Don't apply the CPU affinity hack to nix-shell (and other Perl programs)

As discovered by Todd Veldhuizen, the shell started by nix-shell has
its affinity set to a single CPU.  This is because nix-shell connects
to the Nix daemon, which causes the affinity hack to be applied.  So
we turn this off for Perl programs.

perl/lib/Nix/Store.xs

@@ -20,6 +20,7 @@ void doInit()
     if (!store) {
         try {
             settings.processEnvironment();
+            settings.lockCPU = false;
             store = openStore();
         } catch (Error & e) {
             croak(e.what());

src/libstore/globals.cc

@@ -54,6 +54,7 @@ Settings::Settings()
     gcKeepDerivations = true;
     autoOptimiseStore = false;
     envKeepDerivations = false;
+    lockCPU = getEnv("NIX_AFFINITY_HACK", "1") == "1";
 }
 
 

src/libstore/globals.hh

@@ -183,6 +183,9 @@ struct Settings {
        (to prevent them from being GCed). */
     bool envKeepDerivations;
 
+    /* Whether to lock the Nix client and worker to the same CPU. */
+    bool lockCPU;
+
 private:
     SettingsMap settings, overrides;
 

src/libstore/remote-store.cc

@@ -73,7 +73,7 @@ void RemoteStore::openConnection(bool reserveSpace)
         writeInt(PROTOCOL_VERSION, to);
 
         if (GET_PROTOCOL_MINOR(daemonVersion) >= 14) {
-            int cpu = lockToCurrentCPU();
+            int cpu = settings.lockCPU ? lockToCurrentCPU() : -1;
             if (cpu != -1) {
                 writeInt(1, to);
                 writeInt(cpu, to);

src/libutil/affinity.cc

@@ -33,13 +33,12 @@ void setAffinityTo(int cpu)
 int lockToCurrentCPU()
 {
 #if HAVE_SCHED_SETAFFINITY
-    if (getEnv("NIX_AFFINITY_HACK", "1") == "1") {
     int cpu = sched_getcpu();
     if (cpu != -1) setAffinityTo(cpu);
     return cpu;
-    }
+#else
-#endif
     return -1;
+#endif
 }