[PATCH v3 0/2] Do not lock temporary files via child processes on Windows

This issue was originally reported and fixed in https://github.com/git-for-windows/git/pull/755

The problem is that file handles to temporary files (such as index.lock) were inherited by spawned processes. If those spawned processes do not exit before the parent process wants to delete or rename them, we are in big trouble.

The original use case triggering the bug is a merge driver that does not quit, but listen to subsequent merge requests.

However, the same issue turned up in Lars Schneider's work on making clean/smudge filters batchable (i.e. more efficient by avoiding possibly thousands of child processes, one per file).

Changes since v2:

- O_CLOEXEC is defined in git-compat-util.h unless already defined
- we now handle EINVAL by trying again without O_CLOEXEC

Ben Wijen (2):
  t6026-merge-attr: child processes must not inherit index.lock handles
  mingw: ensure temporary file handles are not inherited by child
    processes

 compat/mingw.h        |  4 ++++
 git-compat-util.h     |  4 ++++
 lockfile.h            |  4 ++++
 t/t6026-merge-attr.sh | 13 +++++++++++++
 tempfile.c            |  7 ++++++-
 tempfile.h            |  4 ++++
 6 files changed, 35 insertions(+), 1 deletion(-)

Published-As: https://github.com/dscho/git/releases/tag/mingw-index-lock-v3
Fetch-It-Via: git fetch https://github.com/dscho/git mingw-index-lock-v3

Interdiff vs v2:

 diff --git a/git-compat-util.h b/git-compat-util.h
 index f52e00b..db89ba7 100644
 --- a/git-compat-util.h
 +++ b/git-compat-util.h
 @@ -667,6 +667,10 @@ 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
 diff --git a/tempfile.c b/tempfile.c
 index db3981d..2990c92 100644
 --- a/tempfile.c
 +++ b/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 | O_CLOEXEC, 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;