Travis updates.

* sg/travis-linux32-sanity:
  travis-ci: don't fail if user already exists on 32 bit Linux build job
  travis-ci: don't run the test suite as root in the 32 bit Linux build
  travis-ci: don't repeat the path of the cache directory
  travis-ci: use 'set -e' in the 32 bit Linux build job
  travis-ci: use 'set -x' for the commands under 'su' in the 32 bit Linux build

The split-index mode had a few corner case bugs fixed.

* tg/split-index-fixes:
  travis: run tests with GIT_TEST_SPLIT_INDEX
  split-index: don't write cache tree with null oid entries
  read-cache: fix reading the shared index for other repos

The 32 bit Linux build job runs in a Docker container, which lends
itself to running and debugging locally, too.  Especially during
debugging one usually doesn't want to start with a fresh container
every time, to save time spent on installing a bunch of dependencies.
However, that doesn't work quite smootly, because the script running
in the container always creates a new user, which then must be removed
every time before subsequent executions, or the build script fails.

Make this process more convenient and don't try to create that user if
it already exists and has the right user ID in the container, so
developers don't have to bother with running a 'userdel' each time
before they run the build script.

The build job on Travis CI always starts with a fresh Docker
container, so this change doesn't make a difference there.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Travis CI runs the 32 bit Linux build job in a Docker container, where
all commands are executed as root by default.  Therefore, ever since
we added this build job in 88dedd5e7 (Travis: also test on 32-bit
Linux, 2017-03-05), we have a bit of code to create a user in the
container matching the ID of the host user and then to run the test
suite as this user.  Matching the host user ID is important, because
otherwise the host user would have no access to any files written by
processes running in the container, notably the logs of failed tests
couldn't be included in the build job's trace log.

Alas, this piece of code never worked, because it sets the variable
holding the user name ($CI_USER) in a subshell, meaning it doesn't
have any effect by the time we get to the point to actually use the
variable to switch users with 'su'.  So all this time we were running
the test suite as root.

Reorganize that piece of code in 'ci/run-linux32-build.sh' a bit to
avoid that problematic subshell and to ensure that we switch to the
right user.  Furthermore, make the script's optional host user ID
option mandatory, so running the build accidentally as root will
become harder when debugging locally.  If someone really wants to run
the test suite as root, whatever the reasons might be, it'll still be
possible to do so by explicitly passing '0' as host user ID.

Finally, one last catch: since commit 7e72cfcee (travis-ci: save prove
state for the 32 bit Linux build, 2017-12-27) the 'prove' test harness
has been writing its state to the Travis CI cache directory from
within the Docker container while running as root.  After this patch
'prove' will run as a regular user, so in future build jobs it won't
be able overwrite a previously written, still root-owned state file,
resulting in build job failures.  To resolve this we should manually
delete caches containing such root-owned files, but that would be a
hassle.  Instead, work this around by changing the owner of the whole
contents of the cache directory to the host user ID.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Some of our 'ci/*' scripts repeat the name or full path of the Travis
CI cache directory, and the following patches will add new places
using that path.

Use a variable to refer to the path of the cache directory instead, so
it's hard-coded only in a single place.

Pay extra attention to the 32 bit Linux build: it runs in a Docker
container, so pass the path of the cache directory from the host to
the container in an environment variable.  Note that an environment
variable passed this way is exported inside the container, therefore
its value is directly available in the 'su' snippet even though that
snippet is single quoted.  Furthermore, use the variable in the
container only if it's been assigned a non-empty value, to prevent
errors when someone is running or debugging the Docker build locally,
because in that case the variable won't be set as there won't be any
Travis CI cache.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

The script 'ci/run-linux32-build.sh' running inside the Docker
container of the 32 bit Linux build job uses an && chain to break the
build if one of the commands fails.  This is problematic for two
reasons:

  - The && chain is broken, because there is this in the middle:

    test -z $HOST_UID || (CI_USER="ci" && useradd -u $HOST_UID $CI_USER) &&

    Luckily it is broken in a way that it didn't lead to false
    successes.  If installing dependencies fails, then the rest of the
    first && chain is skipped and execution resumes  after the ||
    operator.  At that point $HOST_UID is still unset, causing
    'useradd' to error out with "invalid user ID 'ci'", which in turn
    causes the second && chain to abort the script and thus break the
    build.

  - All other 'ci/*' scripts use 'set -e' to break the build if one of
    the commands fails.  This inconsistency among these scripts is
    asking for trouble: I forgot about the && chain more than once
    while working on this patch series.

Enable 'set -e' for the whole script and for the commands executed
under 'su' as well.

While touching every line in the 'su' command block anyway, change
their indentation to use a tab instead of spaces.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Split index mode only has a few dedicated tests, but as the index is
involved in nearly every git operation, this doesn't quite cover all the
ways repositories with split index can break.  To use split index mode
throughout the test suite a GIT_TEST_SPLIT_INDEX environment variable
can be set, which makes git split the index at random and thus
excercises the functionality much more thoroughly.

As this is not turned on by default, it is not executed nearly as often
as the test suite is run, so occationally breakages slip through.  Try
to counteract that by running the test suite with GIT_TEST_SPLIT_INDEX
mode turned on on travis.

To avoid using too many cycles on travis only run split index mode in
the linux-gcc target only.  The Linux build was chosen over the Mac OS
builds because it tends to be much faster to complete.

The linux gcc build was chosen over the linux clang build because the
linux clang build is the fastest build, so it can serve as an early
indicator if something is broken and we want to avoid spending the extra
cycles of running the test suite twice for that.

Helped-by: Lars Schneider <larsxschneider@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Every once in a while our explicit .gitignore files get out of sync
when our build process learns to create new artifacts, like test
helper executables, but the .gitignore files are not updated
accordingly.

Use Travis CI to help catch such issues earlier: check that there are
no untracked files at the end of any build jobs building Git (i.e. the
64 bit Clang and GCC Linux and OSX build jobs, plus the GETTEXT_POISON
and 32 bit Linux build jobs) or its documentation, and fail the build
job if there are any present.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

The Clang and GCC 64 bit Linux build jobs download and store the P4
and Git LFS executables under the current directory, which is the
working tree that we are about to build and test.  This means that Git
commands like 'status' or 'ls-files' would list these files as
untracked.  The next commit is about to make sure that there are no
untracked files present after the build, and the downloaded
executables in the working tree are interfering with those upcoming
checks.

Therefore, let's download P4 and Git LFS in the home directory,
outside of the working tree.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>