Merge branch 'bw/mingw-avoid-inheriting-fd-to-lockfile' into maint

The tempfile (hence its user lockfile) API lets the caller to open
a file descriptor to a temporary file, write into it and then
finalize it by first closing the filehandle and then either
removing or renaming the temporary file.  When the process spawns a
subprocess after obtaining the file descriptor, and if the
subprocess has not exited when the attempt to remove or rename is
made, the last step fails on Windows, because the subprocess has
the file descriptor still open.  Open tempfile with O_CLOEXEC flag
to avoid this (on Windows, this is mapped to O_NOINHERIT).

* bw/mingw-avoid-inheriting-fd-to-lockfile:
  mingw: ensure temporary file handles are not inherited by child processes
  t6026-merge-attr: child processes must not inherit index.lock handles

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

git-compat-util.h

@@ -650,6 +650,10 @@ void *gitmemmem(const void *haystack, size_t haystacklen,
 #define getpagesize() sysconf(_SC_PAGESIZE)
 #endif
 
+#ifndef O_CLOEXEC
+#define O_CLOEXEC 0
+#endif
+
 #ifdef FREAD_READS_DIRECTORIES
 #ifdef fopen
 #undef fopen

lockfile.h

@@ -55,6 +55,10 @@
  *   * calling `fdopen_lock_file()` to get a `FILE` pointer for the
  *     open file and writing to the file using stdio.
  *
+ *   Note that the file descriptor returned by hold_lock_file_for_update()
+ *   is marked O_CLOEXEC, so the new contents must be written by the
+ *   current process, not a spawned one.
+ *
  * When finished writing, the caller can:
  *
  * * Close the file descriptor and rename the lockfile to its final

t/t6026-merge-attr.sh

@@ -181,4 +181,17 @@ test_expect_success 'up-to-date merge without common ancestor' '
 	)
 '
 
+test_expect_success 'custom merge does not lock index' '
+	git reset --hard anchor &&
+	write_script sleep-one-second.sh <<-\EOF &&
+		sleep 1 &
+	EOF
+
+	test_write_lines >.gitattributes \
+		"* merge=ours" "text merge=sleep-one-second" &&
+	test_config merge.ours.driver true &&
+	test_config merge.sleep-one-second.driver ./sleep-one-second.sh &&
+	git merge master
+'
+
 test_done

tempfile.c

@@ -120,7 +120,12 @@ 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 (O_CLOEXEC && tempfile->fd < 0 && errno == EINVAL)
+		/* Try again w/o O_CLOEXEC: the kernel might not support it */
+		tempfile->fd = open(tempfile->filename.buf,
+				    O_RDWR | O_CREAT | O_EXCL, 0666);
 	if (tempfile->fd < 0) {
 		strbuf_reset(&tempfile->filename);
 		return -1;

tempfile.h

@@ -33,6 +33,10 @@
  *   * calling `fdopen_tempfile()` to get a `FILE` pointer for the
  *     open file and writing to the file using stdio.
  *
+ *   Note that the file descriptor returned by create_tempfile()
+ *   is marked O_CLOEXEC, so the new contents must be written by
+ *   the current process, not any spawned one.
+ *
  * When finished writing, the caller can:
  *
  * * Close the file descriptor and remove the temporary file by