gofer: Flush lisafs FDs on inode destruction in shared interop mode.

When the gofer filesystem is in `InteropModeShared`, immediately flush the
`Close` RPC for inode FDs upon inode destruction. This prevents caching of file
descriptors on the server, which could block external processes from unmounting
or releasing disk space. The existing behavior of flushing for deleted regular
files is preserved.

PiperOrigin-RevId: 819357479

Author: ayushr2

pkg/sentry/fsimpl/gofer/lisafs_inode.go

@@ -333,11 +333,15 @@ func (i *lisafsInode) destroy(ctx context.Context, d *dentry) {
 		i.writeFDLisa.Close(ctx, false /* flush */)
 	}
 	if i.controlFD.Ok() {
-		// Close the control FD. Propagate the Close RPCs immediately to the server
-		// if the dentry being destroyed is a deleted regular file. This is to
-		// release the disk space on remote immediately. This will flush the above
-		// read/write lisa FDs as well.
-		flushClose := d.isDeleted() && i.isRegularFile()
+		// Close the control FD.
+		// Propagate the Close RPCs immediately to the server if:
+		// - The dentry being destroyed is a deleted regular file. This is to
+		//   release the disk space on remote immediately.
+		// - The filesystem is shared with external writers, who may try to delete
+		//   files and attempt to unmount the filesystem. gVisor should not be
+		//   caching FDs here as it would prevent external cleanup.
+		// This will flush the Close RPCs for read/write lisa FDs above as well.
+		flushClose := i.fs.opts.interop == InteropModeShared || (d.isDeleted() && i.isRegularFile())
 		i.controlFD.Close(ctx, flushClose)
 	}
 }