Add SyncOpen config option

tail.go

@@ -73,6 +73,7 @@ type Config struct {
 	Location  *SeekInfo // Tail from this location. If nil, start at the beginning of the file
 	ReOpen    bool      // Reopen recreated files (tail -F)
 	MustExist bool      // Fail early if the file does not exist
+	SyncOpen  bool      // Do any seek operation before returning from TailFile.
 	Poll      bool      // Poll for file changes instead of using the default inotify
 	Pipe      bool      // The file is a named pipe (mkfifo)
 
@@ -146,11 +147,24 @@ func TailFile(filename string, config Config) (*Tail, error) {
 		t.watcher = watch.NewInotifyFileWatcher(filename)
 	}
 
-	if t.MustExist {
+	if t.MustExist || t.SyncOpen {
 		var err error
 		t.file, err = OpenFile(t.Filename)
-		if err != nil {
-			return nil, err
+		if t.MustExist {
+			if err != nil {
+				return nil, err
+			}
+		}
+		if t.SyncOpen && err == nil && t.Location != nil {
+			// Do any required seek operation before returning from TailFile.
+			//
+			// If the file didn't already exist, it's reasonable to assume that
+			// we shouldn't seek later when it opens, because its size when we
+			// first tried to open it was zero, so not seeking at all is fine.
+			_, err = t.file.Seek(t.Location.Offset, t.Location.Whence)
+			if err != nil {
+				return nil, err
+			}
 		}
 	}
 
@@ -283,7 +297,7 @@ func (tail *Tail) tailFileSync() {
 	}
 
 	// Seek to requested location on first open of the file.
-	if tail.Location != nil {
+	if tail.Location != nil && !tail.SyncOpen {
 		_, err := tail.file.Seek(tail.Location.Offset, tail.Location.Whence)
 		if err != nil {
 			tail.Killf("Seek error on %s: %s", tail.Filename, err)

tail_test.go

@@ -66,6 +66,23 @@ func TestMustExist(t *testing.T) {
 	tail.Cleanup()
 }
 
+// This test is not very robust because its purpose is to test that a race
+// condition doesn't exist. As such, the failure pattern of this test would
+// be flaky failure, not necessarily easily reproduced failure.
+//
+// Running this same test *without* SyncOpen: true, alongside
+// stress --hdd 8 (to make the filesystem very busy), will fail most
+// of the time. (It fails significantly even without stress.)
+func TestSeeksSynchronously(t *testing.T) {
+	tailTest, cleanup := NewTailTest("seeks-synchronously", t)
+	defer cleanup()
+	tailTest.CreateFile("test.txt", "hello\nworld\n")
+	tail := tailTest.StartTail("test.txt", Config{SyncOpen: true, Location: &SeekInfo{0, io.SeekEnd}})
+	go tailTest.VerifyTailOutput(tail, []string{"more", "data"}, false)
+	tailTest.AppendToFile("test.txt", "more\ndata\n")
+	tailTest.Cleanup(tail, true)
+}
+
 func TestWaitsForFileToExist(t *testing.T) {
 	tailTest, cleanup := NewTailTest("waits-for-file-to-exist", t)
 	defer cleanup()