do not append timestamp to current log file

This patch updates the file_sink to *not* append the timestamp on the
current log file. The timestamp is only appended when the file is
rotated.

This matches the behavior:

- Implemented in Nomad v1.1.4 with hashicorp/nomad#11070
- Requested in hashicorp/nomad#11061
- Referenced in Consul's documentation:
  https://www.consul.io/docs/enterprise/audit-logging

> The following example configures a destination called "My Sink" which
> stores audit events at the file `/tmp/audit.json`.

Nomad's documentation is a little vague but implies the prior behavior
of always appending a timestamp.

While this is a *backward incompatible change* I think it's worth it for
usability and matching Consul's documentation. Nomad's documentation can
be fixed and a notice placed in the Upgrade Guide.

file_sink.go

@@ -159,11 +159,9 @@ func (fs *FileSink) open() error {
 	}
 
 	createTime := time.Now()
-	// New file name as the format:
-	// file rotation enabled: filename-timestamp.extension
-	// file rotation disabled: filename.extension
-	newfileName := fs.newFileName(createTime)
-	newfilePath := filepath.Join(fs.Path, newfileName)
+	// New file name: filename.extension
+	// Old file name: filename-timestamp.extension
+	newfilePath := filepath.Join(fs.Path, fs.FileName)
 
 	var err error
 	fs.f, err = os.OpenFile(newfilePath, os.O_APPEND|os.O_WRONLY|os.O_CREATE, mode)
@@ -198,8 +196,18 @@ func (fs *FileSink) rotate() error {
 		((elapsed > fs.MaxDuration) && (fs.MaxDuration > 0)) {
 
 		fs.f.Close()
+
+		// Move current log file to a timestamped file.
+		rotateTime := time.Now().UnixNano()
+		rotateFileName := fmt.Sprintf(fs.fileNamePattern(), strconv.FormatInt(rotateTime, 10))
+		oldPath := filepath.Join(fs.Path, fs.FileName)
+		newPath := filepath.Join(fs.Path, rotateFileName)
+		if err := os.Rename(oldPath, newPath); err != nil {
+			return fmt.Errorf("failed to rotate log file: %v", err)
+		}
+
 		if err := fs.pruneFiles(); err != nil {
-			return err
+			return fmt.Errorf("failed to prune log files: %w", err)
 		}
 		return fs.open()
 	}

file_sink_test.go

@@ -142,9 +142,25 @@ func TestFileSink_TimeRotate(t *testing.T) {
 	}
 
 	want := 2
-	if got, _ := ioutil.ReadDir(tmpDir); len(got) != want {
+	got, err := os.ReadDir(tmpDir)
+	if err != nil {
+		t.Fatalf("Error listing log directory: %v", err)
+	}
+	if len(got) != want {
 		t.Errorf("Expected %d files, got %v file(s)", want, len(got))
 	}
+	found := false
+	names := []string{}
+	for _, entry := range got {
+		names = append(names, entry.Name())
+		if entry.Name() == fs.FileName {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Errorf("Did not find expected file %q -- found: %v", fs.FileName, names)
+	}
 }
 
 func TestFileSink_ByteRotate(t *testing.T) {
@@ -183,9 +199,25 @@ func TestFileSink_ByteRotate(t *testing.T) {
 	}
 
 	want := 2
-	if got, _ := ioutil.ReadDir(tmpDir); len(got) != want {
+	got, err := os.ReadDir(tmpDir)
+	if err != nil {
+		t.Fatalf("Error listing log directory: %v", err)
+	}
+	if len(got) != want {
 		t.Errorf("Expected %d files, got %v file(s)", want, len(got))
 	}
+	found := false
+	names := []string{}
+	for _, entry := range got {
+		names = append(names, entry.Name())
+		if entry.Name() == fs.FileName {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Errorf("Did not find expected file %q -- found: %v", fs.FileName, names)
+	}
 }
 
 func TestFileSink_open(t *testing.T) {