Please avoid the combination "-B -M" when running "diff" family of
commands, as it can produce incorrect results [*1*] in corner cases.
Use of either "-B" or "-M" by itself is fine, but not both at the
same time.
This problem exists even in Git v1.7.12, and I have no reason to
suspect that it worked correctly in any earlier versions, so I do
not consider it an urgent issue to fix during the pre-release for
upcoming Git v2.3 release.
End of TL;DR.
For a simple reproduction, go to your copy of the kernel tree and do
this:
$ git diff -B -M v2.6.13 v2.6.12 -- \
arch/ppc64/kernel/rtas_pci.c arch/ppc64/kernel/pSeries_pci.c >:patch
$ git reset --hard
$ git checkout v2.6.13
$ git apply --cached --whitespace=nowarn :patch
error: arch/ppc64/kernel/pSeries_pci.c: already exists in index
This is not a bug in "apply", but in "diff". The resulting patch
looks like this:
$ git apply --whitespace=nowarn --numstat --summary :patch
112 5 arch/ppc64/kernel/pSeries_pci.c
rename arch/ppc64/kernel/{rtas_pci.c => pSeries_pci.c} (81%)
That is, it wants to rename rtas_pci.c to pSeries_pci.c with a bit
of editing.
However, what really happens when going from 2.6.13 to 2.6.12 is
this:
$ git diff v2.6.13 v2.6.12 -- \
arch/ppc64/kernel/rtas_pci.c arch/ppc64/kernel/pSeries_pci.c |
git apply --whitespace=nowarn --numstat --summary
478 19 arch/ppc64/kernel/pSeries_pci.c
0 495 arch/ppc64/kernel/rtas_pci.c
delete mode 100644 arch/ppc64/kernel/rtas_pci.c
That is:
#1 rtas_pci.c exists in 2.6.13 but not 2.6.12.
#2 pSeries_pci.c exists in both 2.6.12 and 2.6.13 but is majorly
rewritten; in fact, the difference between rtas_pci.c in
2.6.13 and pSeries_pci.c in 2.6.12 is much smaller than the
difference between pSeries_pci.c from 2.6.13 and 2.6.12.
What seems to happen is that "diff -B" splits the above #2 into
"removal of pSeries_pci.c with contents from 2.6.13" and "creation
of pSeries_pci.c with contents from 2.6.12" and these gets further
combined with the "removal of rtas_pci.c" [*2*]. In the end result,
we incorrectly get "rename rtas_pci.c to create pSeries_pci.c with
some changes". We shouldn't do this because pSeries_pci.c is not
created and is not a new file.
[Footnotes]
*1* "git apply" refuses to apply the output affected by the bug, so
at least this will not lead to silent corruption.
*2* The "-B" option was introduced solely to find a possible
rename/copy source to express this sequence:
$ cp A B
$ edit B slightly ;# optional
$ edit A heavily
as if it was done like this instead:
$ mv A B
$ edit B slightly ;# optional
$ create B from scratch
Without "-B", the change would be expressed as "Heavily edit A,
create B from scratch", and with "-B", we would say "Create B by
copying from A and then edit, and edit A heavily", which would
result in more readable patch (in fact, we would see in the output
of "diff -B -M v2.6.12 v2.6.13" exactly that).
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