t/README: clarify test_must_fail description

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Core GIT Tests
==============

This directory holds many test scripts for core GIT tools.  The
first part of this short document describes how to run the tests
and read their output.

When fixing the tools or adding enhancements, you are strongly
encouraged to add tests in this directory to cover what you are
trying to fix or enhance.  The later part of this short document
describes how your test scripts should be organized.


Running Tests
-------------

The easiest way to run tests is to say "make".  This runs all
the tests.

    *** t0000-basic.sh ***
    *   ok 1: .git/objects should be empty after git-init in an empty repo.
    *   ok 2: .git/objects should have 256 subdirectories.
    *   ok 3: git-update-index without --add should fail adding.
    ...
    *   ok 23: no diff after checkout and git-update-index --refresh.
    * passed all 23 test(s)
    *** t0100-environment-names.sh ***
    *   ok 1: using old names should issue warnings.
    *   ok 2: using old names but having new names should not issue warnings.
    ...

Or you can run each test individually from command line, like
this:

    $ sh ./t3001-ls-files-killed.sh
    *   ok 1: git-update-index --add to add various paths.
    *   ok 2: git-ls-files -k to show killed files.
    *   ok 3: validate git-ls-files -k output.
    * passed all 3 test(s)

You can pass --verbose (or -v), --debug (or -d), and --immediate
(or -i) command line argument to the test, or by setting GIT_TEST_OPTS
appropriately before running "make".

--verbose::
        This makes the test more verbose.  Specifically, the
        command being run and their output if any are also
        output.

--debug::
        This may help the person who is developing a new test.
        It causes the command defined with test_debug to run.

--immediate::
        This causes the test to immediately exit upon the first
        failed test.

--long-tests::
        This causes additional long-running tests to be run (where
        available), for more exhaustive testing.

--valgrind::
        Execute all Git binaries with valgrind and exit with status
        126 on errors (just like regular tests, this will only stop
        the test script when running under -i).  Valgrind errors
        go to stderr, so you might want to pass the -v option, too.

        Since it makes no sense to run the tests with --valgrind and
        not see any output, this option implies --verbose.  For
        convenience, it also implies --tee.

--tee::
        In addition to printing the test output to the terminal,
        write it to files named 't/test-results/$TEST_NAME.out'.
        As the names depend on the tests' file names, it is safe to
        run the tests with this option in parallel.

--with-dashes::
        By default tests are run without dashed forms of
        commands (like git-commit) in the PATH (it only uses
        wrappers from ../bin-wrappers).  Use this option to include
        the build directory (..) in the PATH, which contains all
        the dashed forms of commands.  This option is currently
        implied by other options like --valgrind and
        GIT_TEST_INSTALLED.

--root=<directory>::
        Create "trash" directories used to store all temporary data during
        testing under <directory>, instead of the t/ directory.
        Using this option with a RAM-based filesystem (such as tmpfs)
        can massively speed up the test suite.

You can also set the GIT_TEST_INSTALLED environment variable to
the bindir of an existing git installation to test that installation.
You still need to have built this git sandbox, from which various
test-* support programs, templates, and perl libraries are used.
If your installed git is incomplete, it will silently test parts of
your built version instead.

When using GIT_TEST_INSTALLED, you can also set GIT_TEST_EXEC_PATH to
override the location of the dashed-form subcommands (what
GIT_EXEC_PATH would be used for during normal operation).
GIT_TEST_EXEC_PATH defaults to `$GIT_TEST_INSTALLED/git --exec-path`.


Skipping Tests
--------------

In some environments, certain tests have no way of succeeding
due to platform limitation, such as lack of 'unzip' program, or
filesystem that do not allow arbitrary sequence of non-NUL bytes
as pathnames.

You should be able to say something like

    $ GIT_SKIP_TESTS=t9200.8 sh ./t9200-git-cvsexport-commit.sh

and even:

    $ GIT_SKIP_TESTS='t[0-4]??? t91?? t9200.8' make

to omit such tests.  The value of the environment variable is a
SP separated list of patterns that tells which tests to skip,
and either can match the "t[0-9]{4}" part to skip the whole
test, or t[0-9]{4} followed by ".$number" to say which
particular test to skip.

Note that some tests in the existing test suite rely on previous
test item, so you cannot arbitrarily disable one and expect the
remainder of test to check what the test originally was intended
to check.


Naming Tests
------------

The test files are named as:

        tNNNN-commandname-details.sh

where N is a decimal digit.

First digit tells the family:

        0 - the absolute basics and global stuff
        1 - the basic commands concerning database
        2 - the basic commands concerning the working tree
        3 - the other basic commands (e.g. ls-files)
        4 - the diff commands
        5 - the pull and exporting commands
        6 - the revision tree commands (even e.g. merge-base)
        7 - the porcelainish commands concerning the working tree
        8 - the porcelainish commands concerning forensics
        9 - the git tools

Second digit tells the particular command we are testing.

