relnotes/5.9

filemerge: add support for partial conflict resolution by external tool

A common class of merge conflicts is in imports/#includes/etc. It's
relatively easy to write a tool that can resolve these conflicts,
perhaps by naively just unioning the statements and leaving any
cleanup to other tools to do later [1]. Such specialized tools cannot
generally resolve all conflicts in a file, of course. Let's therefore
call them "partial merge tools". Note that the internal simplemerge
algorithm is such a partial merge tool - one that only resolves
trivial "conflicts" where one side is unchanged or both sides change
in the same way.

One can also imagine having smarter language-aware partial tools that
merge the AST. It may be useful for such tools to interactively let
the user resolve any conflicts it can't resolve itself. However,
having the option of implementing it as a partial merge tool means
that the developer doesn't *need* to create a UI for it. Instead, the
user can resolve any remaining conflicts with their regular merge tool
(e.g. `:merge3` or `meld).

We don't currently have a way to let the user define such partial
merge tools. That's what this patch addresses. It lets the user
configure partial merge tools to run. Each tool can be configured to
run only on files matching certain patterns (e.g. "*.py"). The tool
takes three inputs (local, base, other) and resolves conflicts by
updating these in place. For example, let's say the inputs are these:


base:
```
import sys

def main():
  print('Hello')
```

local:
```
import os
import sys

def main():
  print('Hi')
```

other:
```
import re
import sys

def main():
  print('Howdy')
```

A partial merge tool could now resolve the conflicting imports by
replacing the import statements in *all* files by the following
snippet, while leaving the remainder of the files unchanged.

```
import os
import re
import sys
```

As a result, simplemerge and any regular merge tool that runs after
the partial merge tool(s) will consider the imports to be
non-conflicting and will only present the conflict in `main()` to the
user.

Differential Revision: https://phab.mercurial-scm.org/D12356

== New Features ==

 * `hg config` now has a `--source` option to show where each
   configuration value comes from.
 * Introduced a command (debug-repair-issue6528) to repair repositories affected
   by issue6528 where certain files would show up as modified even if they were
   clean due to an issue in the copy-tracing code.

== Default Format Change ==

These changes affect newly created repositories (or new clone) done with
Mercurial 5.9.


== New Experimental Features ==

 * A `changelogv2` format has been introduced. It is not ready for use yet, but
   will be used later to address some of the weaknesses of the current revlog
   format.
 * Initial experiment and support for `dirstatev2`, a new dirstate format that
   addresses some of the weaknesses of the current dirstate format. Python + C
   and Rust support are being implemented, but the Rust solution is the one
   currently getting the attention for performance.
 * Initial support for `rhg status`. `rhg` is the Rust wrapper executable for hg
   that shortcuts some commands for faster execution speed.

== Bug Fixes ==

 * Fixed committing empty files with `narrow`
 * Allow overriding `pip`'s pep517 compliance to build C or Rust extensions
 * Fixed regression on outgoing email when not specifying revisions
 * Fixed a regression causing bookmarks to disappear when using Rust persistent nodemap
 * Fixed a regression (in 5.9.1) introduced in 5.9 when cloning repos with
   deep filenames
 * Fixed detection of directories becoming symlinks, but only when using the
   Rust extensions.
 * Fixed ignore and include not composing in the Rust status
 * `hg commit --interactive` now handles deselecting edits of a rename
 * Fixed a case where `hg evolve` gives different results when interrupted
 * Fixed a memory leak in phases computation
 * `histedit` and `shelve` don't swallow errors when updating the working copy
   anymore
 * Improve error message when detecting content-divergence with a hidden
   common predecessor
 * No longer re-order parents in filelog, see issue6533
 * Fix revisions affected by issue6533 on the fly during exchange
 * Many Windows fixes for stability and py3 compatibility improvements
 * Many other miscellaneous fixes

== Backwards Compatibility Changes ==


== Internal API Changes ==

The Dirstate API have been updated as the previous function leaked some
internal details and did not distinguish between two important cases: "We are
changing parent and need to adjust the dirstate" and "some command is changing
which file is tracked". To clarify the situation:

* the following functions have been deprecated,

  - `dirstate.add`,
  - `dirstate.normal`,
  - `dirstate.normallookup`,
  - `dirstate.merge`,
  - `dirstate.otherparent`,
  - `dirstate.remove`,
  - `dirstate.drop`,
  - `dirstateitem.__getitem__`,

* these new functions are added for the "adjusting parents" use-case:

  - `dirstate.update_file`,
  - `dirstate.update_file_p1`,

* these new function are added for the "adjusting wc file" use-case":

  - `dirstate.set_tracked`,
  - `dirstate.set_untracked`,
  - `dirstate.set_clean`,
  - `dirstate.set_possibly_dirty`,

See inline documentation of the new functions for details.

* Additionally, the following have been deprecated:
  - `urlutil.getpath` function
  - `localrepository.updatecaches`' `full` argument

* The following have been removed:
  - `revlog.revlogio` has been removed