Support auto-configuration of build-max-jobs

"build-max-jobs" and the "-j" option can now be set to "auto" to use
the number of CPUs in the system. (Unlike build-cores, it doesn't use
0 to imply auto-configuration, because a) magic values are a bad idea
in general; b) 0 is a legitimate value used to disable local
building.)

Fixes #1198.

2 files changed, 12 insertions, 2 deletions

diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 53fa83fe0dee..326202d295fb 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -167,6 +167,10 @@ struct LegacyArgs : public MixCommonArgs
             settings.set("build-fallback", "true");
         });
 
+        mkFlag1('j', "max-jobs", "jobs", "maximum number of parallel builds", [=](std::string s) {
+            settings.set("build-max-jobs", s);
+        });
+
         auto intSettingAlias = [&](char shortName, const std::string & longName,
             const std::string & description, const std::string & dest) {
             mkFlag<unsigned int>(shortName, longName, description, [=](unsigned int n) {
@@ -174,7 +178,6 @@ struct LegacyArgs : public MixCommonArgs
             });
         };
 
-        intSettingAlias('j', "max-jobs", "maximum number of parallel builds", "build-max-jobs");
         intSettingAlias(0, "cores", "maximum number of CPU cores to use inside a build", "build-cores");
         intSettingAlias(0, "max-silent-time", "number of seconds of silence before a build is killed", "build-max-silent-time");
         intSettingAlias(0, "timeout", "number of seconds before a build is killed", "build-timeout");
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 12e2a3cf43dd..07af629260af 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -147,7 +147,14 @@ int Settings::get(const string & name, int def)
 void Settings::update()
 {
     _get(tryFallback, "build-fallback");
-    _get(maxBuildJobs, "build-max-jobs");
+
+    auto s = get("build-max-jobs", std::string("1"));
+    if (s == "auto")
+        maxBuildJobs = std::max(1U, std::thread::hardware_concurrency());
+    else
+        if (!string2Int(s, maxBuildJobs))
+            throw Error("configuration setting ‘build-max-jobs’ should be ‘auto’ or an integer");
+
     _get(buildCores, "build-cores");
     _get(thisSystem, "system");
     _get(maxSilentTime, "build-max-silent-time");