lock: add a way to prevent locks from being inherited

We want to prevent locks from being inherited sometimes (e.g. when there's a
currently running transaction, which will break a lot of assumptions we're
making in here.)

mercurial/lock.py

@@ -40,7 +40,7 b' class lock(object):'
     _host = None
 
     def __init__(self, vfs, file, timeout=-1, releasefn=None, acquirefn=None,
-                 desc=None, parentlock=None):
+                 desc=None, inheritchecker=None, parentlock=None):
         self.vfs = vfs
         self.f = file
         self.held = 0
@@ -48,6 +48,7 b' class lock(object):'
         self.releasefn = releasefn
         self.acquirefn = acquirefn
         self.desc = desc
+        self._inheritchecker = inheritchecker
         self.parentlock = parentlock
         self._parentheld = False
         self._inherited = False
@@ -186,6 +187,8 b' class lock(object):'
         if self._inherited:
             raise error.LockInheritanceContractViolation(
                 'inherit cannot be called while lock is already inherited')
+        if self._inheritchecker is not None:
+            self._inheritchecker()
         if self.releasefn:
             self.releasefn()
         if self._parentheld:

tests/test-lock.py

@@ -8,6 +8,7 b' import types'
 import unittest
 
 from mercurial import (
+    error,
     lock,
     scmutil,
 )
@@ -250,5 +251,21 b' class testlock(unittest.TestCase):'
 
         parentlock.release()
 
+    def testinheritcheck(self):
+        d = tempfile.mkdtemp(dir=os.getcwd())
+        state = teststate(self, d)
+        def check():
+            raise error.LockInheritanceContractViolation('check failed')
+        lock = state.makelock(inheritchecker=check)
+        state.assertacquirecalled(True)
+
+        def tryinherit():
+            with lock.inherit() as lockname:
+                pass
+
+        self.assertRaises(error.LockInheritanceContractViolation, tryinherit)
+
+        lock.release()
+
 if __name__ == '__main__':
     silenttestrunner.main(__name__)