https://bz.mercurial-scm.org/show_bug.cgi?id=5481
Bug ID: 5481
Summary: With aggressivemergedeltas=False, one merge parent is
ignored for delta generation even if the other parent
is unsuitable
Product: Mercurial
Version: default branch
Hardware: All
OS: All
Status: UNCONFIRMED
Severity: feature
Priority: wish
Component: Mercurial
Assignee: bugzi...@mercurial-scm.org
Reporter: gabor.stefa...@nng.com
CC: mercurial-de...@selenic.com
The algorithm for finding the right delta base for a merge is subtly wrong when
aggressivemergedeltas is False. We pick the higher-numbered of the two merge
parent revisions, check if it produces a viable delta, and if not, we fall back
straight to a rev-1 delta without ever looking at the other parent.
In a merge, one of the parents is typically an excellent candidate for the
delta base, while the other may be no worse than a random revision. It's
difficult to predict the optimal parent without actually calculating the deltas
for both.
Since we chose against making aggressivemergedeltas the default, we can't
always guarantee an optimal parent choice. However, there is room for
improvement:
- We select the parent to use based on revision number, or "tipmostness". This
is not a good predictor of which parent is optimal - indeed, in a pull request
scenario, it tends to select the "incoming" parent, which may have branched out
far in the past, and is a rather poor delta base candidate. Instead, we should
compare the DAG distances from a common ancestor, and pick the larger one as
the delta parent.
- If the selected parent turns out to be unsuitable (e.g. too long delta
chain), the other parent is still a better candidate than rev-1. We should fall
back to it, and only try rev-1 if we have exhausted both parents. This behavior
is still different from aggressivemergedeltas, as it accepts the first tested
parent to give a good delta, and only moves on to the other one if the first
delta is rejected, as opposed to always generating both deltas and picking the
smaller one. So, the performance hit should be negligible.
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