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parsers: fail fast if Python has wrong minor version (issue4110)...
parsers: fail fast if Python has wrong minor version (issue4110) This change causes an informative ImportError to be raised when importing the parsers extension module if the minor version of the currently-running Python interpreter doesn't match that of the Python used when compiling the extension module. This change also exposes a parsers.versionerrortext constant in the C implementation of the module. Its presence can be used to determine whether this behavior is present in a version of the module. The value of the constant is the leading text of the ImportError raised and is set to "Python minor version mismatch". Here is an example of what the new error looks like: Traceback (most recent call last): File "test.py", line 1, in <module> import mercurial.parsers ImportError: Python minor version mismatch: The Mercurial extension modules were compiled with Python 2.7.6, but Mercurial is currently using Python with sys.hexversion=33883888: Python 2.5.6 (r256:88840, Nov 18 2012, 05:37:10) [GCC 4.2.1 Compatible Apple Clang 4.1 ((tags/Apple/clang-421.11.66))] at: /opt/local/Library/Frameworks/Python.framework/Versions/2.5/Resources/ Python.app/Contents/MacOS/Python The reason for raising an error in this scenario is that Python's C API is known not to be compatible from minor version to minor version, even if sys.api_version is the same. See for example this Python bug report about incompatibilities between 2.5 and 2.6+: http://bugs.python.org/issue8118 These incompatibilities can cause Mercurial to break in mysterious, unforeseen ways. For example, when Mercurial compiled with Python 2.7 was run with 2.5, the following crash occurred when running "hg status": http://bz.selenic.com/show_bug.cgi?id=4110 After this crash was fixed, running with Python 2.5 no longer crashes, but the following puzzling behavior still occurs: $ hg status ... File ".../mercurial/changelog.py", line 123, in __init__ revlog.revlog.__init__(self, opener, "00changelog.i") File ".../mercurial/revlog.py", line 251, in __init__ d = self._io.parseindex(i, self._inline) File ".../mercurial/revlog.py", line 158, in parseindex index, cache = parsers.parse_index2(data, inline) TypeError: data is not a string which can be reproduced more simply with: import mercurial.parsers as parsers parsers.parse_index2("", True) Both the crash and the TypeError occurred because the Python C API's PyString_Check() returns the wrong value when the C header files from Python 2.7 are run with Python 2.5. This is an example of an incompatibility of the sort mentioned in the Python bug report above. Failing fast with an informative error message results in a better user experience in cases like the above. The information in the ImportError also simplifies troubleshooting for those on Mercurial mailing lists, the bug tracker, etc. This patch only adds the version check to parsers.c, which is sufficient to affect command-line commands like "hg status" and "hg summary". An idea for a future improvement is to move the version-checking C code to a more central location, and have it run when importing all Mercurial extension modules and not just parsers.c.
Simon Heimberg - - Load All Authors

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merge-tools.txt
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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. Here, binary capabilities of the
merge tool are not considered.
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. The merge of the file fails and must be resolved before commit.
.. 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 Mercurial will 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.