cext-revlog: fixed __delitem__ for uninitialized nodetree

This is a bug in a code path that's seldom used, because in practice
(at least in the whole test suite), calls to `del index[i:j]` currently
just don't happen before the nodetree has been initialized.
However, in our current work to replace the nodetree by a Rust implementation,
this is of course systematic.

In `index_slice_del()`, if the slice start is smaller than `self->length`,
the whole of `self->added` has to be cleared.

Before this change, the clearing was done only by the call to
`index_invalidate_added(self, 0)`, that happens only for initialized
nodetrees. Hence the removal was effective only from `start` to `self->length`.

The consequence is index corruption, with bogus results in subsequent calls,
and in particular errors such as `ValueError("parent out of range")`, due to
the fact that parents of entries in `self->added` are now just invalid.

This is detected by the rebase tests, under conditions that the nodetree
of revlog.c is never initialized. The provided specific test is more direct.

Differential Revision: https://phab.mercurial-scm.org/D7603

mercurial/cext/revlog.c

@@ -2522,7 +2522,10 b' static int index_slice_del(indexObject *'
 				index_invalidate_added(self, 0);
 			if (self->ntrev > start)
 				self->ntrev = (int)start;
+		} else if (self->added) {
+			Py_CLEAR(self->added);
 		}
+
 		self->length = start;
 		if (start < self->raw_length) {
 			if (self->cache) {

tests/test-parseindex2.py

@@ -247,6 +247,32 b' class parseindex2tests(unittest.TestCase'
         got = index[-1]
         self.assertEqual(want, got)  # no inline data
 
+    def testdelitemwithoutnodetree(self):
+        index, _junk = parsers.parse_index2(data_non_inlined, False)
+
+        def hexrev(rev):
+            if rev == nullrev:
+                return b'\xff\xff\xff\xff'
+            else:
+                return nodemod.bin('%08x' % rev)
+
+        def appendrev(p1, p2=nullrev):
+            # node won't matter for this test, let's just make sure
+            # they don't collide. Other data don't matter either.
+            node = hexrev(p1) + hexrev(p2) + b'.' * 12
+            index.append((0, 0, 12, 1, 34, p1, p2, node))
+
+        appendrev(4)
+        appendrev(5)
+        appendrev(6)
+        self.assertEqual(len(index), 7)
+
+        del index[1:7]
+
+        # assertions that failed before correction
+        self.assertEqual(len(index), 1)  # was 4
+        self.assertEqual(index.headrevs(), [0])  # gave ValueError
+
 
 if __name__ == '__main__':
     import silenttestrunner