futils: produce improved error messages

src/util/futils.c

@@ -167,12 +167,18 @@ int git_futils_readbuffer_fd(git_str *buf, git_file fd, size_t len)
 	/* p_read loops internally to read len bytes */
 	read_size = p_read(fd, buf->ptr, len);
 
-	if (read_size != (ssize_t)len) {
+	if (read_size < 0) {
 		git_error_set(GIT_ERROR_OS, "failed to read descriptor");
 		git_str_dispose(buf);
 		return -1;
 	}
 
+	if ((size_t)read_size != len) {
+		git_error_set(GIT_ERROR_FILESYSTEM, "could not read (expected %" PRIuZ " bytes, read %" PRIuZ ")", len, (size_t)read_size);
+		git_str_dispose(buf);
+		return -1;
+	}
+
 	buf->ptr[read_size] = '\0';
 	buf->size = read_size;