tests/test-hghave.t

sparse: move code for importing rules from files into core

This is a pretty straightforward port. Some code cleanup was
performed. But no major changes to the logic were made.

I'm not a huge fan of this function because it does multiple
things. I'd like to get things into core first to facilitate
refactoring later.

Please also note the added inline comment about the oddities
of writeconfig() and the try..except to undo it. This is because
of the hackiness in which the sparse matcher is obtained by
various consumers, notably dirstate. We'll need a massive
refactor to address this. That refactor is effectively blocked
on having the sparse dirstate hacks live in core.

  $ . "$TESTDIR/helpers-testrepo.sh"

Testing that hghave does not crash when checking features

  $ hghave --test-features 2>/dev/null

Testing hghave extensibility for third party tools

  $ cat > hghaveaddon.py <<EOF
  > import hghave
  > @hghave.check("custom", "custom hghave feature")
  > def has_custom():
  >     return True
  > EOF

(invocation via run-tests.py)

  $ cat > test-hghaveaddon.t <<EOF
  > #require custom
  >   $ echo foo
  >   foo
  > EOF
  $ ( \
  > testrepohgenv; \
  > $TESTDIR/run-tests.py $HGTEST_RUN_TESTS_PURE test-hghaveaddon.t \
  > )
  .
  # Ran 1 tests, 0 skipped, 0 failed.

(invocation via command line)

  $ unset TESTDIR
  $ hghave custom

(terminate with exit code 2 at failure of importing hghaveaddon.py)

  $ rm hghaveaddon.*
  $ cat > hghaveaddon.py <<EOF
  > importing this file should cause syntax error
  > EOF

  $ hghave custom
  failed to import hghaveaddon.py from '.': invalid syntax (hghaveaddon.py, line 1)
  [2]