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![Glen Choo](/assets/img/avatar_default.png)
These practices largely reflect what we are already doing on the mailing list, which should help new Coccinelle authors and reviewers get up to speed. Signed-off-by: Glen Choo <chooglen@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
125 lines
5.8 KiB
Plaintext
125 lines
5.8 KiB
Plaintext
= coccinelle
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This directory provides Coccinelle (http://coccinelle.lip6.fr/) semantic patches
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that might be useful to developers.
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== Types of semantic patches
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* Using the semantic transformation to check for bad patterns in the code;
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The target 'make coccicheck' is designed to check for these patterns and
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it is expected that any resulting patch indicates a regression.
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The patches resulting from 'make coccicheck' are small and infrequent,
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so once they are found, they can be sent to the mailing list as per usual.
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Example for introducing new patterns:
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67947c34ae (convert "hashcmp() != 0" to "!hasheq()", 2018-08-28)
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b84c783882 (fsck: s/++i > 1/i++/, 2018-10-24)
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Example of fixes using this approach:
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248f66ed8e (run-command: use strbuf_addstr() for adding a string to
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a strbuf, 2018-03-25)
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f919ffebed (Use MOVE_ARRAY, 2018-01-22)
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These types of semantic patches are usually part of testing, c.f.
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0860a7641b (travis-ci: fail if Coccinelle static analysis found something
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to transform, 2018-07-23)
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* Using semantic transformations in large scale refactorings throughout
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the code base.
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When applying the semantic patch into a real patch, sending it to the
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mailing list in the usual way, such a patch would be expected to have a
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lot of textual and semantic conflicts as such large scale refactorings
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change function signatures that are used widely in the code base.
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A textual conflict would arise if surrounding code near any call of such
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function changes. A semantic conflict arises when other patch series in
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flight introduce calls to such functions.
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So to aid these large scale refactorings, semantic patches can be used.
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However we do not want to store them in the same place as the checks for
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bad patterns, as then automated builds would fail.
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That is why semantic patches 'contrib/coccinelle/*.pending.cocci'
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are ignored for checks, and can be applied using 'make coccicheck-pending'.
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This allows to expose plans of pending large scale refactorings without
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impacting the bad pattern checks.
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== Git-specific tips & things to know about how we run "spatch":
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* The "make coccicheck" will piggy-back on
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"COMPUTE_HEADER_DEPENDENCIES". If you've built a given object file
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the "coccicheck" target will consider its depednency to decide if
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it needs to re-run on the corresponding source file.
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This means that a "make coccicheck" will re-compile object files
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before running. This might be unexpected, but speeds up the run in
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the common case, as e.g. a change to "column.h" won't require all
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coccinelle rules to be re-run against "grep.c" (or another file
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that happens not to use "column.h").
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To disable this behavior use the "SPATCH_USE_O_DEPENDENCIES=NoThanks"
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flag.
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* To speed up our rules the "make coccicheck" target will by default
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concatenate all of the *.cocci files here into an "ALL.cocci", and
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apply it to each source file.
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This makes the run faster, as we don't need to run each rule
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against each source file. See the Makefile for further discussion,
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this behavior can be disabled with "SPATCH_CONCAT_COCCI=".
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But since they're concatenated any <id> in the <rulname> (e.g. "@
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my_name", v.s. anonymous "@@") needs to be unique across all our
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*.cocci files. You should only need to name rules if other rules
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depend on them (currently only one rule is named).
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* To speed up incremental runs even more use the "spatchcache" tool
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in this directory as your "SPATCH". It aimns to be a "ccache" for
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coccinelle, and piggy-backs on "COMPUTE_HEADER_DEPENDENCIES".
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It caches in Redis by default, see it source for a how-to.
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In one setup with a primed cache "make coccicheck" followed by a
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"make clean && make" takes around 10s to run, but 2m30s with the
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default of "SPATCH_CONCAT_COCCI=Y".
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With "SPATCH_CONCAT_COCCI=" the total runtime is around ~6m, sped
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up to ~1m with "spatchcache".
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Most of the 10s (or ~1m) being spent on re-running "spatch" on
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files we couldn't cache, as we didn't compile them (in contrib/*
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and compat/* mostly).
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The absolute times will differ for you, but the relative speedup
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from caching should be on that order.
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== Authoring and reviewing coccinelle changes
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* When a .cocci is made, both the Git changes and .cocci file should be
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reviewed. When reviewing such a change, do your best to understand the .cocci
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changes (e.g. by asking the author to explain the change) and be explicit
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about your understanding of the changes. This helps us decide whether input
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from coccinelle experts is needed or not. If you aren't sure of the cocci
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changes, indicate what changes you actively endorse and leave an Acked-by
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(instead of Reviewed-by).
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* Authors should consider that reviewers may not be coccinelle experts, thus the
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the .cocci changes may not be self-evident. A plain text description of the
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changes is strongly encouraged, especially when using more esoteric features
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of the language.
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* .cocci rules should target only the problem it is trying to solve; "collateral
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damage" is not allowed. Reviewers should look out and flag overly-broad rules.
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* Consider the cost-benefit ratio of .cocci changes. In particular, consider the
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effect on the runtime of "make coccicheck", and how often your .cocci check
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will catch something valuable. As a rule of thumb, rules that can bail early
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if a file doesn't have a particular token will have a small impact on runtime,
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and vice-versa.
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* .cocci files used for refactoring should be temporarily kept in-tree to aid
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the refactoring of out-of-tree code (e.g. in-flight topics). Periodically
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evaluate the cost-benefit ratio to determine when the file should be removed.
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For example, consider how many out-of-tree users are left and how much this
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slows down "make coccicheck".
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