vfs: make write without cache more efficient

This updates the out of sequence write code to be more efficient using
a conditional lock with a timeout.

vfs/write.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"os"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/rclone/rclone/fs"
@@ -15,7 +16,8 @@ import (
 type WriteFileHandle struct {
 	baseHandle
 	mu          sync.Mutex
-	closed      bool // set if handle has been closed
+	cond        *sync.Cond // cond lock for out of sequence writes
+	closed      bool       // set if handle has been closed
 	remote      string
 	pipeWriter  *io.PipeWriter
 	o           fs.Object
@@ -42,6 +44,7 @@ func newWriteFileHandle(d *Dir, f *File, remote string, flags int) (*WriteFileHa
 		result: make(chan error, 1),
 		file:   f,
 	}
+	fh.cond = sync.NewCond(&fh.mu)
 	fh.file.addWriter(fh)
 	return fh, nil
 }
@@ -127,14 +130,28 @@ func (fh *WriteFileHandle) writeAt(p []byte, off int64) (n int, err error) {
 		fs.Errorf(fh.remote, "WriteFileHandle.Write: error: %v", EBADF)
 		return 0, ECLOSED
 	}
-	// Wait a short time for sequential writes to appear
-	const maxTries = 1000
-	const sleepTime = 1 * time.Millisecond
-	for try := 1; fh.offset != off && try <= maxTries; try++ {
-		//fs.Debugf(fh.remote, "waiting for in sequence write %d/%d", try, maxTries)
-		fh.mu.Unlock()
-		time.Sleep(sleepTime)
-		fh.mu.Lock()
+	if fh.offset != off {
+		// Set a background timer so we don't wait forever
+		timeout := time.NewTimer(10 * time.Second)
+		done := make(chan struct{})
+		abort := int32(0)
+		go func() {
+			select {
+			case <-timeout.C:
+				// set abort flag an give all the waiting goroutines a kick on timeout
+				atomic.StoreInt32(&abort, 1)
+				fh.cond.Broadcast()
+			case <-done:
+			}
+		}()
+		// Wait for an in-sequence write or abort
+		for fh.offset != off && atomic.LoadInt32(&abort) == 0 {
+			// fs.Debugf(fh.remote, "waiting for in-sequence write to %d", off)
+			fh.cond.Wait()
+		}
+		// tidy up end timer
+		close(done)
+		timeout.Stop()
 	}
 	if fh.offset != off {
 		fs.Errorf(fh.remote, "WriteFileHandle.Write: can't seek in file without --vfs-cache-mode >= writes")
@@ -152,6 +169,7 @@ func (fh *WriteFileHandle) writeAt(p []byte, off int64) (n int, err error) {
 		return 0, err
 	}
 	// fs.Debugf(fh.remote, "WriteFileHandle.Write OK (%d bytes written)", n)
+	fh.cond.Broadcast() // wake everyone up waiting for an in-sequence read
 	return n, nil
 }