revlog: use node tree (native code) for shortest() calculation

I want to rewrite revlog.shortest() to disambiguate only among hex
nodeids and then disambiguate the result with revnums at a higher
level (in scmutil). However, that would slow down `hg log -T
'{shortest(node,1)}\n'` from 5.0s to 6.8s, which I wasn't sure would
be acceptable. So this patch makes revlog.shortest() use the node tree
for finding the length of the shortest prefix that's unambiguous among
nodeids. Once that has been found, it makes it longer until it is also
not ambiguous with a revnum.

This speeds up `hg log -T '{shortest(node,1)}\n'` from 5.0s to 4.0s.

Differential Revision: https://phab.mercurial-scm.org/D3499

mercurial/cext/parsers.c

@@ -713,7 +713,7 b' void dirs_module_init(PyObject *mod);'
 void manifest_module_init(PyObject *mod);
 void revlog_module_init(PyObject *mod);
 
-static const int version = 4;
+static const int version = 5;
 
 static void module_init(PyObject *mod)
 {

mercurial/cext/revlog.c

@@ -1259,6 +1259,55 b' static int nt_partialmatch(indexObject *'
 	return nt_find(self, node, nodelen, 1);
 }
 
+/*
+ * Find the length of the shortest unique prefix of node.
+ *
+ * Return values:
+ *
+ *   -3: error (exception set)
+ *   -2: not found (no exception set)
+ * rest: length of shortest prefix
+ */
+static int nt_shortest(indexObject *self, const char *node)
+{
+	int level, off;
+
+	if (nt_init(self) == -1)
+		return -3;
+	if (nt_populate(self) == -1)
+		return -3;
+
+	for (level = off = 0; level < 40; level++) {
+		int k, v;
+		nodetree *n = &self->nt[off];
+		k = nt_level(node, level);
+		v = n->children[k];
+		if (v < 0) {
+			const char *n;
+			v = -(v + 1);
+			n = index_node(self, v);
+			if (memcmp(node, n, 20) != 0)
+				/*
+				 * Found a unique prefix, but it wasn't for the
+				 * requested node (i.e the requested node does
+				 * not exist).
+				 */
+				return -2;
+			return level + 1;
+		}
+		if (v == 0)
+			return -2;
+		off = v;
+	}
+	/*
+	 * The node was still not unique after 40 hex digits, so this won't
+	 * happen. Also, if we get here, then there's a programming error in
+	 * this file that made us insert a node longer than 40 hex digits.
+	 */
+	PyErr_SetString(PyExc_Exception, "broken node tree");
+	return -3;
+}
+
 static PyObject *index_partialmatch(indexObject *self, PyObject *args)
 {
 	const char *fullnode;
@@ -1307,6 +1356,29 b' static PyObject *index_partialmatch(inde'
 	return PyBytes_FromStringAndSize(fullnode, 20);
 }
 
+static PyObject *index_shortest(indexObject *self, PyObject *args)
+{
+	Py_ssize_t nodelen;
+	PyObject *val;
+	char *node;
+	int length;
+
+	if (!PyArg_ParseTuple(args, "O", &val))
+		return NULL;
+	if (node_check(val, &node, &nodelen) == -1)
+		return NULL;
+
+	self->ntlookups++;
+	length = nt_shortest(self, node);
+	if (length == -3)
+		return NULL;
+	if (length == -2) {
+		raise_revlog_error();
+		return NULL;
+	}
+	return PyInt_FromLong(length);
+}
+
 static PyObject *index_m_get(indexObject *self, PyObject *args)
 {
 	Py_ssize_t nodelen;
@@ -1995,6 +2067,8 b' static PyMethodDef index_methods[] = {'
 	 "insert an index entry"},
 	{"partialmatch", (PyCFunction)index_partialmatch, METH_VARARGS,
 	 "match a potentially ambiguous node ID"},
+	{"shortest", (PyCFunction)index_shortest, METH_VARARGS,
+	 "find length of shortest hex nodeid of a binary ID"},
 	{"stats", (PyCFunction)index_stats, METH_NOARGS,
 	 "stats for the index"},
 	{NULL} /* Sentinel */

mercurial/policy.py

@@ -69,7 +69,7 b' def _importfrom(pkgname, modname):'
     (r'cext', r'bdiff'): 3,
     (r'cext', r'mpatch'): 1,
     (r'cext', r'osutil'): 4,
-    (r'cext', r'parsers'): 4,
+    (r'cext', r'parsers'): 5,
 }
 
 # map import request to other package or module

mercurial/revlog.py

@@ -1526,7 +1526,33 b' class revlog(object):'
                 raise LookupError(node, self.indexfile, _('no node'))
             return not isrev(prefix)
 
+        def maybewdir(prefix):
+            return all(c == 'f' for c in prefix)
+
         hexnode = hex(node)
+
+        def disambiguate(hexnode, minlength):
+            for length in range(minlength, 41):
+                prefix = hexnode[:length]
+                if not isrev(prefix) and not maybewdir(prefix):
+                    return prefix
+
+        if not getattr(self, 'filteredrevs', None):
+            try:
+                length = max(self.index.shortest(node), minlength)
+                return disambiguate(hexnode, length)
+            except RevlogError:
+                if node == wdirid:
+                    for length in range(minlength, 41):
+                        prefix = hexnode[:length]
+                        if isvalid(prefix):
+                            return prefix
+                else:
+                    raise LookupError(node, self.indexfile, _('no node'))
+            except AttributeError:
+                # Fall through to pure code
+                pass
+
         shortest = hexnode
         startlength = max(6, minlength)
         length = startlength