ci: unset GITLAB_FEATURES envvar to not bust xargs(1) limits

We have started to see the following assert happen in our GitLab CI
pipelines for jobs that use Windows with Meson:

  assertion "bc_ctl.arg_max >= LINE_MAX" failed: file "xargs.c", line 512, function: main

The assert in question verifies that we have enough room available to
pass at least `LINE_MAX` many bytes via the command line. The xargs(1)
binary in those jobs comes from Git for Windows, which in turn sources
the binaries from MSYS2, and has the following limits in place:

  $ & "C:/Program Files/Git/usr/bin/bash.exe" -l -c 'xargs --show-limits </dev/null'
  Your environment variables take up 17373 bytes
  POSIX upper limit on argument length (this system): 12579
  POSIX smallest allowable upper limit on argument length (all systems): 4096
  Maximum length of command we could actually use: 18446744073709546822
  Size of command buffer we are actually using: 12579
  Maximum parallelism (--max-procs must be no greater): 2147483647

What's interesting to see is the limit of 16 exabits for the maximum
command line length. This value might seem a bit high, and it is indeed
the result of an underflow: our environment is larger than the POSIX
upper limit on argument length, and the value is computed by subtracting
the former from the latter. So what we get is the result of `2^64 -
(17373 - 12579)`.

This makes it clear that the problem here is the size of our environment
variables. A listing sorted by length yields the following result:

  $ Get-ChildItem "Env:" |
      Sort-Object { $_.Value.Length } -Descending |
      Select-Object Name, @{Name="Length"; Expression={$_.Value.Length}}
  Name                                          Length
  ----                                          ------
  GITLAB_FEATURES                                 6386
  Path                                             706
  PSModulePath                                     229

The GITLAB_FEATURES environment variable makes up for roughly a third of
the complete environment. This variable is a comma-separated list of
features available for the GitLab instance, and seemingly it has been
growing over time as GitLab added more and more features.

Fix the issue by unsetting the environment variable in "ci/lib.sh". This
ensures that the environment variables are now smaller than the upper
limit on argument length again, and that in turn fixes the assert in
xargs(1).

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

ci/lib.sh

@@ -231,6 +231,10 @@ then
 	distro=$(echo "$CI_JOB_IMAGE" | tr : -)
 elif test true = "$GITLAB_CI"
 then
+	# This environment is multiple kB in size and may cause us to exceed
+	# xargs(1) limits on Windows.
+	unset GITLAB_FEATURES
+
 	CI_TYPE=gitlab-ci
 	CI_BRANCH="$CI_COMMIT_REF_NAME"
 	CI_COMMIT="$CI_COMMIT_SHA"