Fix AsyncFile open flags. Fixes #5531

lib/pure/asyncfile.nim

@@ -50,22 +50,21 @@ when defined(windows) or defined(nimdoc):
     case mode
     of fmRead, fmReadWriteExisting:
       OPEN_EXISTING
-    of fmAppend, fmReadWrite, fmWrite:
-      if fileExists(filename):
-        OPEN_EXISTING
-      else:
-        CREATE_NEW
+    of fmReadWrite, fmWrite:
+      CREATE_ALWAYS
+    of fmAppend:
+      OPEN_ALWAYS
 else:
   proc getPosixFlags(mode: FileMode): cint =
     case mode
     of fmRead:
       result = O_RDONLY
     of fmWrite:
-      result = O_WRONLY or O_CREAT
+      result = O_WRONLY or O_CREAT or O_TRUNC
     of fmAppend:
       result = O_WRONLY or O_CREAT or O_APPEND
     of fmReadWrite:
-      result = O_RDWR or O_CREAT
+      result = O_RDWR or O_CREAT or O_TRUNC
     of fmReadWriteExisting:
       result = O_RDWR
     result = result or O_NONBLOCK

lib/windows/winlean.nim

@@ -666,6 +666,7 @@ const
   CREATE_ALWAYS* = 2'i32
   CREATE_NEW* = 1'i32
   OPEN_EXISTING* = 3'i32
+  OPEN_ALWAYS* = 4'i32
   FILE_BEGIN* = 0'i32
   INVALID_SET_FILE_POINTER* = -1'i32
   NO_ERROR* = 0'i32

tests/async/tasyncfile.nim

@@ -41,11 +41,11 @@ proc main() {.async.} =
     await file.write("test2")
     file.close()
     file = openAsync(fn, fmWrite)
-    await file.write("test3")
+    await file.write("t3")
     file.close()
     file = openAsync(fn, fmRead)
     let data = await file.readAll()
-    doAssert data == "test3"
+    doAssert data == "t3"
     file.close()