cmd/prometheus: allow retention percentage with new data path

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>

cmd/prometheus/main.go

@@ -824,7 +824,7 @@ func main() {
 			if cfg.tsdb.MaxBytes > 0 {
 				logger.Warn("storage.tsdb.retention.size is ignored, because storage.tsdb.retention.percentage is specified")
 			}
-			if prom_runtime.FsSize(localStoragePath) == 0 {
+			if storagePathFsSize(localStoragePath) == 0 {
 				fmt.Fprintln(os.Stderr, fmt.Errorf("unable to detect total capacity of metric storage at %s, please disable retention percentage (%g%%)", localStoragePath, cfg.tsdb.MaxPercentage))
 				os.Exit(2)
 			}
@@ -1756,6 +1756,25 @@ func computeExternalURL(u, listenAddr string) (*url.URL, error) {
 	return eu, nil
 }
 
+// storagePathFsSize returns the filesystem size for path or its closest existing parent.
+func storagePathFsSize(path string) uint64 {
+	for {
+		if size := prom_runtime.FsSize(path); size > 0 {
+			return size
+		}
+
+		if _, err := os.Stat(path); !errors.Is(err, os.ErrNotExist) {
+			return 0
+		}
+
+		parent := filepath.Dir(path)
+		if parent == path {
+			return 0
+		}
+		path = parent
+	}
+}
+
 // readyStorage implements the Storage interface while allowing to set the actual
 // storage at a later point in time.
 type readyStorage struct {

cmd/prometheus/main_test.go

@@ -148,6 +148,47 @@ func TestFailedStartupExitCode(t *testing.T) {
 	require.Equal(t, expectedExitStatus, status.ExitStatus())
 }
 
+func TestRetentionPercentageStartsWithMissingStoragePath(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode.")
+	}
+	t.Parallel()
+
+	tmpDir := t.TempDir()
+	if storagePathFsSize(tmpDir) == 0 {
+		t.Skip("skipping test because filesystem size detection is unavailable.")
+	}
+
+	configFile := filepath.Join(tmpDir, "prometheus.yml")
+	storagePath := filepath.Join(tmpDir, "missing", "data")
+
+	require.NoError(t, os.WriteFile(configFile, []byte(`
+storage:
+  tsdb:
+    retention:
+      percentage: 1.5
+`), 0o777))
+
+	port := testutil.RandomUnprivilegedPort(t)
+	prom := prometheusCommandWithLogging(
+		t,
+		configFile,
+		port,
+		"--storage.tsdb.path="+storagePath,
+	)
+	require.NoError(t, prom.Start())
+
+	require.Eventually(t, func() bool {
+		r, err := http.Get(fmt.Sprintf("http://127.0.0.1:%d/metrics", port))
+		if err != nil {
+			return false
+		}
+		defer r.Body.Close()
+		return r.StatusCode == http.StatusOK
+	}, startupTime, 100*time.Millisecond)
+	require.DirExists(t, storagePath)
+}
+
 type senderFunc func(alerts ...*notifier.Alert)
 
 func (s senderFunc) Send(alerts ...*notifier.Alert) {