[PATCH] open_sha1_file: report "most interesting" errno

From: Jeff King <peff@peff.net>
Date: May 15, 2014, 08:54 UTC
Message-ID: <20140515085405.GA27033@sigill.intra.peff.net>

When we try to open a loose object file, we first attempt to open in the local object database, and then try any alternates. This means that the errno value when we return will be from the last place we looked (and due to the way the code is structured, simply ENOENT if we do not have have any alternates).

This can cause confusing error messages, as read_sha1_file checks for ENOENT when reporting a missing object. If errno is something else, we report that. If it is ENOENT, but has_loose_object reports that we have it, then we claim the object is corrupted. For example:

    $ chmod 0 .git/objects/??/*
    $ git rev-list --all
    fatal: loose object b2d6fab18b92d49eac46dc3c5a0bcafabda20131 (stored in .git/objects/b2/d6fab18b92d49eac46dc3c5a0bcafabda20131) is corrupt

This patch instead keeps track of the "most interesting" errno we receive during our search. We consider ENOENT to be the least interesting of all, and otherwise report the first error found (so problems in the object database take precedence over ones in alternates). Here it is with this patch:

    $ git rev-list --all
    fatal: failed to read object b2d6fab18b92d49eac46dc3c5a0bcafabda20131: Permission denied

Signed-off-by: Jeff King <peff@peff.net>
---
 sha1_file.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/sha1_file.c b/sha1_file.c
index 3e9f55f..34d527f 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -1437,19 +1437,23 @@ static int open_sha1_file(const unsigned char *sha1)
 {
 	int fd;
 	struct alternate_object_database *alt;
+	int most_interesting_errno;
 
 	fd = git_open_noatime(sha1_file_name(sha1));
 	if (fd >= 0)
 		return fd;
+	most_interesting_errno = errno;
 
 	prepare_alt_odb();
-	errno = ENOENT;
 	for (alt = alt_odb_list; alt; alt = alt->next) {
 		fill_sha1_path(alt->name, sha1);
 		fd = git_open_noatime(alt->base);
 		if (fd >= 0)
 			return fd;
+		if (most_interesting_errno == ENOENT)
+			most_interesting_errno = errno;
 	}
+	errno = most_interesting_errno;
 	return -1;
 }

-- 
2.0.0.rc1.436.g03cb729