[PATCH v1 2/2] read-cache: make sure file handles are not inherited by child processes

[larsxschneider@gmail.com]

From: Lars Schneider <larsxschneider@gmail.com>

This fix prepares a series with the goal to avoid launching a new
clean/smudge filter process for each file that is filtered. A new
long running filter process is introduced that is used to filter all
files in a single Git invocation.

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. On Linux/OSX the unlink operation
succeeds but the file descriptors still leak into the child process.

Fix this problem by opening files in read-cache with the CLOEXEC flag to
ensure that the file descriptor does not remain open in a newly spawned
process similar to 05d1ed61.

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 491e52d..02f74d3 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -156,7 +156,7 @@ 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];
-- 
2.10.0