On Fri, Sep 06, 2019 at 12:51:56PM -0400, Derrick Stolee wrote: > > This one in theory benefits lots of other callsites, too, since it means > > we'll actually return NULL instead of nonsense like "8". But grepping > > around for calls to this function, I found literally zero of them > > actually bother checking for a NULL result. So there are probably dozens > > of similar segfaults waiting to happen in other code paths. > > Discouraging. > > > > This is sort-of attributable to my 834876630b (get_commit_tree(): return > > NULL for broken tree, 2019-04-09). Before then it was a BUG(). However, > > that state was relatively short-lived. Before 7b8a21dba1 (commit-graph: > > lazy-load trees for commits, 2018-04-06), we'd have similarly returned > > NULL (and anyway, BUG() is clearly wrong since it's a data error). > > > > None of which argues against your patches, but it's kind of sad that the > > issue is present in so many code paths. I wonder if we could be handling > > this in a more central way, but I don't see how short of dying. > > This is due to the mechanical conversion from using commit->tree->oid to > get_commit_tree_oid(commit). Those consumers were not checking if the > tree pointer was NULL, either, but they probably assumed that the > parse_commit() call would have failed earlier. Now that we are using this > method (for performance reasons to avoid creating too many 'struct tree's) > it makes sense to convert some of them to checking the return value more > carefully.
Right, none of this is new at all. We have historically been very loose about assuming that things like commit->tree were valid. And they _usually_ are. Even if we're missing the object on disk, lookup_tree() is happy to assign it a struct (unless the object was already seen as another type!). I think turning that case into an error from parse_commit() would cover a lot of cases easily, without forcing each caller to check for NULL. -Peff