config.c: avoid xmmap error messages

The config-writing code uses xmmap to map the existing
config file, which will die if the map fails. This has two
downsides:

  1. The error message is not very helpful, as it lacks any
     context about the file we are mapping:

       $ mkdir foo
       $ git config --file=foo some.key value
       fatal: Out of memory? mmap failed: No such device

  2. We normally do not die in this code path; instead, we'd
     rather report the error and return an appropriate exit
     status (which is part of the public interface
     documented in git-config.1).

This patch introduces a "gentle" form of xmmap which lets us
produce our own error message. We do not want to use mmap
directly, because we would like to use the other
compatibility elements of xmmap (e.g., handling 0-length
maps portably).

The end result is:

    $ git.compile config --file=foo some.key value
    error: unable to mmap 'foo': No such device
    $ echo $?
    3

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 11 insertions, 4 deletions

diff --git a/sha1_file.c b/sha1_file.c
index d7f1838c13..ca491ab8bb 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -707,8 +707,8 @@ static void mmap_limit_check(size_t length)
 		    (uintmax_t)length, (uintmax_t)limit);
 }
 
-void *xmmap(void *start, size_t length,
-	int prot, int flags, int fd, off_t offset)
+void *xmmap_gently(void *start, size_t length,
+		  int prot, int flags, int fd, off_t offset)
 {
 	void *ret;
 
@@ -719,12 +719,19 @@ void *xmmap(void *start, size_t length,
 			return NULL;
 		release_pack_memory(length);
 		ret = mmap(start, length, prot, flags, fd, offset);
-		if (ret == MAP_FAILED)
-			die_errno("Out of memory? mmap failed");
 	}
 	return ret;
 }
 
+void *xmmap(void *start, size_t length,
+	int prot, int flags, int fd, off_t offset)
+{
+	void *ret = xmmap_gently(start, length, prot, flags, fd, offset);
+	if (ret == MAP_FAILED)
+		die_errno("Out of memory? mmap failed");
+	return ret;
+}
+
 void close_pack_windows(struct packed_git *p)
 {
 	while (p->windows) {