nodefs: do not overwrite the "out.Ino" inode number

...if it has already been set by the filesystem.

"out.Ino" is the user-facing inode number that is displayed by "ls -li".

This change allows loopback filesystems to expose the inode number of
the underlying filesystem.
As a "real" inode number, this one is stable for the lifetime of the file,
even across remounts.

libfuse has the `use_ino` option that allows the inode numbers to be set
arbitrarily as well. This has been used by EncFS for years.

The Linux kernel stores the inode number in `orig_ino`. Grepping in
`fs/fuse` shows that it is only read once in `fuse_update_attributes`,
and only if the `stat` argument is not NULL. There is only one caller
that passes that argument: `fuse_getattr`.

In short, the kernel does not use the inode number for anything but
reporting it via `stat()`, hence setting the inode number is safe.

This functionality will be used for improving log output in gocryptfs,
where file names cannot be used because they are encrypted.

fuse/nodefs/fsconnector.go

@@ -89,7 +89,9 @@ func (c *rawBridge) childLookup(out *fuse.EntryOut, n *Inode, context *fuse.Cont
 	n.Node().GetAttr((*fuse.Attr)(&out.Attr), nil, context)
 	n.mount.fillEntry(out)
 	out.NodeId, out.Generation = c.fsConn().lookupUpdate(n)
+	if out.Ino == 0 {
 		out.Ino = out.NodeId
+	}
 	if out.Nlink == 0 {
 		// With Nlink == 0, newer kernels will refuse link
 		// operations.

fuse/nodefs/fsmount.go

@@ -70,7 +70,9 @@ func (m *fileSystemMount) fillEntry(out *fuse.EntryOut) {
 func (m *fileSystemMount) fillAttr(out *fuse.AttrOut, nodeId uint64) {
 	splitDuration(m.options.AttrTimeout, &out.AttrValid, &out.AttrValidNsec)
 	m.setOwner(&out.Attr)
+	if out.Ino == 0 {
 		out.Ino = nodeId
+	}
 }
 
 func (m *fileSystemMount) getOpenedFile(h uint64) *openedFile {

fuse/nodefs/fsops.go

@@ -99,7 +99,9 @@ func (c *rawBridge) Lookup(header *fuse.InHeader, name string, out *fuse.EntryOu
 
 	child.mount.fillEntry(out)
 	out.NodeId, out.Generation = c.fsConn().lookupUpdate(child)
+	if out.Ino == 0 {
 		out.Ino = out.NodeId
+	}
 
 	return fuse.OK
 }