hghave: allow adding customized features at runtime

Before this patch, there is no way to add customized features to
`hghave` without changing `hghave` and `hghave.py` themselves.

This decreases reusability of `run-tests.py` framework for third party
tools, because they may want to examine custom features at runtime
(e.g.  existence of some external tools).

To allow adding customized features at runtime, this patch makes
`hghave` import `hghaveaddon` module, only when `hghaveaddon.py` file
can be found in directories below:

  - `TESTDIR` for invocation via `run-tests.py`
  - `.` for invocation via command line

The path to the directory where `hghaveaddon.py` should be placed is
added to `sys.path` only while importing `hghaveaddon`, because:

  - `.` may not be added to `PYTHONPATH`

  - adding additional path to `sys.path` may change behavior of
    subsequent `import` for other features

`hghave` is terminated with exit code '2' at failure of `import
hghaveaddon`, because exit code '2' terminates `run-tests.py`
immediately.

This is a one of preparations for issue4677.

tests/hghave

@@ -4,7 +4,7 b' if all features are there, non-zero othe'
 prefixed with "no-", the absence of feature is tested.
 """
 import optparse
-import sys
+import os, sys
 import hghave
 
 checks = hghave.checks
@@ -33,8 +33,30 b' parser.add_option("--list-features", act'
 parser.add_option("-q", "--quiet", action="store_true",
                   help="check features silently")
 
+def _loadaddon(quiet):
+    if 'TESTDIR' in os.environ:
+        # loading from '.' isn't needed, because `hghave` should be
+        # running at TESTTMP in this case
+        path = os.environ['TESTDIR']
+    else:
+        path = '.'
+
+    if not os.path.exists(os.path.join(path, 'hghaveaddon.py')):
+        return
+
+    sys.path.insert(0, path)
+    try:
+        import hghaveaddon
+    except BaseException, inst:
+        if not quiet:
+            sys.stderr.write('failed to import hghaveaddon.py from %r: %s\n'
+                             % (path, inst))
+        sys.exit(2)
+    sys.path.pop(0)
+
 if __name__ == '__main__':
     options, args = parser.parse_args()
+    _loadaddon(options.quiet)
     if options.list_features:
         list_features()
         sys.exit(0)

tests/test-hghave.t

@@ -1,3 +1,39 b''
 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
+  $ run-tests.py test-hghaveaddon.t
+  .
+  # Ran 1 tests, 0 skipped, 0 warned, 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]