[Bug 5628] New: rebasing symlinks and directories leads to broken experience

[mercurial-bugs at mercurial-scm.org]

https://bz.mercurial-scm.org/show_bug.cgi?id=5628

            Bug ID: 5628
           Summary: rebasing symlinks and directories leads to broken
                    experience
           Product: Mercurial
           Version: unspecified
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: bug
          Priority: wish
         Component: rebase
          Assignee: bugzilla at mercurial-scm.org
          Reporter: rm at fb.com
                CC: mercurial-devel at mercurial-scm.org

There is a way to get into a totally broken repo -- one that you can apparently
not update in any way -- using symlinks and rebase.

The behavior today is different than the behavior was when I first discovered
something similar about a year ago. Back then, the rebase would abort and the
user would need to run `hg rebase --abort` to get back to a good state. Today,
the behavior is probably worse: the rebase completes, and the user has no hg
commands they could run to continue to do any work.

Here's a test script:

Set up symlink hell
  $ hg init repo
  $ cd repo
  $ touch base
  $ hg add base
  $ hg commit -m base
  $ ln -s b a
  $ hg add a
  $ hg commit -m a2b
  $ hg up .^
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ mkdir a
  $ touch a/c
  $ hg add a/c
  $ hg commit -m a/c
  created new head
  $ mkdir b
  $ hg rebase -s 2 -d 1 --config extensions.rebase=
  rebasing 2:0044ff20c17d "a/c" (tip)
  saved backup bundle to * (glob)

What The?
  $ hg status
  ! a/c
  ? b/c

Oh No!
  $ hg update .^
  abort: path 'a/c' traverses symbolic link 'a'
  [255]
  $ hg update --clean .^
  abort: path 'a/c' traverses symbolic link 'a'
  [255]
  $ hg update --clean null
  abort: path 'a/c' traverses symbolic link 'a'
  [255]

Haha, you screwed now!

  $ cd ..

Another terrible outcome (mkdir b is removed from above example)
  $ hg init repo2
  $ cd repo2
  $ touch base
  $ hg add base
  $ hg commit -m base
  $ ln -s b a
  $ hg add a
  $ hg commit -m a2b
  $ hg up .^
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ mkdir a
  $ touch a/c
  $ hg add a/c
  $ hg commit -m a/c
  created new head
  $ hg rebase -s 2 -d 1 --config extensions.rebase=
  rebasing 2:0044ff20c17d "a/c" (tip)
  abort: No such file or directory: $TESTTMP/repo2/a/c (glob)
  [255]

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