This patch implements support for reusing existing pseudo-merge commits
when writing bitmaps when there is an existing pseudo-merge bitmap which
has exactly the same set of parents as one that we are about to write.
Note that unstable pseudo-merges are likely to change between
consecutive repacks, and so are generally poor candidates for reuse.
However, stable pseudo-merges (see the configuration option
'bitmapPseudoMerge.<name>.stableThreshold') are by definition unlikely
to change between runs (as they represent long-running branches).
Because there is no index from a *set* of pseudo-merge parents to a
matching pseudo-merge bitmap, we have to construct the bitmap
corresponding to the set of parents for each pending pseudo-merge commit
and see if a matching bitmap exists.
This is technically quadratic in the number of pseudo-merges, but is OK
in practice for a couple of reasons:
- non-matching pseudo-merge bitmaps are rejected quickly as soon as
they differ in a single bit
- already-matched pseudo-merge bitmaps are discarded from subsequent
rounds of search
- the number of pseudo-merges is generally small, even for large
repositories
In order to do this, implement (a) a function that finds a matching
pseudo-merge given some uncompressed bitset describing its parents, (b)
a function that computes the bitset of parents for a given pseudo-merge
commit, and (c) call that function before computing the set of reachable
objects for some pending pseudo-merge.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Now that all of the groundwork has been laid to support reading and
using pseudo-merges, make use of that work in this commit by teaching
the pack-bitmap machinery to use pseudo-merge(s) when available during
traversal.
The basic operation is as follows:
- When enumerating objects on either side of a reachability query,
first see if any subset of the roots satisfies some pseudo-merge
bitmap. If it does, apply that pseudo-merge bitmap.
- If any pseudo-merge bitmap(s) were applied in the previous step, OR
them into the result[^1]. Then repeat the process over all
pseudo-merge bitmaps (we'll refer to this as "cascading"
pseudo-merges). Once this is done, OR in the resulting bitmap.
- If there is no fill-in traversal to be done, return the bitmap for
that side of the reachability query. If there is fill-in traversal,
then for each commit we encounter via show_commit(), check to see if
any unsatisfied pseudo-merges containing that commit as one of its
parents has been made satisfied by the presence of that commit.
If so, OR in the object set from that pseudo-merge bitmap, and then
cascade. If not, continue traversal.
A similar implementation is present in the boundary-based bitmap
traversal routines.
[^1]: Importantly, we cannot OR in the entire set of roots along with
the objects reachable from whatever pseudo-merge bitmaps were
satisfied. This may leave some dangling bits corresponding to any
unsatisfied root(s) getting OR'd into the resulting bitmap, tricking
other parts of the traversal into thinking we already have a
reachability closure over those commit(s) when we do not.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
