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.

doc/manual/command-ref/conf-file.xml

@@ -101,9 +101,9 @@ flag, e.g. <literal>--option gc-keep-outputs false</literal>.</para>

    <listitem><para>This option defines the maximum number of jobs
    that Nix will try to build in parallel.  The default is
    <literal>1</literal>.  You should generally set it to the number
    of CPUs in your system (e.g., <literal>2</literal> on an Athlon 64
    X2).  It can be overridden using the <option
    <literal>1</literal>. The special value <literal>auto</literal>
    causes Nix to use the number of CPUs in your system.  It can be
    overridden using the <option
    linkend='opt-max-jobs'>--max-jobs</option> (<option>-j</option>)
    command line switch.</para></listitem>

doc/manual/command-ref/opt-common.xml

@@ -93,8 +93,9 @@
  <term><option>-j</option></term>

  <listitem><para>Sets the maximum number of build jobs that Nix will
  perform in parallel to the specified number.  The default is
  specified by the <link
  perform in parallel to the specified number.  Specify
  <literal>auto</literal> to use the number of CPUs in the system.
  The default is specified by the <link
  linkend='conf-build-max-jobs'><literal>build-max-jobs</literal></link>
  configuration setting, which itself defaults to
  <literal>1</literal>.  A higher value is useful on SMP systems or to

src/libmain/shared.cc

@@ -167,14 +167,17 @@ 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) {
                settings.set(dest, std::to_string(n));
            });
        };

        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");

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");