[PATCH v5 15/15] 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_CLOEXEX` 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