libstore: change max-jobs default to auto

doc/manual/src/release-notes/rl-next.md

@@ -1 +1,3 @@
 # Release X.Y (202?-??-??)
+
+- The `max-jobs` setting now defaults to `auto`.

src/libstore/globals.cc

@@ -143,15 +143,19 @@ std::vector<Path> getUserConfigFiles()
     return files;
 }
 
+unsigned int Settings::getDefaultThreads()
+{
+    return std::max(1U, std::thread::hardware_concurrency());
+}
+
 unsigned int Settings::getDefaultCores()
 {
-    const unsigned int concurrency = std::max(1U, std::thread::hardware_concurrency());
     const unsigned int maxCPU = getMaxCPU();
 
     if (maxCPU > 0)
       return maxCPU;
     else
-      return concurrency;
+      return getDefaultThreads();
 }
 
 StringSet Settings::getDefaultSystemFeatures()

src/libstore/globals.hh

@@ -56,6 +56,8 @@ const uint32_t maxIdsPerBuild =
 
 class Settings : public Config {
 
+    unsigned int getDefaultThreads();
+
     unsigned int getDefaultCores();
 
     StringSet getDefaultSystemFeatures();
@@ -147,15 +149,14 @@ public:
         "the log to show if a build fails."};
 
     MaxBuildJobsSetting maxBuildJobs{
-        this, 1, "max-jobs",
+        this, getDefaultThreads(), "max-jobs",
         R"(
           This option defines the maximum number of jobs that Nix will try to
-          build in parallel. The default is `1`. The special value `auto`
-          causes Nix to use the number of CPUs in your system. `0` is useful
-          when using remote builders to prevent any local builds (except for
-          `preferLocalBuild` derivation attribute which executes locally
-          regardless). It can be overridden using the `--max-jobs` (`-j`)
-          command line switch.
+          build in parallel. The default is `auto` which causes Nix to use
+          the number of CPUs in your system. `0` is useful when using remote
+          builders to prevent any local builds (except for `preferLocalBuild`
+          derivation attribute which executes locally regardless). It can be
+          overridden using the `--max-jobs` (`-j`) command line switch.
         )",
         {"build-max-jobs"}};