git/contrib/coccinelle

= coccinelle

This directory provides Coccinelle (http://coccinelle.lip6.fr/) semantic patches
that might be useful to developers.

==  Types of semantic patches

 * Using the semantic transformation to check for bad patterns in the code;
   The target 'make coccicheck' is designed to check for these patterns and
   it is expected that any resulting patch indicates a regression.
   The patches resulting from 'make coccicheck' are small and infrequent,
   so once they are found, they can be sent to the mailing list as per usual.

   Example for introducing new patterns:
   67947c34ae (convert "hashcmp() != 0" to "!hasheq()", 2018-08-28)
   b84c783882 (fsck: s/++i > 1/i++/, 2018-10-24)

   Example of fixes using this approach:
   248f66ed8e (run-command: use strbuf_addstr() for adding a string to
               a strbuf, 2018-03-25)
   f919ffebed (Use MOVE_ARRAY, 2018-01-22)

   These types of semantic patches are usually part of testing, c.f.
   0860a7641b (travis-ci: fail if Coccinelle static analysis found something
               to transform, 2018-07-23)

 * Using semantic transformations in large scale refactorings throughout
   the code base.

   When applying the semantic patch into a real patch, sending it to the
   mailing list in the usual way, such a patch would be expected to have a
   lot of textual and semantic conflicts as such large scale refactorings
   change function signatures that are used widely in the code base.
   A textual conflict would arise if surrounding code near any call of such
   function changes. A semantic conflict arises when other patch series in
   flight introduce calls to such functions.

   So to aid these large scale refactorings, semantic patches can be used.
   However we do not want to store them in the same place as the checks for
   bad patterns, as then automated builds would fail.
   That is why semantic patches 'contrib/coccinelle/*.pending.cocci'
   are ignored for checks, and can be applied using 'make coccicheck-pending'.

   This allows to expose plans of pending large scale refactorings without
   impacting the bad pattern checks.

== Git-specific tips & things to know about how we run "spatch":

 * The "make coccicheck" will piggy-back on
   "COMPUTE_HEADER_DEPENDENCIES". If you've built a given object file
   the "coccicheck" target will consider its depednency to decide if
   it needs to re-run on the corresponding source file.

   This means that a "make coccicheck" will re-compile object files
   before running. This might be unexpected, but speeds up the run in
   the common case, as e.g. a change to "column.h" won't require all
   coccinelle rules to be re-run against "grep.c" (or another file
   that happens not to use "column.h").

   To disable this behavior use the "SPATCH_USE_O_DEPENDENCIES=NoThanks"
   flag.

 * To speed up our rules the "make coccicheck" target will by default
   concatenate all of the *.cocci files here into an "ALL.cocci", and
   apply it to each source file.

   This makes the run faster, as we don't need to run each rule
   against each source file. See the Makefile for further discussion,
   this behavior can be disabled with "SPATCH_CONCAT_COCCI=".

   But since they're concatenated any <id> in the <rulname> (e.g. "@
   my_name", v.s. anonymous "@@") needs to be unique across all our
   *.cocci files. You should only need to name rules if other rules
   depend on them (currently only one rule is named).

 * To speed up incremental runs even more use the "spatchcache" tool
   in this directory as your "SPATCH". It aimns to be a "ccache" for
   coccinelle, and piggy-backs on "COMPUTE_HEADER_DEPENDENCIES".

   It caches in Redis by default, see it source for a how-to.

   In one setup with a primed cache "make coccicheck" followed by a
   "make clean && make" takes around 10s to run, but 2m30s with the
   default of "SPATCH_CONCAT_COCCI=Y".

   With "SPATCH_CONCAT_COCCI=" the total runtime is around ~6m, sped
   up to ~1m with "spatchcache".

   Most of the 10s (or ~1m) being spent on re-running "spatch" on
   files we couldn't cache, as we didn't compile them (in contrib/*
   and compat/* mostly).

   The absolute times will differ for you, but the relative speedup
   from caching should be on that order.

== Authoring and reviewing coccinelle changes

* When a .cocci is made, both the Git changes and .cocci file should be
  reviewed. When reviewing such a change, do your best to understand the .cocci
  changes (e.g. by asking the author to explain the change) and be explicit
  about your understanding of the changes. This helps us decide whether input
  from coccinelle experts is needed or not. If you aren't sure of the cocci
  changes, indicate what changes you actively endorse and leave an Acked-by
  (instead of Reviewed-by).

* Authors should consider that reviewers may not be coccinelle experts, thus the
  the .cocci changes may not be self-evident. A plain text description of the
  changes is strongly encouraged, especially when using more esoteric features
  of the language.

* .cocci rules should target only the problem it is trying to solve; "collateral
  damage" is not allowed. Reviewers should look out and flag overly-broad rules.

* Consider the cost-benefit ratio of .cocci changes. In particular, consider the
  effect on the runtime of "make coccicheck", and how often your .cocci check
  will catch something valuable. As a rule of thumb, rules that can bail early
  if a file doesn't have a particular token will have a small impact on runtime,
  and vice-versa.

* .cocci files used for refactoring should be temporarily kept in-tree to aid
  the refactoring of out-of-tree code (e.g. in-flight topics). Periodically
  evaluate the cost-benefit ratio to determine when the file should be removed.
  For example, consider how many out-of-tree users are left and how much this
  slows down "make coccicheck".