Track symlinks between local files via DICE dependencies

[cormacrelf]

Problem

Here's a test case that I was quite alarmed Buck fails at. The use case is reading files at bzl eval time that don't have a .toml suffix, as TOML, for https://github.com/cormacrelf/elk which I have updated to depend on this entire thing at the heart of its strategy only to realise it doesn't work very well at all. (I would be happy with a way to tell load() to load as TOML if we can't get symlinks to work.)

BUCK
blah.lock
blah.lock.toml -> blah.lock   [a symlink]

# blah.lock
value = "original"

# BUCK
load(":blah.lock.toml", "value")

genrule(
    name = "test",
    out = "out.txt",
    cmd = "echo {} > $OUT".format(value),
)

Now, buck does not pick up changes to blah.lock.

buck2 build --out=- :test
{value: original}

echo 'value = "changed"' > blah.lock

buck2 build --out=- :test
{value: original}

# wat?
# now you have to buck2 clean to get it to pick up changes...

Solution

I am not very familiar with the fs operations here. This is pretty likely not to be quite correct and I invite some attention on the implementation. In particular I don't think it handles directory symlinks at all. But the general idea is being symlink-aware when reading a file, and making the DICE dependency graph reflect the symlink graph, so that a change to a real file propagates through any symlinks referring to it.

Basically this PR passes the test above, I make no claim other than that.