remoting: support symlinks attribute in directories

[tdyas]

https://github.com/pantsbuild/pants/pull/10155 refreshed the REv2 protos. The DirectoryNode proto now has a symlinks field that models symbolic links in a directory as a list of SymlinkNode protos. Pants should implement support for that in order to better conform to the REv2 API.

[stuhood]

Oof. Not terribly excited about that, but it is what it is.

[stuhood]

@compyman added a great example of a use case for symlinks over here: https://github.com/pantsbuild/pants/pull/15211#issuecomment-1135155501

[stuhood]

Some thoughts on how this might impact glob matching:

PosixFS does already have a notion of symlink awareness, which currently only controls whether scandir will ever return a symlink.

Glob matching (implemented in GlobMatchingImplementation::expand, which consumes an impl Vfs) is used in two primary cases:

1. By @rule code, via the Snapshot node
   • In this case, the actual Vfs implementation is for Context, which implements the Vfs ops via Nodes (to give them invalidation and memoization).
   • Because we want to track symlinks via Nodes, this case is marked SymlinkBehavior::Aware : otherwise, the Vfs would never receive any read_link calls, because the operating system would already have expanded them.
2. By sandbox outputs capture
   • In this case, the Vfs in use is directly impl Vfs for PosixFS, with less indirection.
   • Currently, this case is marked SymlinkBehavior::Oblivious. That will need to change: see below.

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Since you are adding symlink awareness, the process-sandbox case will need to be marked Aware.

[thejcannon]

@stuhood can we close this now because of https://github.com/pantsbuild/pants/pull/16844?

[stuhood]

Yes!

[compyman]

<3 <3 <3 thanks!