fs: fix ceph copy error truncating readonly files

Trying to copy a read-only file onto a ceph-fuse filesystem fails,
returning an `EACCES` error. This happens when the destination
doesn't exist yet, and a new file is created.

By checking that the error matches, and that the destination file
is empty, we can fix this issue while waiting for a proper Ceph
fix to be upstreamed.

Fixes: libuv#3919
Refs: nodejs/node#37284
Refs: libuv#3117
Refs: libuv#3322

src/unix/fs.c

@@ -1306,7 +1306,19 @@ static ssize_t uv__fs_copyfile(uv_fs_t* req) {
     /* Truncate the file in case the destination already existed. */
     if (ftruncate(dstfd, 0) != 0) {
       err = UV__ERR(errno);
-      goto out;
+
+      /* ftruncate() on ceph-fuse fails with EACCES when the file is created
+       * with read only permissions. Since ftruncate() on a newly created
+       * file is a meaningless operation anyway, detect that condition
+       * and squelch the error.
+       */
+      if (err != UV_EACCES)
+        goto out;
+
+      if (dst_statsbuf.st_size > 0)
+        goto out;
+
+      err = 0;
     }
   }