revision: make get_commit_action() a pure predicate#2169
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get_commit_action() reads as a predicate that decides whether a commit is shown or ignored, but for a line-level log without parent rewriting it also called line_log_process_ranges_arbitrary_commit(), which mutates the tracked line ranges. That hidden side effect makes it unsafe to evaluate ahead of the walk, the way a lookahead would. get_commit_action() was split out of simplify_commit() in beb5af4 (graph API: fix bug in graph_is_interesting(), 2009-08-18) as the show/ignore decision minus the parent rewriting, so the graph renderer could reuse it; line-level log later routed its filtering through it as well, in 3cb9d2b (line-log: more responsive, incremental 'git log -L', 2020-05-11). Besides simplify_commit(), the walk driver, graph_is_interesting() is its only other caller, and it runs only under --graph, which sets rewrite_parents and therefore want_ancestry(); the "-L without ancestry" branch that holds the side effect never fires there, so it is dormant today. The line-level processing folds a commit's tracked ranges onto its parents, which must happen even for a commit that get_commit_action() filters from the output, or the ranges never reach the parents. Move it to simplify_commit() and run it before get_commit_action(), gated by get_commit_action()'s leading checks (already shown, uninteresting, and the like) so a commit ignored by those is not folded, as before; factor those checks out as commit_early_ignore(). get_commit_action() is then side-effect free. commit_early_ignore() runs twice on the -L path, once for that gate and once inside get_commit_action(), but it reads only object flags and pack membership, disjoint from the TREESAME flag the fold sets, so the repeat is harmless. Add a "line-log-peek" subcommand to the revision-walking test helper that evaluates get_commit_action() on a commit the walk has not reached yet, plus a t4211 check that the commit's TREESAME flag is untouched. The flag is watched rather than the commit list because add_line_range() merges ranges by union, which is idempotent, so the side effect never changed which commits a linear -L history shows. Suggested-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>
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