loki.source.file: refactor tail package

go.mod

@@ -305,7 +305,6 @@ require (
 	google.golang.org/api v0.254.0
 	google.golang.org/grpc v1.76.0
 	google.golang.org/protobuf v1.36.10
-	gopkg.in/tomb.v1 v1.0.0-20141024135613-dd632973f1e7
 	gopkg.in/yaml.v2 v2.4.0
 	gopkg.in/yaml.v3 v3.0.1
 	gotest.tools v2.2.0+incompatible

internal/component/loki/source/file/internal/tail/block.go

@@ -0,0 +1,117 @@
+package tail
+
+import (
+	"context"
+	"os"
+	"runtime"
+
+	"github.com/grafana/dskit/backoff"
+
+	"github.com/grafana/alloy/internal/component/loki/source/file/internal/tail/fileext"
+)
+
+// blockUntilExists blocks until the file specified in cfg exists or the context is canceled.
+// It polls the file system at intervals defined by WatcherConfig polling frequencies.
+// Returns an error if the context is canceled or an unrecoverable error occurs.
+func blockUntilExists(ctx context.Context, cfg *Config) error {
+	backoff := backoff.New(ctx, backoff.Config{
+		MinBackoff: cfg.WatcherConfig.MinPollFrequency,
+		MaxBackoff: cfg.WatcherConfig.MaxPollFrequency,
+	})
+
+	for backoff.Ongoing() {
+		if _, err := os.Stat(cfg.Filename); err == nil {
+			return nil
+		} else if !os.IsNotExist(err) {
+			return err
+		}
+		backoff.Wait()
+	}
+
+	return backoff.Err()
+}
+
+// event represents a file system event detected during polling.
+type event int
+
+const (
+	eventNone      event = iota // no event detected
+	eventTruncated              // file was truncated (size decreased)
+	eventModified               // file was modified (size increased or modification time changed)
+	eventDeleted                // file was deleted, moved, or renamed
+)
+
+// blockUntilEvent blocks until it detects a file system event for the given file or the context is canceled.
+// It polls the file system to detect modifications, truncations, deletions, or renames.
+// The pos parameter is the current file position and is used to detect truncation events.
+// Returns the detected event type and any error encountered. Returns eventNone if the context is canceled.
+func blockUntilEvent(ctx context.Context, f *os.File, prevSize int64, cfg *Config) (event, error) {
+	// NOTE: it is important that we stat the open file here. Later we do os.Stat(cfg.Filename)
+	// and use os.IsSameFile to detect if file was rotated.
+	origFi, err := f.Stat()
+	if err != nil {
+		// If file no longer exists we treat it as a delete event.
+		if os.IsNotExist(err) {
+			return eventDeleted, nil
+		}
+		return eventNone, err
+	}
+
+	backoff := backoff.New(ctx, backoff.Config{
+		MinBackoff: cfg.WatcherConfig.MinPollFrequency,
+		MaxBackoff: cfg.WatcherConfig.MaxPollFrequency,
+	})
+
+	prevModTime := origFi.ModTime()
+
+	for backoff.Ongoing() {
+		deletePending, err := fileext.IsDeletePending(f)
+
+		// DeletePending is a windows state where the file has been queued
+		// for delete but won't actually get deleted until all handles are
+		// closed. It's a variation on the NotifyDeleted call below.
+		//
+		// IsDeletePending may fail in cases where the file handle becomes
+		// invalid, so we treat a failed call the same as a pending delete.
+		if err != nil || deletePending {
+			return eventDeleted, nil
+		}
+
+		fi, err := os.Stat(cfg.Filename)
+		if err != nil {
+			// Windows cannot delete a file if a handle is still open (tail keeps one open)
+			// so it gives access denied to anything trying to read it until all handles are released.
+			if os.IsNotExist(err) || (runtime.GOOS == "windows" && os.IsPermission(err)) {
+				// File does not exist (has been deleted).
+				return eventDeleted, nil
+			}
+			return eventNone, err
+		}
+
+		// File got moved/renamed?
+		if !os.SameFile(origFi, fi) {
+			return eventDeleted, nil
+		}
+
+		// File got truncated?
+		currentSize := fi.Size()
+		if prevSize > 0 && prevSize > currentSize {
+			return eventTruncated, nil
+		}
+
+		// File got bigger?
+		if prevSize < currentSize {
+			return eventModified, nil
+		}
+
+		// File was appended to (changed)?
+		if fi.ModTime() != prevModTime {
+			return eventModified, nil
+		}
+
+		// File hasn't changed so wait until next retry.
+		backoff.Wait()
+	}
+
+	return eventNone, backoff.Err()
+}

