filemerge: add config knob to check capabilities of internal merge tools
For historical reason, Mercurial assumes capabilities of internal
merge tools as below while examining rules to decide merge tool,
regardless of actual capabilities of them.
=============== ====== ========
specified via binary symlinks
=============== ====== ========
--tool o o
HGMERGE o o
merge-patterns o (*) x (*)
ui.merge x (*) x (*)
=============== ====== ========
This causes:
- unintentional internal merge tool is chosen for binary files via
merge-patterns section of configuration file
- explicit configuration of internal merge tool for symlinks is
ignored unintentionally
But on the other hand, simple "check capability strictly" might break
backward compatibility (e.g. existing merge automations), because it
changes the result of merge tool selection.
Therefore, this patch adds config knob "merge.strict-capability-check"
to control whether capabilities of internal merge tools should be
checked strictly or not.
If this configuration is true, capabilities of internal merge tools
are checked strictly in (*) cases above.
To merge files Mercurial uses merge tools.
A merge tool combines two different versions of a file into a merged
file. Merge tools are given the two files and the greatest common
ancestor of the two file versions, so they can determine the changes
made on both branches.
Merge tools are used both for :hg:`resolve`, :hg:`merge`, :hg:`update`,
:hg:`backout` and in several extensions.
Usually, the merge tool tries to automatically reconcile the files by
combining all non-overlapping changes that occurred separately in
the two different evolutions of the same initial base file. Furthermore, some
interactive merge programs make it easier to manually resolve
conflicting merges, either in a graphical way, or by inserting some
conflict markers. Mercurial does not include any interactive merge
programs but relies on external tools for that.
Available merge tools
=====================
External merge tools and their properties are configured in the
merge-tools configuration section - see hgrc(5) - but they can often just
be named by their executable.
A merge tool is generally usable if its executable can be found on the
system and if it can handle the merge. The executable is found if it
is an absolute or relative executable path or the name of an
application in the executable search path. The tool is assumed to be
able to handle the merge if it can handle symlinks if the file is a
symlink, if it can handle binary files if the file is binary, and if a
GUI is available if the tool requires a GUI.
There are some internal merge tools which can be used. The internal
merge tools are:
.. internaltoolsmarker
Internal tools are always available and do not require a GUI but will by default
not handle symlinks or binary files.
Choosing a merge tool
=====================
Mercurial uses these rules when deciding which merge tool to use:
1. If a tool has been specified with the --tool option to merge or resolve, it
is used. If it is the name of a tool in the merge-tools configuration, its
configuration is used. Otherwise the specified tool must be executable by
the shell.
2. If the ``HGMERGE`` environment variable is present, its value is used and
must be executable by the shell.
3. If the filename of the file to be merged matches any of the patterns in the
merge-patterns configuration section, the first usable merge tool
corresponding to a matching pattern is used.
4. If ui.merge is set it will be considered next. If the value is not the name
of a configured tool, the specified value is used and must be executable by
the shell. Otherwise the named tool is used if it is usable.
5. If any usable merge tools are present in the merge-tools configuration
section, the one with the highest priority is used.
6. If a program named ``hgmerge`` can be found on the system, it is used - but
it will by default not be used for symlinks and binary files.
7. If the file to be merged is not binary and is not a symlink, then
internal ``:merge`` is used.
8. Otherwise, ``:prompt`` is used.
For historical reason, Mercurial assumes capabilities of internal
merge tools as below while examining rules above, regardless of actual
capabilities of them.
==== =============== ====== =======
step specified via binary symlink
==== =============== ====== =======
1. --tool o o
2. HGMERGE o o
3. merge-patterns o (*) x (*)
4. ui.merge x (*) x (*)
==== =============== ====== =======
If ``merge.strict-capability-check`` configuration is true, Mercurial
checks capabilities of internal merge tools strictly in (*) cases
above. It is false by default for backward compatibility.
.. note::
After selecting a merge program, Mercurial will by default attempt
to merge the files using a simple merge algorithm first. Only if it doesn't
succeed because of conflicting changes will Mercurial actually execute the
merge program. Whether to use the simple merge algorithm first can be
controlled by the premerge setting of the merge tool. Premerge is enabled by
default unless the file is binary or a symlink.
See the merge-tools and ui sections of hgrc(5) for details on the
configuration of merge tools.