testsuite: use xdist for parallel test running

This is capped at 12 because 3.7 seconds of startup is painful enough
and 5.5 seconds with 24 was more annoying.

Change-Id: I327db40fd98deaa5330cd9cf6de99fb07b2c1cb0

doc/manual/rl-next/pytest-suite.md

@@ -0,0 +1,10 @@
+---
+synopsis: "The beginnings of a new pytest-based functional test suite"
+category: Development
+cls: [2036, 2037]
+credits: jade
+---
+
+The existing integration/functional test suite is based on a large volume of shell scripts.
+This often makes it somewhat challenging to debug at the best of times.
+The goal of the pytest test suite is to make tests have more obvious dependencies on files and to make tests more concise and easier to write, as well as making new testing methods like snapshot testing easy.

package.nix

@@ -245,6 +245,7 @@ stdenv.mkDerivation (finalAttrs: {
     [
       python3
       python3.pkgs.pytest
+      python3.pkgs.pytest-xdist
       meson
       ninja
       cmake
@@ -479,6 +480,7 @@ stdenv.mkDerivation (finalAttrs: {
             # FIXME: these have to be added twice due to the nix shell using a
             # wrapped python instead of build inputs for its python inputs
             p.pytest
+            p.pytest-xdist
 
             p.yapf
             p.python-frontmatter

tests/functional2/README.md

@@ -18,7 +18,7 @@ Its primary goal is to make tests more concise, more self-contained, easier to w
 - [ ] Web server fixture: we don't test our network functionality because background processes are hard and this is simply goofy.
   We could just test it.
 - [ ] Nix daemon fixture.
-- [ ] Parallelism via pytest-xdist.
+- [x] Parallelism via pytest-xdist.
 
 [pytest-expect-test]: https://pypi.org/project/pytest-expect-test/
 [pytest-insta]: https://pypi.org/project/pytest-insta/

tests/functional2/meson.build

@@ -1,3 +1,10 @@
+xdist_opts = [
+  # auto number of workers, max 12 jobs
+  '-n', 'auto', '--maxprocesses=12',
+  # group tests by module or class; ensures that any setup work occurs as little as possible
+  '--dist=loadscope',
+]
+
 # surprisingly, this actually works even if PATH is set to something before
 # meson gets hold of it. neat!
 functional2_env = environment()
@@ -7,7 +14,10 @@ test(
   'functional2',
   python,
   args : [
-    '-m', 'pytest', meson.current_source_dir()
+    '-m', 'pytest',
+    '-v',
+    xdist_opts,
+    meson.current_source_dir()
   ],
   env : functional2_env,
   # FIXME: Although we can trivially use TAP here with pytest-tap, due to a meson bug, it is unusable.