draft attempt at enabling specific number of CPUs.

reprotest/build.py

@@ -333,12 +333,19 @@ def aslr(ctx, build, vary):
 
 def num_cpus(ctx, build, vary):
     _ = build
-    _ = _.append_setup_exec_raw('CPU_MAX=$(nproc)')
+    if ctx.num_cpus.cpus:
+        _ = _.append_setup_exec_raw('CPU_MAX=$(nproc)')
+    else:
+        _ = _.append_setup_exec_raw('CPU_MAX=%s' % ctx.num_cpus.cpus)
     _ = _.append_setup_exec_raw('CPU_MIN=$({ echo $CPU_MAX; echo %s; } | sort -n | head -n1)' % ctx.min_cpus)
     if ctx.min_cpus <= 0:
         raise ValueError("--min-cpus must be a positive integer: " % ctx.min_cpus)
     if not vary:
         _ = _.append_setup_exec_raw('CPU_NUM=$CPU_MIN')
+    if ctx.num_cpus.cpus:
+        _ = _.append_setup_exec_raw('CPU_NUM=$(if [ $CPU_MIN = $CPU_MAX ]; \
+            then echo $CPU_MIN; echo >&2 "only 1 CPU is available; num_cpus is ineffective"; \
+            else echo %s; fi)' % ctx.num_cpus.cpus)
     else:
         # random number between min_cpus and $(nproc)
         _ = _.append_setup_exec_raw('CPU_NUM=$(if [ $CPU_MIN = $CPU_MAX ]; \
@@ -582,6 +589,11 @@ class LocalesVariation(collections.namedtuple('_LocaleVariation', 'locale')):
     def default(cls):
         return cls(mdiffconf.strlist_set(";"))
 
+class NumCpusVariation(collections.namedtuple('_LocaleVariation', 'cpus')):
+    @classmethod
+    def default(cls):
+        return None
+
 class VariationSpec(mdiffconf.ImmutableNamespace):
     @classmethod
     def default(cls, variations=VARIATIONS):
@@ -591,6 +603,7 @@ class VariationSpec(mdiffconf.ImmutableNamespace):
             "time": TimeVariation.default(),
             "domain_host": DomainHostVariation.default(),
             "locales": LocalesVariation.default(),
+            "num_cpus": NumCpusVariation.default(),
         }
         return cls(**{k: default_overrides.get(k, True) for k in variations})