builtin/merge.c contains this important requirement for merge strategies:
...the index must be in sync with the head commit. The strategies are
responsible to ensure this.
However, Documentation/git-merge.txt says:
...[merge will] abort if there are any changes registered in the index
relative to the `HEAD` commit. (One exception is when the changed
index entries are in the state that would result from the merge
already.)
Interestingly, prior to commit c0be8aa06b ("Documentation/git-merge.txt:
Partial rewrite of How Merge Works", 2008-07-19),
Documentation/git-merge.txt said much more:
...the index file must match the tree of `HEAD` commit...
[NOTE]
This is a bit of a lie. In certain special cases [explained
in detail]...
Otherwise, merge will refuse to do any harm to your repository
(that is...your working tree...and index are left intact).
So, this suggests that the exceptions existed because there were special
cases where it would case no harm, and potentially be slightly more
convenient for the user. While the current text in git-merge.txt does
list a condition under which it would be safe to proceed despite the index
not matching HEAD, it does not match what is actually implemented, in
three different ways:
* The exception is written to describe what unpack-trees allows. Not
all merge strategies allow such an exception, though, making this
description misleading. 'ours' and 'octopus' merges have strictly
enforced index==HEAD for a while, and the commit previous to this
one made 'recursive' do so as well.
* If someone did a three-way content merge on a specific file using
versions from the relevant commits and staged it prior to running
merge, then that path would technically satisfy the exception listed
in git-merge.txt. unpack-trees.c would still error out on the path,
though, because it defers the three-way content merge logic to other
parts of the code (resolve, octopus, or recursive) and has no way of
checking whether the index entry from before the merge will match
the end result of the merge.
* The exception as implemented in unpack-trees actually only checked
that the index matched the MERGE_HEAD version of the file and that
HEAD matched the merge base. Assuming no renames, that would indeed
provide cases where the index matches the end result we'd get from a
merge. But renames means unpack-trees is checking that it instead
matches something other than what the final result will be, risking
either erroring out when we shouldn't need to, or not erroring out
when we should and overwriting the user's staged changes.
In addition to the wording behind this exception being misleading, it is
also somewhat surprising to see how many times the code for the special
cases were wrong or the check to make sure the index matched head was
forgotten altogether:
* Prior to commit ee6566e8d7 ("[PATCH] Rewrite read-tree", 2005-09-05),
there were many cases where an unclean index entry was allowed (look for
merged_entry_allow_dirty()); it appears that in those cases, the merge
would have simply overwritten staged changes with the result of the
merge. Thus, the merge result would have been correct, but the user's
uncommitted changes could be thrown away without warning.
* Prior to commit 160252f816 ("git-merge-ours: make sure our index
matches HEAD", 2005-11-03), the 'ours' merge strategy did not check
whether the index matched HEAD. If it didn't, the resulting merge
would include all the staged changes, and thus wasn't really an 'ours'
strategy.
* Prior to commit 3ec62ad9ff ("merge-octopus: abort if index does not
match HEAD", 2016-04-09), 'octopus' merges did not check whether the
index matched HEAD, also resulting in any staged changes from before
the commit silently being folded into the resulting merge. commit
a6ee883b8e ("t6044: new merge testcases for when index doesn't match
HEAD", 2016-04-09) was also added at the same time to try to test to
make sure all strategies did the necessary checking for the requirement
that the index match HEAD. Sadly, it didn't catch all the cases, as
evidenced by the remainder of this list...
* Prior to commit 65170c07d4 ("merge-recursive: avoid incorporating
uncommitted changes in a merge", 2017-12-21), merge-recursive simply
relied on unpack_trees() to do the necessary check, but in one special
case it avoided calling unpack_trees() entirely and accidentally ended
up silently including any staged changes from before the merge in the
resulting merge commit.
* The commit immediately before this one in this series noted that the
exceptions were written in a way that assumed no renames, making it
unsafe for merge-recursive to use. merge-recursive was modified to
use its own check to enforce that index==HEAD.
This history makes it very tempting to go into builtin/merge.c and replace
the comment that strategies must enforce that index matches HEAD with code
that just enforces it. At this point, that would only affect the
'resolve' strategy; all other strategies have each been modified to
manually enforce it. (However, note that index==HEAD is not strictly
enforced for fast-forward merges, as those are not considered a merge
strategy and they trigger in builtin/merge.c before the section in the
code where the relevant comment is found.)
But, even if we don't take the step of just fixing these problems by
enforcing index==HEAD for all strategies, we at least need to update this
misleading documentation in git-merge.txt. For now, just modify the claim
in Documentation/git-merge.txt to fix the error. The precise details
around combination of merges strategies and special cases probably is not
relevant to most users, so simply state that exceptions may exist but are
narrow and vary depending upon which merge strategy is in use.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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