[PATCH v2 25/39] chardev/char-file: Add FILE_SHARE_WRITE when openning the file for win32

[Bin Meng]

From: Xuzhou Cheng <xuzhou.cheng@windriver.com>

The combination of GENERIC_WRITE and FILE_SHARE_READ options does
not allow the same file to be opened again by CreateFile() from
another QEMU process with the same options when the previous QEMU
process still holds the file handle opened.

This was triggered by running the test_multifd_tcp_cancel() case on
Windows, which cancels the migration, and launches another QEMU
process to migrate with the same file opened for write. Chances are
that the previous QEMU process does not quit before the new QEMU
process runs hence the old one still holds the file handle that does
not allow shared write permission then the new QEMU process will fail.

As per [1] we should add FILE_SHARE_WRITE to the share mode to allow
such use case. This change makes the behavior be consistent with the
POSIX platforms.

[1] 
https://docs.microsoft.com/en-us/windows/win32/fileio/creating-and-opening-files

Signed-off-by: Xuzhou Cheng <xuzhou.cheng@windriver.com>
Signed-off-by: Bin Meng <bin.meng@windriver.com>
---

Changes in v2:
- Update commit message to include the use case why we should set
  FILE_SHARE_WRITE when openning the file for win32

 chardev/char-file.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/chardev/char-file.c b/chardev/char-file.c
index 2fd80707e5..66385211eb 100644
--- a/chardev/char-file.c
+++ b/chardev/char-file.c
@@ -60,8 +60,8 @@ static void qmp_chardev_open_file(Chardev *chr,
         flags = CREATE_ALWAYS;
     }
 
-    out = CreateFile(file->out, accessmode, FILE_SHARE_READ, NULL, flags,
-                     FILE_ATTRIBUTE_NORMAL, NULL);
+    out = CreateFile(file->out, accessmode, FILE_SHARE_READ | FILE_SHARE_WRITE,
+                     NULL, flags, FILE_ATTRIBUTE_NORMAL, NULL);
     if (out == INVALID_HANDLE_VALUE) {
         error_setg(errp, "open %s failed", file->out);
         return;
-- 
2.34.1