Bug: git checkout @{u} breaks git branch if the upstream isn't a local branch

Bug: git checkout @{u} breaks git branch if the upstream isn't a local branch

[Leszek Swirski]

Consider a repository with branch master, with upstream origin/master.
If one is on master, and runs "git checkout @{u}" to get the upstream,
then git branch stops working. The reason is that we end up on a
detached HEAD, and git branch searches recent reflog entries to figure
out where the HEAD was detached from (wt-status.c:
wt_status_get_detached_from). Since @{u} is not a local branch, the
checkout is inserted into the reflog as "moving from master to @{u}"
rather than "moving from master to origin/master". This is because
setup_branch_path (checkout.c) only updates the branch name for local
branches, keeping the input name (here, "@{u}") otherwise. So, git
branch sees "@{u}", decides that we are detached from "@{u}", tries to
lookup "HEAD@{u}" and fails with "fatal: HEAD does not point to a
branch"

I assume that either the reflog entry should be amended to print
something more informative (moving from master to origin/master, or at
least master@{u}, or the SHA...), or the reflog search in
wt_status_get_detached_from should do something cleverer with relative
refs.

- Leszek

Re: Bug: git checkout @{u} breaks git branch if the upstream isn't a local branch

[Junio C Hamano]

Leszek Swirski <leszeks@google.com> writes:

> I assume that either the reflog entry should be amended to print
> something more informative (moving from master to origin/master, or at
> least master@{u}, or the SHA...), or the reflog search in
> wt_status_get_detached_from should do something cleverer with relative
> refs.

Nicely analyzed. A record "moving to @{u}" in the reflog may not
leave sufficient information for a later operation to tell what
branch was refered to with the @{u} reference back when "checkout"
was run.  I suspect that checking out @{4}, @{1.day.ago}, etc. may
share the same issue.

"git checkout @{-1}" aka "git checkout -" may not have any problem
after such a checkout, as it only uses "moving from" side of the
reflog entry.