internal/component/loki/source/file/internal/tail/block_test.go

@@ -0,0 +1,189 @@
+package tail
+
+import (
+	"context"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/grafana/alloy/internal/component/loki/source/file/internal/tail/fileext"
+)
+
+func TestBlockUntilExists(t *testing.T) {
+	watcherConfig := WatcherConfig{
+		MinPollFrequency: 5 * time.Millisecond,
+		MaxPollFrequency: 5 * time.Millisecond,
+	}
+
+	t.Run("should block until file exists", func(t *testing.T) {
+		filename := filepath.Join(t.TempDir(), "eventually")
+
+		go func() {
+			time.Sleep(10 * time.Millisecond)
+			createFileWithPath(t, filename, "")
+		}()
+
+		err := blockUntilExists(context.Background(), &Config{
+			Filename:      filename,
+			WatcherConfig: watcherConfig,
+		})
+		require.NoError(t, err)
+	})
+
+	t.Run("should exit when context is canceled", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+		go func() {
+			time.Sleep(10 * time.Millisecond)
+			cancel()
+		}()
+
+		err := blockUntilExists(ctx, &Config{
+			Filename:      filepath.Join(t.TempDir(), "never"),
+			WatcherConfig: watcherConfig,
+		})
+		require.ErrorIs(t, err, context.Canceled)
+	})
+}
+
+func TestBlockUntilEvent(t *testing.T) {
+	watcherConfig := WatcherConfig{
+		MinPollFrequency: 5 * time.Millisecond,
+		MaxPollFrequency: 5 * time.Millisecond,
+	}
+
+	t.Run("should return modified event when file is written to", func(t *testing.T) {
+		f := createEmptyFile(t, "startempty")
+		defer f.Close()
+
+		go func() {
+			time.Sleep(10 * time.Millisecond)
+			_, err := f.WriteString("updated")
+			require.NoError(t, err)
+		}()
+
+		event, err := blockUntilEvent(context.Background(), f, 0, &Config{
+			Filename:      f.Name(),
+			WatcherConfig: watcherConfig,
+		})
+		require.NoError(t, err)
+		require.Equal(t, eventModified, event)
+	})
+
+	t.Run("should return modified event if mod time is updated", func(t *testing.T) {
+		f := createEmptyFile(t, "startempty")
+		defer f.Close()
+
+		go func() {
+			time.Sleep(10 * time.Millisecond)
+			require.NoError(t, os.Chtimes(f.Name(), time.Now(), time.Now()))
+		}()
+
+		event, err := blockUntilEvent(context.Background(), f, 0, &Config{
+			Filename:      f.Name(),
+			WatcherConfig: watcherConfig,
+		})
+		require.NoError(t, err)
+		require.Equal(t, eventModified, event)
+	})
+
+	t.Run("should return deleted event if file is deleted", func(t *testing.T) {
+		f := createEmptyFile(t, "startempty")
+		require.NoError(t, f.Close())
+
+		// NOTE: important for windows that we open with correct flags.
+		f, err := fileext.OpenFile(f.Name())
+		require.NoError(t, err)
+		defer f.Close()
+
+		go func() {
+			time.Sleep(10 * time.Millisecond)
+			removeFile(t, f.Name())
+		}()
+
+		event, err := blockUntilEvent(context.Background(), f, 0, &Config{
+			Filename:      f.Name(),
+			WatcherConfig: watcherConfig,
+		})
+		require.NoError(t, err)
+		require.Equal(t, eventDeleted, event)
+	})
+
+	t.Run("should return deleted event if file is deleted before", func(t *testing.T) {
+		f := createEmptyFile(t, "startempty")
+		require.NoError(t, f.Close())
+
+		// NOTE: important for windows that we open with correct flags.
+		f, err := fileext.OpenFile(f.Name())
+		require.NoError(t, err)
+		defer f.Close()
+
+		removeFile(t, f.Name())
+
+		event, err := blockUntilEvent(context.Background(), f, 0, &Config{
+			Filename:      f.Name(),
+			WatcherConfig: watcherConfig,
+		})
+		require.NoError(t, err)
+		require.Equal(t, eventDeleted, event)
+	})
+
+	t.Run("should return truncated event", func(t *testing.T) {
+		f := createFileWithContent(t, "truncate", "content")
+		defer f.Close()
+
+		offset, err := f.Seek(0, io.SeekEnd)
+		require.NoError(t, err)
+
+		go func() {
+			time.Sleep(10 * time.Millisecond)
+			require.NoError(t, f.Truncate(0))
+		}()
+
+		event, err := blockUntilEvent(context.Background(), f, offset, &Config{
+			Filename:      f.Name(),
+			WatcherConfig: watcherConfig,
+		})
+		require.NoError(t, err)
+		require.Equal(t, eventTruncated, event)
+	})
+
+	t.Run("should exit when context is canceled", func(t *testing.T) {
+		f := createEmptyFile(t, "startempty")
+		defer f.Close()
+
+		ctx, cancel := context.WithCancel(context.Background())
+		go func() {
+			time.Sleep(10 * time.Millisecond)
+			cancel()
+		}()
+
+		event, err := blockUntilEvent(ctx, f, 0, &Config{
+			Filename:      f.Name(),
+			WatcherConfig: watcherConfig,
+		})
+		require.ErrorIs(t, err, context.Canceled)
+		require.Equal(t, eventNone, event)
+	})
+}
+
+func createEmptyFile(t *testing.T, name string) *os.File {
+	path := filepath.Join(t.TempDir(), name)
+	f, err := os.Create(path)
+	require.NoError(t, err)
+	return f
+}
+
+func createFileWithContent(t *testing.T, name, content string) *os.File {
+	path := createFile(t, name, content)
+	f, err := os.OpenFile(path, os.O_RDWR, 0)
+	require.NoError(t, err)
+	return f
+}
+
+func createFileWithPath(t *testing.T, path, content string) {
+	require.NoError(t, os.WriteFile(path, []byte(content), 0600))
+}

