[libcxx-dev] [monorepo] Much improved downstream zipping tool available

[Björn Pettersson A via libcxx-dev]

> -----Original Message-----
> From: David Greene <dag at cray.com>
> Sent: den 29 januari 2019 19:33
> To: Björn Pettersson A <bjorn.a.pettersson at ericsson.com>
> Cc: llvm-dev at lists.llvm.org; cfe-dev at lists.llvm.org; openmp-
> dev at lists.llvm.org; clangd-dev at lists.llvm.org; libclc-
> dev at lists.llvm.org; libcxx-dev at lists.llvm.org; lldb-dev at lists.llvm.org
> Subject: Re: [monorepo] Much improved downstream zipping tool available
> 
> Björn Pettersson A <bjorn.a.pettersson at ericsson.com> writes:
> 
> > In the new monorepo UC1 may or may not be a parent to UL1.
> > We could actually have something like this:
> >
> >   UL4->UC2->UL3->UL2->UL1->UL0->UC1
> >
> > Our DL1 commit should preferably have UL1 as parent after
> > conversion
> >
> >   UL4->UC2->UL3->UL2->UL1->UL0->UC1
> >                        |
> >                  ...->DL1
> >
> > but since it also includes DC1 (via submodule reference) we
> > want to zip in DC1 before DL1, right?
> >
> >   UL4->UC2->UL3->UL2->UL1->UL0->UC1
> >                        |
> >             ...->DC1->DL1
> >
> > The problem is that DC1 is based on UC1, so we would get something
> > like this
> >
> >   UL4->UC2->UL3->UL2->UL1->UL0->UC1
> >                        |         |
> >             ...->DC1->DL1        |
> >                   ^              |
> >                   |              |
> >                    --------------
> >
> > Which is not correct, since then we also get the UL0 commit
> > as predecessor to DL1.
> 
> To be clear, is DC1 a commit that updates the clang submodule to UC1
> and
> DL1 a separate local commit to llvm that merges in UL1?


In llvm (split) we have:

  UL4->UL3->UL2->UL1->UL0
                   \
         ...->DL2->DL1

In clang (split) we have:

  UC4->UC3->UC2->UC1->UC0
                   \
         ...->DC2->DC1


DL1 is a commit that updates the clang submodule to DC1 (and in this
scenario at the same time merges UL1 and DL2 in llvm).


> 
> When zip-downstream-fork.py runs, it *always* uses the exact trees in
> use by each downstream commit, whether from submodules or the umbrella
> itself.  It tries very hard to maintain the state of the trees as they
> appeared in the umbrella repository.
> 
> Since in your case llvm isn't a submodule (it's the "umbrella"), DL1
> will absolutely have the tree from UL1, not UL0.  This is how
> migrate-downstream-fork.py works and zip-downstream-fork.py won't touch
> the llvm tree since it's not a submodule.  The commit DL1 doesn't
> update
> any submodules so it will just use the clang tree from DC1.
> 
> I haven't tested this case explicitly but I would expect the resulting
> history graph to look as you diagrammed above (reformatted to make it
> clear there isn't a cycle):
> 
>    UL4->UC2->UL3->UL2->UL1->UL0->UC1 <- monorepo/master
>                         |         |
>                         \         |
>                          `-----------.
>                                   |   \
>                            ... ->DC1->DL1 <- zip/master
> 
> The "redundant" edge here is indicating that the state of the llvm tree
> at DL1 is based on UL1, not UL0.  All other projects will be in the
> state at UC1 (assuming you don't have other submodules under llvm).  I
> know it looks strange but this is the best I could come up with because
> in general there is no guarantee that submodule updates were in any way
> correlated with when upstream commits were made (as you discovered!).
> There's some discussion of this issue on the documentation I posted
> [1],
> as well as in header comments in zip-downstream-fork.py.

How does git know that it should follow the parent relation from
DL1 to UL1 for the llvm subdir, and not the UL0->UC1->DC1->DL1
path? I mean, if I check out commit DC1 I will see the contribution
from UL0 in the llvm subdir, and DL1 includes the changes from DC1.

(I understand that we never really want to check out the old clang commits
after the migration, it will be the DLx commits that matters and that
should have a synced view between the different subdirs, it is also the
DLx commits that may have old release labels in our downstream release
track)