MDEV-10907 MTR and server writes can interleave in the error log

Ensure atomic appends to the error log by using CreateFile with
FILE_APPEND_DATA flag to open error log file (both MTR and server)

1 files changed, 3 insertions, 3 deletions

diff --git a/mysys/my_fopen.c b/mysys/my_fopen.c
index 52f61649bb3..cc1019365ac 100644
--- a/mysys/my_fopen.c
+++ b/mysys/my_fopen.c
@@ -102,6 +102,7 @@ static FILE *my_win_freopen(const char *path, const char *mode, FILE *stream)
   HANDLE osfh;
 
   DBUG_ASSERT(path && stream);
+  DBUG_ASSERT(strchr(mode, 'a')); /* We use FILE_APPEND_DATA below */
 
   /* Services don't have stdout/stderr on Windows, so _fileno returns -1. */
   if (fd < 0)
@@ -112,15 +113,14 @@ static FILE *my_win_freopen(const char *path, const char *mode, FILE *stream)
     fd= _fileno(stream);
   }
 
-  if ((osfh= CreateFile(path, GENERIC_READ | GENERIC_WRITE,
+  if ((osfh= CreateFile(path, GENERIC_READ | FILE_APPEND_DATA,
                         FILE_SHARE_READ | FILE_SHARE_WRITE |
                         FILE_SHARE_DELETE, NULL,
                         OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL,
                         NULL)) == INVALID_HANDLE_VALUE)
     return NULL;
 
-  if ((handle_fd= _open_osfhandle((intptr_t)osfh,
-                                  _O_APPEND | _O_TEXT)) == -1)
+  if ((handle_fd= _open_osfhandle((intptr_t)osfh, _O_TEXT)) == -1)
   {
     CloseHandle(osfh);
     return NULL;