On Wed, May 29, 2019 at 07:54:44PM +0200, René Scharfe wrote: > Am 29.05.19 um 03:17 schrieb Jeff King: > > On Wed, May 29, 2019 at 01:34:32AM +0200, René Scharfe wrote: > >> Parsing trees with symlinks twice is not ideal, but keeps the set > >> structure simple -- a standard oidset suffices. > > > > If blobs comes after trees (and they usually do in a pack), you can do > > it in a single pass by marking the blob as a symlink target, and then > > when we actually see that blob's contents, marking it as either OK or > > problematic. And then the finish() step just correlates those with the > > tree. > > Good idea. Is that ordering guaranteed? (Stumbling about the "usually" > in your first sentence.) It's not guaranteed. Our implementation of pack-objects does order blobs after trees, but I suspect this could be violated in rare cases with some of the delta-island pack layering stuff. I think it makes sense to be sure that the receiver is correct no matter what, but optimize for this expected case (that's what I tried to do with the .gitmodules checks). > An ordering where dependent objects (like trees) follow the objects they > reference would be better suited for these kinds of checks.. But worse for others (e.g., like .gitmodules where it's cheap to identify a candidate blob, but the blob check is involved; there it's much more optimal to see the tree first). > > But here the problem is in the tree, not the blob. So we're not finding > > suspect blobs, but rather re-checking each tree. And no matter what we > > do (whether it's visiting the object again, or creating a set or mapping > > with the object names) is going to be linear there. And a repository > > with a symlink in the root tree is going to revisit or put in our > > mapping every single root tree. > > That's true, potentially it needs remember and/or reprocess all trees, > meaning this check may double the run time of fsck in the worst case. > Example from the wild: The kernel repo currently has 36 symlinks and > 6+ million objects are checked in total, and the symlink check processes > 18943 trees_with_symlinks entries there. That sounds about right. It's basically every version of every tree that has a symlink. Did it make a noticeable difference in timing? Indexing the whole kernel history is already a horribly slow process. :) > > TBH, I'm not sure this fsck check was worth it even without the > > implementation complexity. > > Hmm. git status reports such truncated symlinks as changed, so the > issue *is* already detectable. Hmm, yeah. That makes sense, since the filesystem (well, really the syscall API layer) cannot represent the data we are feeding it. -Peff
