freebsd stable - Jan 2021 - HEADS UP: FreeBSD src repo transitioning to git this weekend

On Sun, 2021-01-03 at 19:02:24 +0100, Helge Oldach
wrote:
>Hi all,
>
>I have deep cloned main from https://git.freebsd.org/src.git as
>described in the mini primer and started pulling updates. Now I'm
>stumbling over a bit of confusion:
>
>| hmo at p48 /usr/src $ git log -p e35a01eec6926bfb5c088ca8961079b51a067bf3
>| commit e35a01eec6926bfb5c088ca8961079b51a067bf3
>| Merge: 2ff66a91552 96b88ac701b
>| Author: Philip Paeps <philip at FreeBSD.org>
>| Date:   Wed Dec 30 12:50:26 2020 +0800
>|
>|     contrib/tzdata: import tzdata 2020f
>|
>|     Merge commit '96b88ac701b35ce68425046d4be8f51cb75b5d5b' into
main
>|
>|     Changes: https://github.com/eggert/tz/blob/2020f/NEWS
>|
>|     MFC after:    1 day
>|
>| commit 96b88ac701b35ce68425046d4be8f51cb75b5d5b
>| Author: Philip Paeps <philip at FreeBSD.org>
>| Date:   Wed Dec 30 12:45:24 2020 +0800
>|
>|     Import tzdata 2020f
>|
>| diff --git a/Makefile b/Makefile
>| index 5064a190c5a..1136af9298f 100644
>| --- a/Makefile
>| +++ b/Makefile
>| @@ -945,7 +945,10 @@ check_public: $(VERSION_DEPS)
>|                 mkdir public.dir
>|                 ln $(VERSION_DEPS) public.dir
>
>What confuses me is that the diff refers to just "Makefile". Same
for
>the other files in this commit. The diff just looks like the vendor
>commit and not like the merged commit. This is kind of confusing as
>clearly this commit does not refer to the main source Makefile, but just
>to contrib/tzdata/Makefile.
Well, you're looking at the diff of the vendor import, not the diff of 
the merge commit. The vendor area doesn't have a prefix in its tree, see 
the output of
git ls-tree -r 96b88ac701b35ce68425046d4be8f51cb75b5d5b
That is the vendor area tree for tzdata.

What trips you up is that git log -p does _not_ show diffs for merge 
commits by default, because with 2 parents for a commit, what's the diff 
really? Look carefully at the 2nd line, it says "Merge: 2ff66a91552
96b88ac701b" indicating that this is a merge commit.

To get a meaningful diff output, you sadly have to hand-hold git. What 
always works is comparing 2 trees directly, so (add -p to see full 
diff):
% git diff-tree 2ff66a91552 e35a01eec6926bfb5c088ca8961079b51a067bf3
:040000 040000 8feda71c4bc2d2deb3a6c3dcf19ca1272ef51c71
6e6f976db44b4430645b0c6d7a169d5e5564acec M      contrib

This has to assume that the first parent shown was the previous commit 
on main, but it could well have been the other way round. But you also 
list the commit for 96b88ac701b35ce68425046d4be8f51cb75b5d5b so it's 
clear that this had to be the one.

You can short-cut this, assuming that the first parent will always be on 
main (I think this is true in the conversion).

% git diff e35a01eec^1..e35a01eec

(^1 means first parent of the commit, and is often equivalent to ~1 
which means "go 1 back").

At which point you can read git-log(1) and see that there's also a 
--first-parent option, which will only walk the first parent and 
properly works in tandem with -p.

hth
Uli