[PATCH 2/2] mingw: ensure temporary file handles are not inherited by child processes

[Johannes Schindelin <johannes.schindelin@gmx.de>]

From: Ben Wijen <ben@wijen.net>

When the index is locked and child processes inherit the handle to
said lock and the parent process wants to remove the lock before the
child process exits, on Windows there is a problem: it won't work
because files cannot be deleted if a process holds a handle on them.
The symptom:

    Rename from 'xxx/.git/index.lock' to 'xxx/.git/index' failed.
    Should I try again? (y/n)

Spawning child processes with bInheritHandles==FALSE would not work
because no file handles would be inherited, not even the hStdXxx
handles in STARTUPINFO (stdin/stdout/stderr).

Opening every file with O_NOINHERIT does not work, either, as e.g.
git-upload-pack expects inherited file handles.

This leaves us with the only way out: creating temp files with the
O_NOINHERIT flag. This flag is Windows-specific, however. For our
purposes, it is equivalent our purposes) to O_CLOEXEC (which does not
exist on Windows), so let's just open temporary files with the
O_CLOEXEC flag and map that flag to O_NOINHERIT on Windows.

This fixes the test that we just introduced to demonstrate the problem.

Signed-off-by: Ben Wijen <ben@wijen.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 compat/mingw.h        | 4 ++++
 t/t6026-merge-attr.sh | 2 +-
 tempfile.c            | 2 +-
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/compat/mingw.h b/compat/mingw.h
index 95e128f..753e641 100644
--- a/compat/mingw.h
+++ b/compat/mingw.h
@@ -67,6 +67,10 @@ typedef int pid_t;
 #define F_SETFD 2
 #define FD_CLOEXEC 0x1
 
+#if !defined O_CLOEXEC && defined O_NOINHERIT
+#define O_CLOEXEC	O_NOINHERIT
+#endif
+
 #ifndef EAFNOSUPPORT
 #define EAFNOSUPPORT WSAEAFNOSUPPORT
 #endif
diff --git a/t/t6026-merge-attr.sh b/t/t6026-merge-attr.sh
index 3d28c78..dd8f88d 100755
--- a/t/t6026-merge-attr.sh
+++ b/t/t6026-merge-attr.sh
@@ -181,7 +181,7 @@ test_expect_success 'up-to-date merge without common ancestor' '
 	)
 '
 
-test_expect_success !MINGW 'custom merge does not lock index' '
+test_expect_success 'custom merge does not lock index' '
 	git reset --hard anchor &&
 	write_script sleep-one-second.sh <<-\EOF &&
 		sleep 1 &
diff --git a/tempfile.c b/tempfile.c
index 0af7ebf..db3981d 100644
--- a/tempfile.c
+++ b/tempfile.c
@@ -120,7 +120,7 @@ int create_tempfile(struct tempfile *tempfile, const char *path)
 	prepare_tempfile_object(tempfile);
 
 	strbuf_add_absolute_path(&tempfile->filename, path);
-	tempfile->fd = open(tempfile->filename.buf, O_RDWR | O_CREAT | O_EXCL, 0666);
+	tempfile->fd = open(tempfile->filename.buf, O_RDWR | O_CREAT | O_EXCL | O_CLOEXEC, 0666);
 	if (tempfile->fd < 0) {
 		strbuf_reset(&tempfile->filename);
 		return -1;
-- 
2.9.2.691.g78954f3