distinguish error versus short read from read_in_full()

Many callers of read_in_full() expect to see the exact
number of bytes requested, but their error handling lumps
together true read errors and short reads due to unexpected
EOF.

We can give more specific error messages by separating these
cases (showing errno when appropriate, and otherwise
describing the short read).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/get-tar-commit-id.c

@@ -26,8 +26,10 @@ int cmd_get_tar_commit_id(int argc, const char **argv, const char *prefix)
 		usage(builtin_get_tar_commit_id_usage);
 
 	n = read_in_full(0, buffer, HEADERSIZE);
+	if (n < 0)
+		die_errno("git get-tar-commit-id: read error");
 	if (n != HEADERSIZE)
-		die("git get-tar-commit-id: read error");
+		die_errno("git get-tar-commit-id: EOF before reading tar header");
 	if (header->typeflag[0] != 'g')
 		return 1;
 	if (!skip_prefix(content, "52 comment=", &comment))

bulk-checkin.c

@@ -115,7 +115,10 @@ static int stream_to_pack(struct bulk_checkin_state *state,
 
 		if (size && !s.avail_in) {
 			ssize_t rsize = size < sizeof(ibuf) ? size : sizeof(ibuf);
-			if (read_in_full(fd, ibuf, rsize) != rsize)
+			ssize_t read_result = read_in_full(fd, ibuf, rsize);
+			if (read_result < 0)
+				die_errno("failed to read from '%s'", path);
+			if (read_result != rsize)
 				die("failed to read %d bytes from '%s'",
 				    (int)rsize, path);
 			offset += rsize;

packfile.c

@@ -444,6 +444,7 @@ static int open_packed_git_1(struct packed_git *p)
 	unsigned char sha1[20];
 	unsigned char *idx_sha1;
 	long fd_flag;
+	ssize_t read_result;
 
 	if (!p->index_data && open_pack_index(p))
 		return error("packfile %s index unavailable", p->pack_name);
@@ -485,7 +486,10 @@ static int open_packed_git_1(struct packed_git *p)
 		return error("cannot set FD_CLOEXEC");
 
 	/* Verify we recognize this pack file format. */
-	if (read_in_full(p->pack_fd, &hdr, sizeof(hdr)) != sizeof(hdr))
+	read_result = read_in_full(p->pack_fd, &hdr, sizeof(hdr));
+	if (read_result < 0)
+		return error_errno("error reading from %s", p->pack_name);
+	if (read_result != sizeof(hdr))
 		return error("file %s is far too short to be a packfile", p->pack_name);
 	if (hdr.hdr_signature != htonl(PACK_SIGNATURE))
 		return error("file %s is not a GIT packfile", p->pack_name);
@@ -502,7 +506,10 @@ static int open_packed_git_1(struct packed_git *p)
 			     p->num_objects);
 	if (lseek(p->pack_fd, p->pack_size - sizeof(sha1), SEEK_SET) == -1)
 		return error("end of packfile %s is unavailable", p->pack_name);
-	if (read_in_full(p->pack_fd, sha1, sizeof(sha1)) != sizeof(sha1))
+	read_result = read_in_full(p->pack_fd, sha1, sizeof(sha1));
+	if (read_result < 0)
+		return error_errno("error reading from %s", p->pack_name);
+	if (read_result != sizeof(sha1))
 		return error("packfile %s signature is unavailable", p->pack_name);
 	idx_sha1 = ((unsigned char *)p->index_data) + p->index_size - 40;
 	if (hashcmp(sha1, idx_sha1))