Fixes #5532 win async write (#5791)

* nimgrab tool bugfix: don't divide by zero
* fixes #5532 (asyncfile write on Windows)
* add a comment about what has been tried instead

1 files changed, 6 insertions, 4 deletions

diff --git a/lib/pure/asyncfile.nim b/lib/pure/asyncfile.nim
index 9bd060e30..8fb30075c 100644
--- a/lib/pure/asyncfile.nim
+++ b/lib/pure/asyncfile.nim
@@ -339,13 +339,17 @@ proc writeBuffer*(f: AsyncFile, buf: pointer, size: int): Future[void] =
         if not retFuture.finished:
           if errcode == OSErrorCode(-1):
             assert bytesCount == size.int32
-            f.offset.inc(size)
             retFuture.complete()
           else:
             retFuture.fail(newException(OSError, osErrorMsg(errcode)))
     )
+    # passing -1 here should work according to MSDN, but doesn't. For more
+    # information see
+    # http://stackoverflow.com/questions/33650899/does-asynchronous-file-
+    #   appending-in-windows-preserve-order
     ol.offset = DWord(f.offset and 0xffffffff)
     ol.offsetHigh = DWord(f.offset shr 32)
+    f.offset.inc(size)
 
     # According to MSDN we're supposed to pass nil to lpNumberOfBytesWritten.
     let ret = writeFile(f.fd.Handle, buf, size.int32, nil,
@@ -364,7 +368,6 @@ proc writeBuffer*(f: AsyncFile, buf: pointer, size: int): Future[void] =
         retFuture.fail(newException(OSError, osErrorMsg(osLastError())))
       else:
         assert bytesWritten == size.int32
-        f.offset.inc(size)
         retFuture.complete()
   else:
     var written = 0
@@ -410,7 +413,6 @@ proc write*(f: AsyncFile, data: string): Future[void] =
         if not retFuture.finished:
           if errcode == OSErrorCode(-1):
             assert bytesCount == data.len.int32
-            f.offset.inc(data.len)
             retFuture.complete()
           else:
             retFuture.fail(newException(OSError, osErrorMsg(errcode)))
@@ -420,6 +422,7 @@ proc write*(f: AsyncFile, data: string): Future[void] =
     )
     ol.offset = DWord(f.offset and 0xffffffff)
     ol.offsetHigh = DWord(f.offset shr 32)
+    f.offset.inc(data.len)
 
     # According to MSDN we're supposed to pass nil to lpNumberOfBytesWritten.
     let ret = writeFile(f.fd.Handle, buffer, data.len.int32, nil,
@@ -441,7 +444,6 @@ proc write*(f: AsyncFile, data: string): Future[void] =
         retFuture.fail(newException(OSError, osErrorMsg(osLastError())))
       else:
         assert bytesWritten == data.len.int32
-        f.offset.inc(data.len)
         retFuture.complete()
   else:
     var written = 0