Third digit (optionally) tells the particular switch or group of switches
we are testing.

If you create files under t/ directory (i.e. here) that is not
the top-level test script, never name the file to match the above
pattern.  The Makefile here considers all such files as the
top-level test script and tries to run all of them.  A care is
especially needed if you are creating a common test library
file, similar to test-lib.sh, because such a library file may
not be suitable for standalone execution.


Writing Tests
-------------

The test script is written as a shell script.  It should start
with the standard "#!/bin/sh" with copyright notices, and an
assignment to variable 'test_description', like this:

        #!/bin/sh
        #
        # Copyright (c) 2005 Junio C Hamano
        #

        test_description='xxx test (option --frotz)

        This test registers the following structure in the cache
        and tries to run git-ls-files with option --frotz.'


Source 'test-lib.sh'
--------------------

After assigning test_description, the test script should source
test-lib.sh like this:

        . ./test-lib.sh

This test harness library does the following things:

 - If the script is invoked with command line argument --help
   (or -h), it shows the test_description and exits.

 - Creates an empty test directory with an empty .git/objects
   database and chdir(2) into it.  This directory is 't/trash directory'
   if you must know, but I do not think you care.

 - Defines standard test helper functions for your scripts to
   use.  These functions are designed to make all scripts behave
   consistently when command line arguments --verbose (or -v),
   --debug (or -d), and --immediate (or -i) is given.


End with test_done
------------------

Your script will be a sequence of tests, using helper functions
from the test harness library.  At the end of the script, call
'test_done'.


Test harness library
--------------------

There are a handful helper functions defined in the test harness
library for your script to use.

 - test_expect_success <message> <script>

   This takes two strings as parameter, and evaluates the
   <script>.  If it yields success, test is considered
   successful.  <message> should state what it is testing.

   Example:

        test_expect_success \
            'git-write-tree should be able to write an empty tree.' \
            'tree=$(git-write-tree)'

 - test_expect_failure <message> <script>

   This is NOT the opposite of test_expect_success, but is used
   to mark a test that demonstrates a known breakage.  Unlike
   the usual test_expect_success tests, which say "ok" on
   success and "FAIL" on failure, this will say "FIXED" on
   success and "still broken" on failure.  Failures from these
   tests won't cause -i (immediate) to stop.

 - test_debug <script>

   This takes a single argument, <script>, and evaluates it only
   when the test script is started with --debug command line
   argument.  This is primarily meant for use during the
   development of a new test script.

 - test_done

   Your test script must have test_done at the end.  Its purpose
   is to summarize successes and failures in the test script and
   exit with an appropriate error code.

 - test_tick

   Make commit and tag names consistent by setting the author and
   committer times to defined stated.  Subsequent calls will
   advance the times by a fixed amount.

 - test_commit <message> [<filename> [<contents>]]

   Creates a commit with the given message, committing the given
   file with the given contents (default for both is to reuse the
   message string), and adds a tag (again reusing the message
   string as name).  Calls test_tick to make the SHA-1s
   reproducible.

 - test_merge <message> <commit-or-tag>

   Merges the given rev using the given message.  Like test_commit,
   creates a tag and calls test_tick before committing.

 - test_must_fail <git-command>

   Run a git command and ensure it fails in a controlled way.  Use
   this instead of "! <git-command>".  When git-command dies due to a
   segfault, test_must_fail diagnoses it as an error; "! <git-command>"
   treats it as just another expected failure, which would let such a
   bug go unnoticed.

Tips for Writing Tests
----------------------

As with any programming projects, existing programs are the best
source of the information.  However, do _not_ emulate
t0000-basic.sh when writing your tests.  The test is special in
that it tries to validate the very core of GIT.  For example, it
knows that there will be 256 subdirectories under .git/objects/,
and it knows that the object ID of an empty tree is a certain
40-byte string.  This is deliberately done so in t0000-basic.sh
because the things the very basic core test tries to achieve is
to serve as a basis for people who are changing the GIT internal
drastically.  For these people, after making certain changes,
not seeing failures from the basic test _is_ a failure.  And
such drastic changes to the core GIT that even changes these
otherwise supposedly stable object IDs should be accompanied by
an update to t0000-basic.sh.

However, other tests that simply rely on basic parts of the core
GIT working properly should not have that level of intimate
knowledge of the core GIT internals.  If all the test scripts
hardcoded the object IDs like t0000-basic.sh does, that defeats
the purpose of t0000-basic.sh, which is to isolate that level of
validation in one place.  Your test also ends up needing
updating when such a change to the internal happens, so do _not_
do it and leave the low level of validation to t0000-basic.sh.