From af030f74f5c9c49d337ad768b307e50d3cfd60fe Mon Sep 17 00:00:00 2001
From: Nick Craig-Wood <nick@craig-wood.com>
Date: Fri, 10 May 2019 13:22:55 +0100
Subject: [PATCH] vfs: make WriteAt for non cached files work with
 non-sequential writes

This makes WriteAt for non cached files wait a short time if it gets
an out of order write (which would normally cause an error) to see if
the gap will be filled with an in order write.

The makes the SFTP backend work fine with --vfs-cache-mode off
---
 vfs/write.go | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/vfs/write.go b/vfs/write.go
index fd4c2cf11..4cf5df9b2 100644
--- a/vfs/write.go
+++ b/vfs/write.go
@@ -121,11 +121,20 @@ func (fh *WriteFileHandle) WriteAt(p []byte, off int64) (n int, err error) {
 
 // Implementatino of WriteAt - call with lock held
 func (fh *WriteFileHandle) writeAt(p []byte, off int64) (n int, err error) {
-	// fs.Debugf(fh.remote, "WriteFileHandle.Write len=%d", len(p))
+	// defer log.Trace(fh.remote, "len=%d off=%d", len(p), off)("n=%d, fh.off=%d, err=%v", &n, &fh.offset, &err)
 	if fh.closed {
 		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 {
 		fs.Errorf(fh.remote, "WriteFileHandle.Write: can't seek in file without --vfs-cache-mode >= writes")
 		return 0, ESPIPE