[PATCH v6 13/13] read-cache: make sure file handles are not inherited by child processes

[larsxschneider@gmail.com]

From: Lars Schneider <larsxschneider@gmail.com>

Consider the case of a file that requires filtering and is present in
branch A but not in branch B. If A is the current HEAD and we checkout B
then the following happens:

1. ce_compare_data() opens the file
2.   index_fd() detects that the file requires to run a clean filter and
     calls index_stream_convert_blob()
4.     index_stream_convert_blob() calls convert_to_git_filter_fd()
5.       convert_to_git_filter_fd() calls apply_filter() which creates a
         new long running filter process (in case it is the first file
         of this kind to be filtered)
6.       The new filter process inherits all file handles. This is the
         default on Linux/OSX and is explicitly defined in the
         `CreateProcessW` call in `mingw.c` on Windows.
7. ce_compare_data() closes the file
8. Git unlinks the file as it is not present in B

The unlink operation does not work on Windows because the filter process
has still an open handle to the file. Apparently that is no problem on
Linux/OSX. Probably because "[...] the two file descriptors share open
file status flags" (see fork(2)).

Fix this problem by opening files in read-cache with the `O_CLOEXEC`
flag to ensure that the file descriptor does not remain open in a newly
spawned process. `O_CLOEXEC` is defined as `O_NOINHERIT` on Windows. A
similar fix for temporary file handles was applied on Git for Windows
already:
https://github.com/git-for-windows/git/commit/667b8b51ec850c3e1c7d75dee69dc13c29d1f162

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
---
 read-cache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/read-cache.c b/read-cache.c
index db27766..f481dee 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -159,7 +159,7 @@ void fill_stat_cache_info(struct cache_entry *ce, struct stat *st)
 static int ce_compare_data(const struct cache_entry *ce, struct stat *st)
 {
 	int match = -1;
-	int fd = open(ce->name, O_RDONLY);
+	int fd = open(ce->name, O_RDONLY | O_CLOEXEC);
 
 	if (fd >= 0) {
 		unsigned char sha1[20];
-- 
2.9.2