[PATCH 1/3] compat/mingw: share file handles created via `CreateFileW()`

[Patrick Steinhardt <ps@pks.im>]

Unless told otherwise, Windows will keep other processes from reading,
writing and deleting files when one has an open handle that was created
via `CreateFileW()`. This behaviour can be altered via `FILE_SHARE_*`
flags such that other processes _can_ read and/or modify such a file.
This sharing mechanism is quite important in the context of Git, as we
assume POSIX semantics all over the place.

There are two calls where we don't set up those flags though:

  - We don't set `FILE_SHARE_DELETE` when creating a file for appending
    via `mingw_open_append()`. This makes it impossible to delete the
    file from another process or to replace it via an atomic rename.

  - When opening a file such that we can change its access/modification
    times. This makes it impossible to perform any kind of operation
    on this file at all from another process. While we only open the
    file for a short amount of time to update its timestamps, this still
    opens us up for a race condition with another process.

Adapt both of these callsites to pass all three sharing flags.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 compat/mingw.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/compat/mingw.c b/compat/mingw.c
index 0e851ecae29..e326c6fcd2d 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -502,7 +502,7 @@ static int mingw_open_append(wchar_t const *wfilename, int oflags, ...)
 	 * to append to the file.
 	 */
 	handle = CreateFileW(wfilename, FILE_APPEND_DATA,
-			FILE_SHARE_WRITE | FILE_SHARE_READ,
+			FILE_SHARE_WRITE | FILE_SHARE_READ | FILE_SHARE_DELETE,
 			NULL, create, FILE_ATTRIBUTE_NORMAL, NULL);
 	if (handle == INVALID_HANDLE_VALUE) {
 		DWORD err = GetLastError();
@@ -1006,7 +1006,7 @@ int mingw_utime (const char *file_name, const struct utimbuf *times)
 
 	osfilehandle = CreateFileW(wfilename,
 				   FILE_WRITE_ATTRIBUTES,
-				   0 /*FileShare.None*/,
+				   FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
 				   NULL,
 				   OPEN_EXISTING,
 				   (attrs != INVALID_FILE_ATTRIBUTES &&
-- 
2.47.0.118.gfd3785337b.dirty