fellow_log: fellow_io_p(read|write)_sync to return -errno upon error

to match the result of io_uring

Ref a9700c63
Ref #65

src/fellow_log.c

@@ -979,10 +979,10 @@ fellow_io_pread_sync(const struct fellow_fd *ffd, void *bufa,
 
 	do {
 		r = pread(ffd->fd, buf, sz, off);
-		if (r <= 0) {
-			rdsz = r;
+		if (r < 0)
+			rdsz = -errno;
+		if (r <= 0)
 			break;
-		}
 		rdsz += r;
 		off += r;
 		buf += r;
@@ -1008,6 +1008,8 @@ fellow_io_pwrite_sync(const struct fellow_fd *ffd, const void *buf,
 	assert (sz <= INT32_MAX);
 
 	wrsz = pwrite(ffd->fd, buf, sz, off);
+	if (wrsz < 0)
+		wrsz = -errno;
 	assert(wrsz >= INT32_MIN);
 	assert(wrsz <= INT32_MAX);
 	return ((int32_t)wrsz);