Make fstat work on file descriptor with no name

[hoodmane]

This also is a bit more efficient: rather than taking the stream and converting it to a path, then calling lookupPath to find the node and calling getattr on it, just lookup stream.node and call getattr on it directly.

This requires that we add stream_ops.getattr in addition to node_ops.getattr -- the first to implement lookup from file descriptors to file attributes, the second to implement lookup from file system path to file attributes. Most file systems do not need a stream_ops.getattr because stream.node contains all the information about the file. However, in the nodefs, stream holds the nfd. If the file descriptor points to an unlinked file, the nfd is needed to access the native file information but the node doesn't have it. Thus, the nodefs needs both node_ops.getattr for the path based functions and stream_ops.getattr for the file descriptor based functions and similar for setattr. This requires a significant refactor. The common logic is moved into helper functions.

Resolves #16414.

[hoodmane]

Looks like to fix this in NODEFS, we need to make stream_ops.getattr and stream_ops.setattr and have fstat, fchmod, fchown, and ftruncate use them instead of the corresponding node_ops if they exist.

[sbc100]

Is this a common pattern where we try to do an operation first on the stream and then on the node if it fails?

Do we need the fallback?

[hoodmane]

Well we have a map from file descriptors to streams, and a map from paths to nodes. Each stream has a reference to its node. The difficulty is that not every node has a valid path.

Most file systems store all relevant information about a node on the node itself. However, in the nodefs, each stream has a distinct native file descriptor. In the case where the node has no valid path, the only way to talk to the native object is with fstat and friends using the native file descriptor. So in that case we need these stream_ops. For every other FS, it's sufficient to have setattr on the node_ops.

Also, we do need the node_ops case because if we call stat instead of fstat then we have a path but no file descriptor. We could open the path to get the file descriptor and then stat it and close it. But we're better off just having a separate handler for gettattr/setattr on a path/node than on a file descriptor/stream. But only nodefs seems to need it.

[sbc100]

@hoodmane, I assume you are aware of the WasmFS project within emscripten: WasmFS. The goal here is to eventually replace the current FS completely, if possible. All these recent changes you have been making a much appreciate, but I wonder if you have looked at WasmFS yet, and what issues you ran into. I wonder if it would make sense to focus more effect there, if that is where we will ultimately end up? As you can see by the list of open issues there are still some limitations, and I wonder how much work it would be to be get to a place where you could use it in your project(s)?

[hoodmane]

@sbc100 if you want, I could split this into a first PR with all changes except for nodefs and a second PR with the nodefs changes.

[hoodmane]

Closing in favor of #23364, #23470, and #23480.