[PATCH] fsck: give accurate error message on empty loose object files

Since 3ba7a065527a (A loose object is not corrupt if it
cannot be read due to EMFILE), "git fsck" on a repository with an empty
loose object file complains with the error message

  fatal: failed to read object <sha1>: Invalid argument

This comes from a failure of mmap on this empty file, which sets errno to
EINVAL. Instead of calling xmmap on empty file, we display a clean error
message ourselves, and return a NULL pointer. The new message is

  error: object file .git/objects/09/<rest-of-sha1> is empty
  fatal: loose object <sha1> (stored in .git/objects/09/<rest-of-sha1>) is corrupt

The second line was already there before the regression in 3ba7a065527a,
and the first is an additional message, that should help diagnosing the
problem for the user.

Signed-off-by: Matthieu Moy <redacted>
---
 sha1_file.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/sha1_file.c b/sha1_file.c
index 88f2151..fafc187 100644
--- a/sha1_file.c
+++ b/sha1_file.c

@@ -1202,6 +1202,11 @@ void *map_sha1_file(const unsigned char *sha1, unsigned long *size)
 
 		if (!fstat(fd, &st)) {
 			*size = xsize_t(st.st_size);
+			if (*size == 0) {
+				/* mmap() is forbidden on empty files */
+				error("object file %s is empty", sha1_file_name(sha1));
+				return NULL;
+			}
 			map = xmmap(NULL, *size, PROT_READ, MAP_PRIVATE, fd, 0);
 		}
 		close(fd);

-- 
1.7.9.111.gf3fb0.dirty