util: ability to change capacity when copying lrucachedict

util: ability to change capacity when copying lrucachedict

This will allow us to easily replace an lrucachedict with one
with a higher or lower capacity as consumers deem necessary.

IMO it is easier to just create a new cache instance than to
muck with the capacity of an existing cache. Mutating an existing
cache's capacity feels more prone to bugs.

Differential Revision: https://phab.mercurial-scm.org/D4500

--- a/mercurial/util.py	Thu Sep 06 11:37:27 2018 -0700
+++ b/mercurial/util.py	Thu Sep 06 11:40:20 2018 -0700
@@ -1311,8 +1311,19 @@
 
         self._cache.clear()
 
-    def copy(self):
-        result = lrucachedict(self.capacity)
+    def copy(self, capacity=None):
+        """Create a new cache as a copy of the current one.
+
+        By default, the new cache has the same capacity as the existing one.
+        But, the cache capacity can be changed as part of performing the
+        copy.
+
+        Items in the copy have an insertion/access order matching this
+        instance.
+        """
+
+        capacity = capacity or self.capacity
+        result = lrucachedict(capacity)
 
         # We copy entries by iterating in oldest-to-newest order so the copy
         # has the correct ordering.
@@ -1322,6 +1333,8 @@
         while n.key is _notset and n is not self._head:
             n = n.prev
 
+        # We could potentially skip the first N items when decreasing capacity.
+        # But let's keep it simple unless it is a performance problem.
         for i in range(len(self._cache)):
             result[n.key] = n.value
             n = n.prev

--- a/tests/test-lrucachedict.py	Thu Sep 06 11:37:27 2018 -0700
+++ b/tests/test-lrucachedict.py	Thu Sep 06 11:40:20 2018 -0700
@@ -118,5 +118,59 @@
         for key in ('a', 'b', 'c', 'd'):
             self.assertEqual(d[key], 'v%s' % key)
 
+    def testcopydecreasecapacity(self):
+        d = util.lrucachedict(5)
+        d['a'] = 'va'
+        d['b'] = 'vb'
+        d['c'] = 'vc'
+        d['d'] = 'vd'
+
+        dc = d.copy(2)
+        for key in ('a', 'b'):
+            self.assertNotIn(key, dc)
+        for key in ('c', 'd'):
+            self.assertIn(key, dc)
+            self.assertEqual(dc[key], 'v%s' % key)
+
+        dc['e'] = 've'
+        self.assertNotIn('c', dc)
+        for key in ('d', 'e'):
+            self.assertIn(key, dc)
+            self.assertEqual(dc[key], 'v%s' % key)
+
+        # Original should remain unchanged.
+        for key in ('a', 'b', 'c', 'd'):
+            self.assertIn(key, d)
+            self.assertEqual(d[key], 'v%s' % key)
+
+    def testcopyincreasecapacity(self):
+        d = util.lrucachedict(5)
+        d['a'] = 'va'
+        d['b'] = 'vb'
+        d['c'] = 'vc'
+        d['d'] = 'vd'
+
+        dc = d.copy(6)
+        for key in ('a', 'b', 'c', 'd'):
+            self.assertIn(key, dc)
+            self.assertEqual(dc[key], 'v%s' % key)
+
+        dc['e'] = 've'
+        dc['f'] = 'vf'
+        for key in ('a', 'b', 'c', 'd', 'e', 'f'):
+            self.assertIn(key, dc)
+            self.assertEqual(dc[key], 'v%s' % key)
+
+        dc['g'] = 'vg'
+        self.assertNotIn('a', dc)
+        for key in ('b', 'c', 'd', 'e', 'f', 'g'):
+            self.assertIn(key, dc)
+            self.assertEqual(dc[key], 'v%s' % key)
+
+        # Original should remain unchanged.
+        for key in ('a', 'b', 'c', 'd'):
+            self.assertIn(key, d)
+            self.assertEqual(d[key], 'v%s' % key)
+
 if __name__ == '__main__':
     silenttestrunner.main(__name__)