worktree: check the result of read_in_full()

We try to read "len" bytes into a buffer and just assume
that it happened correctly. In practice this should usually
be the case, since we just stat'd the file to get the
length.  But we could be fooled by transient errors or by
other processes racily truncating the file.

Let's be more careful. There's a slim chance this could
catch a real error, but it also prevents people and tools
from getting worried while reading the code.

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

builtin/worktree.c

@@ -40,6 +40,7 @@ static int prune_worktree(const char *id, struct strbuf *reason)
 	char *path;
 	int fd;
 	size_t len;
+	ssize_t read_result;
 
 	if (!is_directory(git_path("worktrees/%s", id))) {
 		strbuf_addf(reason, _("Removing worktrees/%s: not a valid directory"), id);
@@ -59,8 +60,24 @@ static int prune_worktree(const char *id, struct strbuf *reason)
 	}
 	len = xsize_t(st.st_size);
 	path = xmallocz(len);
-	read_in_full(fd, path, len);
+
+	read_result = read_in_full(fd, path, len);
+	if (read_result < 0) {
+		strbuf_addf(reason, _("Removing worktrees/%s: unable to read gitdir file (%s)"),
+			    id, strerror(errno));
+		close(fd);
+		free(path);
+		return 1;
+	}
 	close(fd);
+
+	if (read_result != len) {
+		strbuf_addf(reason,
+			    _("Removing worktrees/%s: short read (expected %"PRIuMAX" bytes, read %"PRIuMAX")"),
+			    id, (uintmax_t)len, (uintmax_t)read_result);
+		free(path);
+		return 1;
+	}
 	while (len && (path[len - 1] == '\n' || path[len - 1] == '\r'))
 		len--;
 	if (!len) {