testing: allow Hypothesis to enable extensions

This adds support for testing extensions, including both tests
that extensions don't change behaviour and test for specific
commands.

We use the precondition system to determine what commands are
available to us. If we never use any commands enabled by an
extension then that extension is *skippable* and should not
have changed the behaviour of the test. We thus rerun the test
with an environment variable which is designed to turn off the
extension.

tests/test-verify-repo-operations.py

@@ -37,7 +37,8 b' import silenttestrunner'
 import subprocess
 
 from hypothesis.errors import HypothesisException
-from hypothesis.stateful import rule, RuleBasedStateMachine, Bundle
+from hypothesis.stateful import (
+    rule, RuleBasedStateMachine, Bundle, precondition)
 from hypothesis import settings, note, strategies as st
 from hypothesis.configuration import set_hypothesis_home_dir
 
@@ -155,6 +156,9 b' class verifyingstatemachine(RuleBasedSta'
         self.mkdirp("repo1")
         self.cd("repo1")
         self.hg("init")
+        self.extensions = {}
+        self.all_extensions = set()
+        self.non_skippable_extensions = set()
 
     def teardown(self):
         """On teardown we clean up after ourselves as usual, but we also
@@ -185,6 +189,17 b' class verifyingstatemachine(RuleBasedSta'
         e = None
         if not self.failed:
             try:
+                for ext in (
+                    self.all_extensions - self.non_skippable_extensions
+                ):
+                    try:
+                        os.environ["SKIP_EXTENSION"] = ext
+                        output = subprocess.check_output([
+                            runtests, path, "--local",
+                        ], stderr=subprocess.STDOUT)
+                        assert "Ran 1 test" in output, output
+                    finally:
+                        del os.environ["SKIP_EXTENSION"]
                 output = subprocess.check_output([
                     runtests, path, "--local", "--pure"
                 ], stderr=subprocess.STDOUT)
@@ -471,6 +486,50 b' class verifyingstatemachine(RuleBasedSta'
             else:
                 self.hg("update", "--", branch)
 
+    # Section: Extension management
+    def hasextension(self, extension):
+        repo = self.currentrepo
+        return repo in self.extensions and extension in self.extensions[repo]
+
+    def commandused(self, extension):
+        assert extension in self.all_extensions
+        self.non_skippable_extensions.add(extension)
+
+    @rule(extension=st.sampled_from((
+        'shelve', 'mq', 'blackbox',
+    )))
+    def addextension(self, extension):
+        self.all_extensions.add(extension)
+        extensions = self.extensions.setdefault(self.currentrepo, set())
+        if extension in extensions:
+            return
+        extensions.add(extension)
+        if not os.path.exists(hgrc):
+            self.command("touch", hgrc)
+        with open(hgrc, 'a') as o:
+            line = "[extensions]\n%s=\n" % (extension,)
+            o.write(line)
+        for l in line.splitlines():
+            self.log.append((
+                '$ if test "$SKIP_EXTENSION" != "%s" ; '
+                'then echo %r >> %s; fi') % (
+                    extension, l, hgrc,))
+
+    # Section: Commands from the shelve extension
+    @rule()
+    @precondition(lambda self: self.hasextension("shelve"))
+    def shelve(self):
+        self.commandused("shelve")
+        with acceptableerrors("nothing changed"):
+            self.hg("shelve")
+
+    @rule()
+    @precondition(lambda self: self.hasextension("shelve"))
+    def unshelve(self):
+        self.commandused("shelve")
+        with acceptableerrors("no shelved changes to apply"):
+            self.hg("unshelve")
+
 settings.register_profile(
     'default',  settings(
         timeout=300,