mtest: move determine_worker_count to utils, generalize

It is useful to apply a limit to the number of processes even outside "meson test",
and specifically for clang tools.  In preparation for this, generalize
determine_worker_count() to accept a variable MESON_NUM_PROCESSES instead of
MESON_TESTTHREADS, and use it throughout instead of multiprocessing.cpu_count().

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 12 insertions, 3 deletions

diff --git a/docs/markdown/Unit-tests.md b/docs/markdown/Unit-tests.md
index 898366095..13f6093f2 100644
--- a/docs/markdown/Unit-tests.md
+++ b/docs/markdown/Unit-tests.md
@@ -83,16 +83,18 @@ possible.
 
 By default Meson uses as many concurrent processes as there are cores
 on the test machine. You can override this with the environment
-variable `MESON_TESTTHREADS` like this.
+variable `MESON_TESTTHREADS` or, *since 1.7.0*, `MESON_NUM_PROCESSES`:
 
 ```console
-$ MESON_TESTTHREADS=5 meson test
+$ MESON_NUM_PROCESSES=5 meson test
 ```
 
-Setting `MESON_TESTTHREADS` to 0 enables the default behavior (core
+Setting `MESON_NUM_PROCESSES` to 0 enables the default behavior (core
 count), whereas setting an invalid value results in setting the job
 count to 1.
 
+If both environment variables are present, `MESON_NUM_PROCESSES` prevails.
+
 ## Priorities
 
 *(added in version 0.52.0)*
diff --git a/docs/markdown/snippets/num-processes.md b/docs/markdown/snippets/num-processes.md
new file mode 100644
index 000000000..e4ffdd1dc
--- /dev/null
+++ b/docs/markdown/snippets/num-processes.md
@@ -0,0 +1,7 @@
+## Control the number of child processes with an environment variable
+
+Previously, `meson test` checked the `MESON_TESTTHREADS` variable to control
+the amount of parallel jobs to run; this was useful when `meson test` is
+invoked through `ninja test` for example.  With this version, a new variable
+`MESON_NUM_PROCESSES` is supported with a broader scope: in addition to
+`meson test`, it is also used by the `external_project` module.