internal/component/loki/source/file/internal/tail/config.go

@@ -0,0 +1,41 @@
+package tail
+
+import (
+	"time"
+
+	"golang.org/x/text/encoding"
+)
+
+// Config holds configuration for tailing a file.
+type Config struct {
+	// Filename is the path to the file to tail.
+	Filename string
+	// Offset is the byte offset in the file where tailing should start.
+	// If 0, tailing starts from the beginning of the file.
+	Offset int64
+
+	// Decoder is an optional text decoder for non-UTF-8 encoded files.
+	// If the file is not UTF-8, the tailer must use the correct decoder
+	// or the output text may be corrupted. For example, if the file is
+	// "UTF-16 LE" encoded, the tailer would not separate new lines properly
+	// and the output could appear as garbled characters.
+	Decoder *encoding.Decoder
+
+	// WatcherConfig controls how the file system is polled for changes.
+	WatcherConfig WatcherConfig
+}
+
+// WatcherConfig controls the polling behavior for detecting file system events.
+type WatcherConfig struct {
+	// MinPollFrequency and MaxPollFrequency specify the polling frequency range
+	// for detecting file system events. The actual polling frequency will vary
+	// within this range based on backoff behavior.
+	MinPollFrequency, MaxPollFrequency time.Duration
+}
+
+// defaultWatcherConfig holds the default polling configuration used when
+// WatcherConfig is not explicitly provided in Config.
+var defaultWatcherConfig = WatcherConfig{
+	MinPollFrequency: 250 * time.Millisecond,
+	MaxPollFrequency: 250 * time.Millisecond,
+}