SCons: Default `num_jobs` to max CPUs minus 1 if not specified

This doesn't change the behavior when `--jobs`/`-j` is specified as a
command-line argument or in `SCONSFLAGS`.

The SCons hack used to know if `num_jobs` was set by the user is derived
from the MongoDB setup.

We use `os.cpu_count()` for portability (available since Python 3.4).

With 4 CPUs or less, we use the max. With more than 4 we use max - 1 to
preserve some bandwidth for the user's other programs.

(cherry picked from commit ea211225754ba3fc1c2afb290bf26bd714125b24)

.github/actions/godot-build/action.yml

@@ -32,5 +32,5 @@ runs:
           SCONS_CACHE_LIMIT: ${{ inputs.scons-cache-limit }}
       run: |
         echo "Building with flags:" ${{ env.SCONSFLAGS }}
-        scons p=${{ inputs.platform }} target=${{ inputs.target }} tools=${{ inputs.tools }} tests=${{ inputs.tests }} --jobs=2 ${{ env.SCONSFLAGS }}
+        scons p=${{ inputs.platform }} target=${{ inputs.target }} tools=${{ inputs.tools }} tests=${{ inputs.tests }} ${{ env.SCONSFLAGS }}
         ls -l bin/

SConstruct

@@ -360,6 +360,25 @@ if selected_platform in platform_list:
 
     env = env_base.Clone()
 
+    # Default num_jobs to local cpu count if not user specified.
+    # SCons has a peculiarity where user-specified options won't be overridden
+    # by SetOption, so we can rely on this to know if we should use our default.
+    initial_num_jobs = env.GetOption("num_jobs")
+    altered_num_jobs = initial_num_jobs + 1
+    env.SetOption("num_jobs", altered_num_jobs)
+    # os.cpu_count() requires Python 3.4+.
+    if hasattr(os, "cpu_count") and env.GetOption("num_jobs") == altered_num_jobs:
+        cpu_count = os.cpu_count()
+        if cpu_count is None:
+            print("Couldn't auto-detect CPU count to configure build parallelism. Specify it with the -j argument.")
+        else:
+            safer_cpu_count = cpu_count if cpu_count <= 4 else cpu_count - 1
+            print(
+                "Auto-detected %d CPU cores available for build parallelism. Using %d cores by default. You can override it with the -j argument."
+                % (cpu_count, safer_cpu_count)
+            )
+            env.SetOption("num_jobs", safer_cpu_count)
+
     if env["compiledb"]:
         # Generating the compilation DB (`compile_commands.json`) requires SCons 4.0.0 or later.
         from SCons import __version__ as scons_raw_version