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:
- `FILE_SHARE_READ` allows a concurrent process to open the file for
reading.
- `FILE_SHARE_WRITE` allows a concurrent process to open the file for
writing.
- `FILE_SHARE_DELETE` allows a concurrent process to delete the file
or to replace it via an atomic rename.
This sharing mechanism is quite important in the context of Git, as we
assume POSIX semantics all over the place. But there are two callsites
where we don't pass all three of these flags:
- 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. The
function was introduced via d641097589 (mingw: enable atomic
O_APPEND, 2018-08-13) and has been using `FILE_SHARE_READ |
FILE_SHARE_WRITE` since the inception. There aren't any indicators
that the omission of `FILE_SHARE_DELETE` was intentional.
- We don't set any sharing flags in `mingw_utime()`, which changes the
access and modification of a file. 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.
`mingw_utime()` was originally implemented via `_wopen()`, which
doesn't give you full control over the sharing mode. Instead, it
calls `_wsopen()` with `_SH_DENYNO`, which ultimately translates to
`FILE_SHARE_READ | FILE_SHARE_WRITE`. It was then refactored via
090a3085bc (t/helper/test-chmtime: update mingw to support chmtime
on directories, 2022-03-02) to use `CreateFileW()`, but we stopped
setting any sharing flags at all, which seems like an unintentional
side effect. By restoring `FILE_SHARE_READ | FILE_SHARE_WRITE` we
thus fix this and get back the old behaviour of `_wopen()`.
The fact that we didn't set the equivalent of `FILE_SHARE_DELETE`
can be explained, as well: neither `_wopen()` nor `_wsopen()` allow
you to do so. So overall, it doesn't seem intentional that we didn't
allow deletions here, either.
Adapt both of these callsites to pass all three sharing flags.
Signed-off-by: Patrick Steinhardt <ps@xxxxxx>
---
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
