avoid looking at errno for short read_in_full() returns

When a caller tries to read a particular set of bytes via
read_in_full(), there are three possible outcomes:

  1. An error, in which case -1 is returned and errno is
     set.

  2. A short read, in which fewer bytes are returned and
     errno is unspecified (we never saw a read error, so we
     may have some random value from whatever syscall failed
     last).

  3. The full read completed successfully.

Many callers handle cases 1 and 2 together by just checking
the result against the requested size. If their combined
error path looks at errno (e.g., by calling die_errno), they
may report a nonsense value.

Let's fix these sites by having them distinguish between the
two error cases. That avoids the random errno confusion, and
lets us give more detailed error messages.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

pack-write.c

@@ -213,14 +213,19 @@ void fixup_pack_header_footer(int pack_fd,
 	git_SHA_CTX old_sha1_ctx, new_sha1_ctx;
 	struct pack_header hdr;
 	char *buf;
+	ssize_t read_result;
 
 	git_SHA1_Init(&old_sha1_ctx);
 	git_SHA1_Init(&new_sha1_ctx);
 
 	if (lseek(pack_fd, 0, SEEK_SET) != 0)
 		die_errno("Failed seeking to start of '%s'", pack_name);
-	if (read_in_full(pack_fd, &hdr, sizeof(hdr)) != sizeof(hdr))
+	read_result = read_in_full(pack_fd, &hdr, sizeof(hdr));
+	if (read_result < 0)
 		die_errno("Unable to reread header of '%s'", pack_name);
+	else if (read_result != sizeof(hdr))
+		die_errno("Unexpected short read for header of '%s'",
+			  pack_name);
 	if (lseek(pack_fd, 0, SEEK_SET) != 0)
 		die_errno("Failed seeking to start of '%s'", pack_name);
 	git_SHA1_Update(&old_sha1_ctx, &hdr, sizeof(hdr));

sha1_file.c

@@ -1757,10 +1757,15 @@ static int index_core(unsigned char *sha1, int fd, size_t size,
 		ret = index_mem(sha1, "", size, type, path, flags);
 	} else if (size <= SMALL_FILE_SIZE) {
 		char *buf = xmalloc(size);
-		if (size == read_in_full(fd, buf, size))
-			ret = index_mem(sha1, buf, size, type, path, flags);
+		ssize_t read_result = read_in_full(fd, buf, size);
+		if (read_result < 0)
+			ret = error_errno("read error while indexing %s",
+					  path ? path : "<unknown>");
+		else if (read_result != size)
+			ret = error("short read while indexing %s",
+				    path ? path : "<unknown>");
 		else
-			ret = error_errno("short read");
+			ret = index_mem(sha1, buf, size, type, path, flags);
 		free(buf);
 	} else {
 		void *buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);