revlog: extract function for getting node from known-to-exist rev

Many of the calls to index_node() (which converts a rev to a nodeid)
are done with a rev that's know to exist. If the function fails,
there's something really wrong and we should just abort. This was done
in only one place. This patch starts by extracting that code to a
function that we can reuse in later patches.

Differential Revision: https://phab.mercurial-scm.org/D3456

mercurial/cext/revlog.c

@@ -248,6 +248,20 b' static const char *index_node(indexObjec'
 	return data ? data + 32 : NULL;
 }
 
+/*
+ * Return the 20-byte SHA of the node corresponding to the given rev. The
+ * rev is assumed to be existing. If not, an exception is set.
+ */
+static const char *index_node_existing(indexObject *self, Py_ssize_t pos)
+{
+	const char *node = index_node(self, pos);
+	if (node == NULL) {
+		PyErr_Format(PyExc_IndexError, "could not access rev %d",
+		             (int)pos);
+	}
+	return node;
+}
+
 static int nt_insert(indexObject *self, const char *node, int rev);
 
 static int node_check(PyObject *obj, char **node, Py_ssize_t *nodelen)
@@ -1282,10 +1296,8 b' static PyObject *index_partialmatch(inde'
 		return PyBytes_FromStringAndSize(nullid, 20);
 	}
 
-	fullnode = index_node(self, rev);
+	fullnode = index_node_existing(self, rev);
 	if (fullnode == NULL) {
-		PyErr_Format(PyExc_IndexError,
-			     "could not access rev %d", rev);
 		return NULL;
 	}
 	return PyBytes_FromStringAndSize(fullnode, 20);