index: add pointer from nodetree back to index This is always a cycle right now, but it will not be for the nodetree instances I'm planning to add later (see earlier patch). Differential Revision: https://phab.mercurial-scm.org/D4112

index: remove side-effect from failed nt_new() As pointed out by Yuya in the review of D4108, if realloc() fails, we would end up with an invalid nodetree instance (with nt->nodes set to NULL), which means that if it was later accessed again it would likely segfault. It's probably unlikely that much else happens in the process if it ran out memory, but we should of course do our best to handle it. This patch makes it so we don't update the nodetree in this case. Differential Revision: https://phab.mercurial-scm.org/D4154

index: remove side-effect from failed nt_init() As pointed out by Yuya in the review of D4108, if we run into the "overflow in nt_init" case (which I think normally happens only in repos with at least 2^26=64Mi revisions), we would leave the node tree half-initialized. Differential Revision: https://phab.mercurial-scm.org/D4153