treemanifest: rework lazy-copying code (issue4840)

The old lazy-copy code formed a chain of copied manifests with each
copy. Under typical operation, the stack never got more than a couple
of manifests deep and was fine. Under conditions like hgsubversion or
convert, the stack could get hundreds of manifests deep, and
eventually overflow the recursion limit for Python. I was able to
consistently reproduce this by converting an hgsubversion clone of
svn's history to treemanifests.

This may result in fewer manifests staying in memory during operations
like convert when treemanifests are in use, and should make those
operations faster since there will be significantly fewer noop
function calls going on.

A previous attempt (never mailed) of mine to fix this problem tried to
simply have all treemanifests only have a loadfunc - that caused
somewhat weird problems because the gettext() callable passed into
read() wasn't idempotent, so the easy solution is to have a loadfunc
and a copyfunc.

mercurial/manifest.py

@@ -442,13 +442,14 b' def _splittopdir(f):'
     else:
         return '', f
 
-_noop = lambda: None
+_noop = lambda s: None
 
 class treemanifest(object):
     def __init__(self, dir='', text=''):
         self._dir = dir
         self._node = revlog.nullid
-        self._load = _noop
+        self._loadfunc = _noop
+        self._copyfunc = _noop
         self._dirty = False
         self._dirs = {}
         # Using _lazymanifest here is a little slower than plain old dicts
@@ -479,7 +480,7 b' class treemanifest(object):'
     def __repr__(self):
         return ('<treemanifest dir=%s, node=%s, loaded=%s, dirty=%s at 0x%x>' %
                 (self._dir, revlog.hex(self._node),
-                 bool(self._load is _noop),
+                 bool(self._loadfunc is _noop),
                  self._dirty, id(self)))
 
     def dir(self):
@@ -598,6 +599,14 b' class treemanifest(object):'
             self._files[f] = n[:21] # to match manifestdict's behavior
         self._dirty = True
 
+    def _load(self):
+        if self._loadfunc is not _noop:
+            lf, self._loadfunc = self._loadfunc, _noop
+            lf(self)
+        elif self._copyfunc is not _noop:
+            cf, self._copyfunc = self._copyfunc, _noop
+            cf(self)
+
     def setflag(self, f, flags):
         """Set the flags (symlink, executable) for path f."""
         assert 'd' not in flags
@@ -615,19 +624,19 b' class treemanifest(object):'
         copy = treemanifest(self._dir)
         copy._node = self._node
         copy._dirty = self._dirty
-        def _load_for_copy():
+        if self._copyfunc is _noop:
+            def _copyfunc(s):
             self._load()
             for d in self._dirs:
-                copy._dirs[d] = self._dirs[d].copy()
-            copy._files = dict.copy(self._files)
-            copy._flags = dict.copy(self._flags)
-            copy._load = _noop
-        copy._load = _load_for_copy
-        if self._load == _noop:
-            # Chaining _load if it's _noop is functionally correct, but the
-            # chain may end up excessively long (stack overflow), and
-            # will prevent garbage collection of 'self'.
-            copy._load()
+                    s._dirs[d] = self._dirs[d].copy()
+                s._files = dict.copy(self._files)
+                s._flags = dict.copy(self._flags)
+            if self._loadfunc is _noop:
+                _copyfunc(copy)
+            else:
+                copy._copyfunc = _copyfunc
+        else:
+            copy._copyfunc = self._copyfunc
         return copy
 
     def filesnotin(self, m2):
@@ -834,13 +843,10 b' class treemanifest(object):'
         return _text(sorted(dirs + files), usemanifestv2)
 
     def read(self, gettext, readsubtree):
-        def _load_for_read():
-            # Mark as loaded already here, so __setitem__ and setflag() don't
-            # cause infinite loops when they try to load.
-            self._load = _noop
-            self.parse(gettext(), readsubtree)
-            self._dirty = False
-        self._load = _load_for_read
+        def _load_for_read(s):
+            s.parse(gettext(), readsubtree)
+            s._dirty = False
+        self._loadfunc = _load_for_read
 
     def writesubtrees(self, m1, m2, writesubtree):
         self._load() # for consistency; should never have any effect here