revlog: C implementation of delta chain resolution

I've seen revlog._deltachain() appear in a number of performance
profiles. I suspect there are 2 reasons for this:

1. Delta chain resolution performs many index lookups, thus triggering
   population of index tuples. Creating possibly tens of thousands of
   PyObject will have overhead.
2. Delta chain resolution is a tight loop.

By moving delta chain resolution to C, we can defer instantiation
of full index entry tuples and make the loop faster courtesy of
not running in Python.

We can measure the impact to delta chain resolution via
`hg perflogrevision` using the mozilla-central repo with a recent
manifest having delta chain length of 33726:

$ hg perfrevlogrevision -m 364895
! full
! wall 0.367585 comb 0.370000 user 0.340000 sys 0.030000 (best of 27)
! wall 0.357581 comb 0.360000 user 0.350000 sys 0.010000 (best of 28)
! deltachain
! wall 0.010644 comb 0.010000 user 0.010000 sys 0.000000 (best of 270)
! wall 0.000292 comb 0.000000 user 0.000000 sys 0.000000 (best of 8729)

$ hg perfrevlogrevision --cache -m 364895
! deltachain
! wall 0.003904 comb 0.000000 user 0.000000 sys 0.000000 (best of 712)
! wall 0.000284 comb 0.000000 user 0.000000 sys 0.000000 (best of 9926)

The first test measures savings from both not instantiating index
entries and moving to C. The second test (which doesn't clear the
index caches) essentially isolates the benefits of moving from Python
to C. It still shows a 13.7x speedup (versus 36.4x). And there are
multiple milliseconds of savings within the critical path for resolving
revision data. I think that justifies the existence of C code.

A more striking example of the benefits of this change can be
demonstrated by timing `hg debugdeltachain -m` for the mozilla-central
repo:

$ time hg debugdeltachain -m > /dev/null
before:        1057.4s
after:          503.3s
PyPy2.7 5.8.0:  220.0s

It's worth noting that the C code isn't as optimal as it could be.
We're still instantiating a new PyObject for every revision. A future
optimization would be to reuse the PyObject on the cached index tuple.
We could potentially also get wins by using a memory array of raw
integers. There is also room for a delta chain cache on revlog
instances. Of course, the best optimization is to implement revlog
reading outside of Python so Python doesn't need to be concerned
about the relatively expensive index entries and operations on them.

mercurial/cext/revlog.c

@@ -816,6 +816,139 b' bail:'
 	return NULL;
 }
 
+static inline int index_baserev(indexObject *self, int rev)
+{
+	const char *data;
+
+	if (rev >= self->length - 1) {
+		PyObject *tuple = PyList_GET_ITEM(self->added,
+			rev - self->length + 1);
+		return (int)PyInt_AS_LONG(PyTuple_GET_ITEM(tuple, 3));
+	}
+	else {
+		data = index_deref(self, rev);
+		if (data == NULL) {
+			return -2;
+		}
+
+		return getbe32(data + 16);
+	}
+}
+
+static PyObject *index_deltachain(indexObject *self, PyObject *args)
+{
+	int rev, generaldelta;
+	PyObject *stoparg;
+	int stoprev, iterrev, baserev = -1;
+	int stopped;
+	PyObject *chain = NULL, *value = NULL, *result = NULL;
+	const Py_ssize_t length = index_length(self);
+
+	if (!PyArg_ParseTuple(args, "iOi", &rev, &stoparg, &generaldelta)) {
+		return NULL;
+	}
+
+	if (PyInt_Check(stoparg)) {
+		stoprev = (int)PyInt_AsLong(stoparg);
+		if (stoprev == -1 && PyErr_Occurred()) {
+			return NULL;
+		}
+	}
+	else if (stoparg == Py_None) {
+		stoprev = -2;
+	}
+	else {
+		PyErr_SetString(PyExc_ValueError,
+			"stoprev must be integer or None");
+		return NULL;
+	}
+
+	if (rev < 0 || rev >= length - 1) {
+		PyErr_SetString(PyExc_ValueError, "revlog index out of range");
+		return NULL;
+	}
+
+	chain = PyList_New(0);
+	if (chain == NULL) {
+		return NULL;
+	}
+
+	baserev = index_baserev(self, rev);
+
+	/* This should never happen. */
+	if (baserev == -2) {
+		PyErr_SetString(PyExc_IndexError, "unable to resolve data");
+		goto bail;
+	}
+
+	iterrev = rev;
+
+	while (iterrev != baserev && iterrev != stoprev) {
+		value = PyInt_FromLong(iterrev);
+		if (value == NULL) {
+			goto bail;
+		}
+		if (PyList_Append(chain, value)) {
+			Py_DECREF(value);
+			goto bail;
+		}
+		Py_DECREF(value);
+
+		if (generaldelta) {
+			iterrev = baserev;
+		}
+		else {
+			iterrev--;
+		}
+
+		if (iterrev < 0) {
+			break;
+		}
+
+		if (iterrev >= length - 1) {
+			PyErr_SetString(PyExc_IndexError, "revision outside index");
+			return NULL;
+		}
+
+		baserev = index_baserev(self, iterrev);
+
+		/* This should never happen. */
+		if (baserev == -2) {
+			PyErr_SetString(PyExc_IndexError, "unable to resolve data");
+			goto bail;
+		}
+	}
+
+	if (iterrev == stoprev) {
+		stopped = 1;
+	}
+	else {
+		value = PyInt_FromLong(iterrev);
+		if (value == NULL) {
+			goto bail;
+		}
+		if (PyList_Append(chain, value)) {
+			Py_DECREF(value);
+			goto bail;
+		}
+		Py_DECREF(value);
+
+		stopped = 0;
+	}
+
+	if (PyList_Reverse(chain)) {
+		goto bail;
+	}
+
+	result = Py_BuildValue("OO", chain, stopped ? Py_True : Py_False);
+	Py_DECREF(chain);
+	return result;
+
+bail:
+	Py_DECREF(chain);
+	return NULL;
+}
+
 static inline int nt_level(const char *node, Py_ssize_t level)
 {
 	int v = node[level>>1];
@@ -1828,6 +1961,8 b' static PyMethodDef index_methods[] = {'
 	 "get head revisions"}, /* Can do filtering since 3.2 */
 	{"headrevsfiltered", (PyCFunction)index_headrevs, METH_VARARGS,
 	 "get filtered head revisions"}, /* Can always do filtering */
+	{"deltachain", (PyCFunction)index_deltachain, METH_VARARGS,
+	 "determine revisions with deltas to reconstruct fulltext"},
 	{"insert", (PyCFunction)index_insert, METH_VARARGS,
 	 "insert an index entry"},
 	{"partialmatch", (PyCFunction)index_partialmatch, METH_VARARGS,

mercurial/revlog.py

@@ -570,6 +570,12 b' class revlog(object):'
         revs in ascending order and ``stopped`` is a bool indicating whether
         ``stoprev`` was hit.
         """
+        # Try C implementation.
+        try:
+            return self.index.deltachain(rev, stoprev, self._generaldelta)
+        except AttributeError:
+            pass
+
         chain = []
 
         # Alias to prevent attribute lookup in tight loop.