view tests/test-branch-tag-confict.t @ 23702:c48924787eaa

filectx.parents: enforce changeid of parent to be in own changectx ancestors

Because of the way filenodes are computed, you can have multiple changesets
"introducing" the same file revision. For example, in the changeset graph
below, changeset 2 and 3 both change a file -to- and -from- the same content.

  o 3: content = new
  |
  | o 2: content = new
  |/
  o 1: content = old

In such cases, the file revision is create once, when 2 is added, and just reused
for 3. So the file change in '3' (from "old" to "new)" has no linkrev pointing
to it).  We'll call this situation "linkrev-shadowing". As the linkrev is used for
optimization purposes when walking a file history, the linkrev-shadowing
results in an unexpected jump to another branch during such a walk.. This leads to
multiple bugs with log, annotate and rename detection.

One element to fix such bugs is to ensure that walking the file history sticks on
the same topology as the changeset's history. For this purpose, we extend the
logic in 'basefilectx.parents' so that it always defines the proper changeset
to associate the parent file revision with. This "proper" changeset has to be an
ancestor of the changeset associated with the child file revision.

This logic is performed in the '_adjustlinkrev' function. This function is
given the starting changeset and all the information regarding the parent file
revision. If the linkrev for the file revision is an ancestor of the starting
changeset, the linkrev is valid and will be used. If it is not, we detected a
topological jump caused by linkrev shadowing, we are going to walk the
ancestors of the starting changeset until we find one setting the file to the
revision we are trying to create.

The performance impact appears acceptable:

- We are walking the changelog once for each filelog traversal (as there should
  be no overlap between searches),

- changelog traversal itself is fairly cheap, compared to what is likely going
  to be perform on the result on the filelog traversal,

- We only touch the manifest for ancestors touching the file, And such
  changesets are likely to be the one introducing the file. (except in
  pathological cases involving merge),

- We use manifest diff instead of full manifest unpacking to check manifest
  content, so it does not involve applying multiple diffs in most case.

- linkrev shadowing is not the common case.

Tests for fixed issues in log, annotate and rename detection have been
added.

But this changeset does not solve all problems. It fixes -ancestry-
computation, but if the linkrev-shadowed changesets is the starting one, we'll
still get things wrong. We'll have to fix the bootstrapping of such operations
in a later changeset. Also, the usage of `hg log FILE`  without --follow still
has issues with linkrev pointing to hidden changesets, because it relies on the
`filelog` revset which implement its own traversal logic that is still to be
fixed.

Thanks goes to:
- Matt Mackall: for nudging me in the right direction
- Julien Cristau and Rémi Cardona: for keep telling me linkrev bug were an
  evolution show stopper for 3 years.
- Durham Goode: for finding a new linkrev issue every few weeks
- Mads Kiilerich: for that last rename bug who raise this topic over my
  anoyance limit.

Initial setup.

  $ hg init repo
  $ cd repo
  $ touch thefile
  $ hg ci -A -m 'Initial commit.'
  adding thefile

Create a tag.

  $ hg tag branchortag

Create a branch with the same name as the tag.

  $ hg branch branchortag
  marked working directory as branch branchortag
  (branches are permanent and global, did you want a bookmark?)
  $ hg ci -m 'Create a branch with the same name as a tag.'

This is what we have:

  $ hg log
  changeset:   2:10519b3f489a
  branch:      branchortag
  tag:         tip
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     Create a branch with the same name as a tag.
  
  changeset:   1:2635c45ca99b
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     Added tag branchortag for changeset f57387372b5d
  
  changeset:   0:f57387372b5d
  tag:         branchortag
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     Initial commit.
  
Update to the tag:

  $ hg up 'tag(branchortag)'
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ hg parents
  changeset:   0:f57387372b5d
  tag:         branchortag
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     Initial commit.
  
Updating to the branch:

  $ hg up 'branch(branchortag)'
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg parents
  changeset:   2:10519b3f489a
  branch:      branchortag
  tag:         tip
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     Create a branch with the same name as a tag.
  

  $ cd ..