[PATCH v3 42/54] chardev/char-file: Add FILE_SHARE_WRITE when opening 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.

There is another test case boot-serial-test that triggers the same
issue. The qtest executable created a serial chardev file to be
passed to the QEMU executable. The serial file was created by
g_file_open_tmp(), which internally opens the file with
FILE_SHARE_WRITE security attribute, and based on [1], there is
only one case that allows the first call to CreateFile() with
GENERIC_READ & FILE_SHARE_WRITE, and second call to CreateFile()
with GENERIC_WRITE & FILE_SHARE_READ. All other combinations
require FILE_SHARE_WRITE in the second call. But there is no way
for the second call (in this case the QEMU executable) to know
what combination was passed to the first call, so we will have to
add FILE_SHARE_WRITE to the second call.

For both scenarios we should add FILE_SHARE_WRITE in the chardev
file backend driver. This change also makes the behavior to 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 v3:
- Add another case "boot-serial-test" to justify the change

Changes in v2:
- Update commit message to include the use case why we should set
  FILE_SHARE_WRITE when opening 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