ([]model.BackupMetric, bool, error) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
-	var metrics []model.BackupMetric
-	if err := readJSON(s.backupMetricsPath(), &metrics); errors.Is(err, os.ErrNotExist) {
-		return []model.BackupMetric{}, nil
-	} else if err != nil {
-		return nil, err
+	path := s.backupMetricsPath()
+	b, err := os.ReadFile(path)
+	legacySource := false
+	if errors.Is(err, os.ErrNotExist) {
+		path = s.legacyBackupMetricsPath()
+		b, err = os.ReadFile(path)
+		legacySource = true
+		if errors.Is(err, os.ErrNotExist) {
+			return []model.BackupMetric{}, false, nil
+		}
 	}
-	return metrics, nil
+	if err != nil {
+		return nil, false, err
+	}
+	b = bytes.TrimSpace(b)
+	if len(b) == 0 {
+		return []model.BackupMetric{}, false, nil
+	}
+	if b[0] == '[' {
+		var metrics []model.BackupMetric
+		if err := json.Unmarshal(b, &metrics); err != nil {
+			return nil, true, fmt.Errorf("decode %s: %w", path, err)
+		}
+		return metrics, true, nil
+	}
+	lines := bytes.Split(b, []byte{'\n'})
+	metrics := make([]model.BackupMetric, 0, len(lines))
+	for index, line := range lines {
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 {
+			continue
+		}
+		var metric model.BackupMetric
+		if err := json.Unmarshal(line, &metric); err != nil {
+			return nil, legacySource, fmt.Errorf("decode %s line %d: %w", path, index+1, err)
+		}
+		metrics = append(metrics, metric)
+	}
+	return metrics, legacySource, nil
 }
 
 func (s *Store) SaveBackupMetrics(metrics []model.BackupMetric) error {
@@ -88,12 +121,45 @@ func (s *Store) SaveBackupMetrics(metrics []model.BackupMetric) error {
 	if len(metrics) > 5000 {
 		metrics = metrics[len(metrics)-5000:]
 	}
-	return writeJSONAtomic(s.backupMetricsPath(), metrics, 0600)
+	var data bytes.Buffer
+	encoder := json.NewEncoder(&data)
+	for _, metric := range metrics {
+		if err := encoder.Encode(metric); err != nil {
+			return err
+		}
+	}
+	return writeBytesAtomic(s.backupMetricsPath(), data.Bytes(), 0600)
+}
+
+func (s *Store) AppendBackupMetric(metric model.BackupMetric) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	b, err := json.Marshal(metric)
+	if err != nil {
+		return err
+	}
+	b = append(b, '\n')
+	file, err := os.OpenFile(s.backupMetricsPath(), os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0600)
+	if err != nil {
+		return err
+	}
+	if _, err := file.Write(b); err != nil {
+		file.Close()
+		return err
+	}
+	if err := file.Sync(); err != nil {
+		file.Close()
+		return err
+	}
+	return file.Close()
 }
 
 func (s *Store) configPath() string { return filepath.Join(s.dir, "config.json") }
 func (s *Store) runsPath() string   { return filepath.Join(s.dir, "runs.json") }
 func (s *Store) backupMetricsPath() string {
+	return filepath.Join(s.dir, "backup-metrics.jsonl")
+}
+func (s *Store) legacyBackupMetricsPath() string {
 	return filepath.Join(s.dir, "backup-metrics.json")
 }
 
@@ -114,6 +180,10 @@ func writeJSONAtomic(path string, value any, mode os.FileMode) error {
 		return err
 	}
 	b = append(b, '\n')
+	return writeBytesAtomic(path, b, mode)
+}
+
+func writeBytesAtomic(path string, b []byte, mode os.FileMode) error {
 	tmp, err := os.CreateTemp(filepath.Dir(path), ".urbm-*")
 	if err != nil {
 		return err

internal/store/store_test.go

@@ -43,10 +43,13 @@ func TestBackupMetricsPersistSeparatelyFromRuns(t *testing.T) {
 	if err := s.SaveRuns(nil); err != nil {
 		t.Fatal(err)
 	}
-	loaded, err := s.LoadBackupMetrics()
+	loaded, legacy, err := s.LoadBackupMetrics()
 	if err != nil {
 		t.Fatal(err)
 	}
+	if legacy {
+		t.Fatal("compacted metric file was detected as legacy JSON array")
+	}
 	if len(loaded) != 1 || loaded[0].RunID != "run-1" || loaded[0].BytesProcessed != 42 {
 		t.Fatalf("backup metrics = %#v", loaded)
 	}
@@ -58,3 +61,32 @@ func TestBackupMetricsPersistSeparatelyFromRuns(t *testing.T) {
 		t.Fatalf("backup metrics mode = %o", info.Mode().Perm())
 	}
 }
+
+func TestBackupMetricsMigrateFromJSONArrayAndAppend(t *testing.T) {
+	s := New(t.TempDir())
+	finished := time.Date(2026, 7, 13, 1, 30, 0, 0, time.UTC)
+	legacyJSON := `[{"schemaVersion":1,"runId":"old","jobId":"job-1","finishedAt":"2026-07-13T01:30:00Z","status":"success","bytesProcessed":42,"filesProcessed":7}]`
+	if err := os.WriteFile(s.legacyBackupMetricsPath(), []byte(legacyJSON), 0600); err != nil {
+		t.Fatal(err)
+	}
+	metrics, legacy, err := s.LoadBackupMetrics()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !legacy || len(metrics) != 1 || metrics[0].RunID != "old" {
+		t.Fatalf("legacy metrics = %#v, legacy = %v", metrics, legacy)
+	}
+	if err := s.SaveBackupMetrics(metrics); err != nil {
+		t.Fatal(err)
+	}
+	if err := s.AppendBackupMetric(model.BackupMetric{SchemaVersion: model.SchemaVersion, RunID: "new", JobID: "job-1", FinishedAt: finished.Add(time.Hour), Status: "success"}); err != nil {
+		t.Fatal(err)
+	}
+	metrics, legacy, err = s.LoadBackupMetrics()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if legacy || len(metrics) != 2 || metrics[1].RunID != "new" {
+		t.Fatalf("appended metrics = %#v, legacy = %v", metrics, legacy)
+	}
+}