Do not ignore O_TRUNC flag

Instead of silently ignoring this return an error code. There is no reason why this
couldn't be supported, this is just a quick fix to avoid silent incorrect behavior.

We should also check if there are other open() flags that we may need to handle.

See also issue s3ql#182.

Changes.txt

@@ -3,6 +3,10 @@ UNRELEASED CHANGES
   * `s3qlctrl upload-meta` now works properly again (previously, only the first
     invocation had an effect).
 
+  * The O_TRUNC flag of the open() syscall is no longer silently ignored, but
+    the open() call fails with ENOTSUP. A proper implementation will hopefully
+    follow soon.
+
 
 2020-07-15, S3QL 3.5.0
 

src/s3ql/fs.py

@@ -932,6 +932,10 @@ async def open(self, id_, flags, ctx):
             and (self.failsafe or self.inodes[id_].locked)):
             raise FUSEError(errno.EPERM)
 
+        if flags & os.O_TRUNC:
+            log.warning('open() with O_TRUNC is not yet supported.')
+            raise FUSEError(errno.ENOTSUP)
+
         return pyfuse3.FileInfo(fh=id_, keep_cache=True)
 
     async def access(self, id_, mode, ctx):