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[PATCH] Makefile: add pending semantic patches

From: SZEDER Gábor <szeder.dev@xxxxxxxxx>

Add a description and place on how to use coccinelle for large refactorings
that happen only once.

Based-on-work-by: SZEDER Gábor <szeder.dev@xxxxxxxxx>
Signed-off-by: Stefan Beller <sbeller@xxxxxxxxxx>

I consider including this patch in a resend instead.
It outlays the basics of such a new workflow, which we can refine later.


 Makefile                  |  7 +++--
 contrib/coccinelle/README | 60 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index b08d5ea258..e5abfe4cef 100644
--- a/Makefile
+++ b/Makefile
@@ -2739,9 +2739,12 @@ endif
 	then \
 		echo '    ' SPATCH result: $@; \
-coccicheck: $(addsuffix .patch,$(wildcard contrib/coccinelle/*.cocci))
+coccicheck: $(addsuffix .patch,$(filter-out %.pending.cocci,$(wildcard contrib/coccinelle/*.cocci)))
-.PHONY: coccicheck
+# See contrib/coccinelle/README
+coccicheck-pending: $(addsuffix .patch,$(wildcard contrib/coccinelle/*.pending.cocci))
+.PHONY: coccicheck coccicheck-pending
 ### Installation rules
diff --git a/contrib/coccinelle/README b/contrib/coccinelle/README
index 9c2f8879c2..fa09d1abcc 100644
--- a/contrib/coccinelle/README
+++ b/contrib/coccinelle/README
@@ -1,2 +1,62 @@
 This directory provides examples of Coccinelle (http://coccinelle.lip6.fr/)
 semantic patches that might be useful to developers.
+There are two types of semantic patches:
+ * Using the semantic transformation to check for bad patterns in the code;
+   This is what the original target 'make coccicheck' is designed to do 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,
+   using the process as follows:
+   1) Figure out what kind of large scale refactoring we need
+      -> This is usually done by another unrelated series.
+   2) Create the sematic patch needed for the large scale refactoring
+      and store it in contrib/coccinelle/*.pending.cocci
+      -> The suffix containing 'pending' is important to differentiate
+      this case from the other use case of checking for bad patterns.
+   3) Apply the semantic patch only partially, where needed for the patch series
+      that motivates the large scale refactoring and then build that series
+      on top of it.
+      By applying the semantic patch only partially where needed, the series
+      is less likely to conflict with other series in flight.
+      To make it possible to apply the semantic patch partially, there needs
+      to be mechanism for backwards compatibility to keep those places working
+      where the semantic patch is not applied. This can be done via a
+      wrapper function that has the exact name and signature as the function
+      to be changed.
+   4) Send the series as usual, including only the needed parts of the
+      large scale refactoring
+   Later steps (not necessarily by the original author) are to apply the
+   semantic patch in a way that do not produce lots of conflicts, for example
+   by consulting `git diff --numstat origin/master..origin/pu` and the changes
+   of the semantic patch.