Do not GC the current active incremental session directory

[iximeow]

when building a relatively large repo (https://github.com/oxidecomputer/omicron) on illumos under heavy CPU pressure, i saw some rustc invocations die like:

[..]/target/debug/incremental/<crate>-<hash>/<name>/dep-graph.part.bin: No such file or directory (os error 2)

a bit of debugging later and it seems that if the system is very slow, Unix-flavored flock::Lock::new() doesn't quite get the mutual exclusion garbage_collect_session_directories expects. before this patch i could reproduce this with the crate nexus_db_queries (in that repo) by pinning the full cargo build to one core and having a busy loop fighting on that same core. with this patch i cannot reproduce the issue. i took a look at how flock::Lock is used and i think this is the only problematic use, so i figure i'll propose this change particularly since i don't think file locking can be made.. good... for Unix in general.

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In setup_dep_graph, we set up a session directory for the current incremental compilation session, load the dep graph, and then GC stale incremental compilation sessions for the crate. The freshly-created session directory ends up in this list of potentially-GC'd directories but in practice is not typically even considered for GC because the new directory is neither finalized nor is_old_enough_to_be_collected.

Unfortunately, is_old_enough_to_be_collected is a simple time check, and if load_dep_graph is slow enough it's possible for the freshly-created session directory to be tens of seconds old already. Then, old enough to be eligible to GC, we try to flock::Lock it as proof it is not owned by anyone else, and so is a stale working directory.

Because we hold the lock in the same process, the behavior of flock::Lock is dependent on platform-specifics about file locking APIs. fcntl(F_SETLK)-style locks used on non-Linux Unices do not provide mutual exclusion internal to a process. fcntl_locking(2) on Linux describes some relevant problems:

       The record locks described above are associated with the process
       (unlike the open file description locks described below).  This
       has some unfortunate consequences:

       *  If a process closes any file descriptor referring to a file,
          then all of the process's locks on that file are released, [...]

       *  The threads in a process share locks.  In other words, a
          multithreaded program can't use record locking to ensure that
          threads don't simultaneously access the same region of a file.

fcntl-locks will appear to succeed to lock the fresh incremental compilation directory, at which point we can remove it just before using it later for incremental compilation. Saving incremental compilation state later fails and takes rustc with it with an error like

[..]/target/debug/incremental/crate-<hash>/<name>/dep-graph.part.bin: No such file or directory (os error 2)

The release-lock-on-close behavior has uncomfortable consequences for the freshly-opened file description for the lock, but I think in practice isn't an issue. If we would close the file, we failed to acquire the lock, so someone else had the lock ad we're not releasing locks prematurely.

flock(LOCK_EX) doesn't seem to have these same issues, and because flock::Lock::new always opens a new file description when locking, I don't think Linux can have this issue.

From reading LockFileEx on MSDN I think Windows has locking semantics similar to flock, but I haven't tested there at all.

My conclusion is that there is no way to write a pure-POSIX flock::Lock::new which guarantees mutual exclusion across different file descriptions of the same file in the same process, and flock::Lock::new must not be used for that purpose. So, instead, avoid considering the current incremental session directory for GC in the first place. Our own sess is evidence we're alive and using it.

[rustbot]

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