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[RFC PATCH 0/5] Improve path collision conflict resolutions

This series improves conflict resolutions for conflict types that involve
two (possibly entirely unrelated) files colliding at the same path.  These
conflict types are:
  * add/add
  * rename/add
  * rename/rename(2to1)

Improving these conflict types has some subtle (though significant)
performance ramifications, but for now, I am just concentrating on
providing the user with better information for their conflict resolution.
Performance considerations will be addressed in a future patch series.

Before mentioning the improvements, it may be worth noting that these three
types are actually more similar than might at first be apparent: for the
cases involving renames, any trace of the rename-source path is deleted
from both the index and the working copy (modulo a small technicality
mentioned in patch 2), leaving just the question of how to represent two
colliding files in both the index and the working copy for all three cases.

There are three important changes this patch series introduces:

  1) Consolidating the code for these three types of conflict resolutions
     into a single function that all three code paths can call.

  2) Doing content merges for a rename before looking at the path collision
     between a rename and some other file.  In particular (in what I most
     suspect others might have an objection to from this series), record
     that content-merged file -- which may have conflict markers -- in the
     index at the appropriate higher stage.

  3) Smarter behavior for recording the conflict in the working tree: first
     checking whether the two colliding files are similar, and then based
     on that, deciding whether to handle the path collision via a two-way
     merge of the different files or to instead record the two different
     files at separate temporary paths.

In more detail:


The consolidation seems fairly self-explanatory, but it had a bigger effect
than you'd expect: it made it clear that the rename/add conflict resolution
is broken in multiple ways, and it also made it easier to reason about what
_should_ be done for rename/add conflicts (something I had struggled with
when I looked at that particular conflict type in the past).  See patch 3
for more details.

Sidenote: I was kind of surprised that rename/add could have been broken
for this long, unnoticed.  Does no one ever hit that conflict in real life?
It looks like we did not have very good test coverage for rename/add
conflicts; a brief search seems to show that we only had a few testcases
triggering that conflict type, and all of them were added by me.  Patch 1
tries to address the testcase problem by adding some tests that try to
check the index and working copy more strictly for all three conflict


Previously, rename/rename(2to1) conflict resolution for the colliding path
would just accept the index changes made by unpack_trees(), meaning that
each of the higher order stages in the index for the path collision would
implicitly ignore any changes to each renamed file from the other side of
history.  Since, as noted above, all traces of the rename-source path were
removed from both the index and the working tree, this meant that the index
was missing information about changes to such files.  If the user tried to
resolve the conflict using the index rather than the working copy, they
would end up with a silent loss of changes.

I "fixed" this by doing the three-way content merge for each renamed-file,
and then recorded THAT in the index at either stage 2 or 3 as appropriate.
Since that merge might have conflict markers, that could mean recording in
the index a file with conflict markers as though it were a given side.
(See patch 2 for a more detailed explanation.)  I figure this might be the
most controversial change I made.  I can think of a few alternatives, but I
liked all of them less.  Opinions?

This change did not require any significant changes to the testsuite; the
difference between the old and new behavior was essentially untested.

(rename/add was even worse: not recording _any_ higher order stages in the
index, and thus partially hiding the fact that the path was involved in a
conflict at all.)


Given the similarity between the conflict types, the big question for
handling the conflict in the working tree was whether the two colliding
files should be two-way merged and recorded in place (as add/add does,
which seems to be preferable if the two files are similar), or whether the
two files should be recorded into separate files (as rename/add and
rename/rename(2to1) do, which seems to be preferable if the two files are
dissimilar).  The code handling the different types of conflicts appear to
have been written with different assumptions about whether the colliding
files would be similar.

But, rather than make an assumption about whether the two files will be
similar, why not just check and then make the best choice based on that?
Thus, this code makes use of estimate_similarity(), and uses that to decide
whether to do a two-way content merge or writing unrelated files out to
differently named temporary files.

This logical change did require changing one to two dozen testcases in the
testsuite; I think this is more logical behavior and that the testcases
were toy examples utilized to test other things, but maybe someone else has
an argument for why add/add conflicts should always be two-way merged
regardless of file dissimilarity?

Other notes:

This series builds on en/rename-directory-detection; there are too many
conflicts to resolve if I tried to just base on master.

Elijah Newren (5):
  Add testcases for improved file collision conflict handling
  merge-recursive: new function for better colliding conflict
  merge-recursive: fix rename/add conflict handling
  merge-recursive: improve handling for rename/rename(2to1) conflicts
  merge-recursive: improve handling for add/add conflicts

 diff.h                               |   4 +
 diffcore-rename.c                    |   6 +-
 merge-recursive.c                    | 383 +++++++++++++++++++++++------------
 t/t2023-checkout-m.sh                |   2 +-
 t/t3418-rebase-continue.sh           |  27 ++-
 t/t3504-cherry-pick-rerere.sh        |  19 +-
 t/t4200-rerere.sh                    |  12 +-
 t/t6020-merge-df.sh                  |   4 +-
 t/t6024-recursive-merge.sh           |  35 ++--
 t/t6025-merge-symlinks.sh            |   9 +-
 t/t6031-merge-filemode.sh            |   4 +-
 t/t6036-recursive-corner-cases.sh    |  19 +-
 t/t6042-merge-rename-corner-cases.sh | 212 ++++++++++++++++++-
 t/t6043-merge-rename-directories.sh  |  15 +-
 t/t7060-wtstatus.sh                  |   1 +
 t/t7064-wtstatus-pv2.sh              |   4 +-
 t/t7506-status-submodule.sh          |  11 +-
 t/t7610-mergetool.sh                 |  28 +--
 18 files changed, 588 insertions(+), 207 deletions(-)