fix #15148 (#15149)

* fix #15148

Co-authored-by: alaviss <leorize+oss@disroot.org>

lib/pure/asyncfile.nim

@@ -379,7 +379,7 @@ proc writeBuffer*(f: AsyncFile, buf: pointer, size: int): Future[void] =
 
     proc cb(fd: AsyncFD): bool =
       result = true
-      let remainderSize = size-written
+      let remainderSize = size - written
       var cbuf = cast[cstring](buf)
       let res = write(fd.cint, addr cbuf[written], remainderSize.cint)
       if res < 0:
@@ -409,7 +409,7 @@ proc write*(f: AsyncFile, data: string): Future[void] =
   var copy = data
   when defined(windows) or defined(nimdoc):
     var buffer = alloc0(data.len)
-    copyMem(buffer, addr copy[0], data.len)
+    copyMem(buffer, copy.cstring, data.len)
 
     var ol = newCustom()
     ol.data = CompletionData(fd: f.fd, cb:
@@ -454,8 +454,15 @@ proc write*(f: AsyncFile, data: string): Future[void] =
 
     proc cb(fd: AsyncFD): bool =
       result = true
-      let remainderSize = data.len-written
-      let res = write(fd.cint, addr copy[written], remainderSize.cint)
+
+      let remainderSize = data.len - written
+
+      let res =
+        if data.len == 0:
+          write(fd.cint, copy.cstring, 0)
+        else:
+          write(fd.cint, addr copy[written], remainderSize.cint)
+
       if res < 0:
         let lastError = osLastError()
         if lastError.int32 != EAGAIN:

tests/async/t15148.nim

@@ -0,0 +1,12 @@
+import asyncdispatch, asyncfile, os
+
+const Filename = "t15148.txt"
+
+proc saveEmpty() {.async.} =
+  let
+    text = ""
+    file = openAsync(Filename, fmWrite)
+  await file.write(text)
+  file.close()
+
+waitFor saveEmpty()