Don't substitute derivations that have preferLocalBuild set

In particular this means that "trivial" derivations such as writeText
are not substituted, reducing the number of GET requests to the binary
cache by about 200 on a typical NixOS configuration.

src/libstore/build.cc

@@ -1072,7 +1072,7 @@ void DerivationGoal::haveDerivation()
     /* We are first going to try to create the invalid output paths
        through substitutes.  If that doesn't work, we'll build
        them. */
-    if (settings.useSubstitutes)
+    if (settings.useSubstitutes && !willBuildLocally(drv))
         foreach (PathSet::iterator, i, invalidOutputs)
             addWaitee(worker.makeSubstitutionGoal(*i, repair));
 
@@ -1273,6 +1273,12 @@ static bool canBuildLocally(const string & platform)
 }
 
 
+bool willBuildLocally(Derivation & drv)
+{
+    return drv.env["preferLocalBuild"] == "1" && canBuildLocally(drv.platform);
+}
+
+
 void DerivationGoal::tryToBuild()
 {
     trace("trying to build");
@@ -1337,11 +1343,10 @@ void DerivationGoal::tryToBuild()
 
     /* Don't do a remote build if the derivation has the attribute
        `preferLocalBuild' set. */
-    bool preferLocalBuild =
-        drv.env["preferLocalBuild"] == "1" && canBuildLocally(drv.platform);
+    bool buildLocally = willBuildLocally(drv);
 
     /* Is the build hook willing to accept this job? */
-    if (!preferLocalBuild) {
+    if (!buildLocally) {
         switch (tryBuildHook()) {
             case rpAccept:
                 /* Yes, it has started doing so.  Wait until we get
@@ -1364,7 +1369,7 @@ void DerivationGoal::tryToBuild()
        derivation prefers to be done locally, do it even if
        maxBuildJobs is 0. */
     unsigned int curBuilds = worker.getNrLocalBuilds();
-    if (curBuilds >= settings.maxBuildJobs && !(preferLocalBuild && curBuilds == 0)) {
+    if (curBuilds >= settings.maxBuildJobs && !(buildLocally && curBuilds == 0)) {
         worker.waitForBuildSlot(shared_from_this());
         outputLocks.unlock();
         return;

src/libstore/misc.cc

@@ -120,7 +120,8 @@ void queryMissing(StoreAPI & store, const PathSet & targets,
                 if (invalid.empty()) continue;
 
                 todoDrv.insert(*i);
-                if (settings.useSubstitutes) query.insert(invalid.begin(), invalid.end());
+                if (settings.useSubstitutes && !willBuildLocally(drv))
+                    query.insert(invalid.begin(), invalid.end());
             }
 
             else {
@@ -143,7 +144,7 @@ void queryMissing(StoreAPI & store, const PathSet & targets,
 
             PathSet outputs;
             bool mustBuild = false;
-            if (settings.useSubstitutes) {
+            if (settings.useSubstitutes && !willBuildLocally(drv)) {
                 foreach (DerivationOutputs::iterator, j, drv.outputs) {
                     if (!wantOutput(j->first, i2.second)) continue;
                     if (!store.isValidPath(j->second.path)) {

src/libstore/misc.hh

@@ -32,5 +32,7 @@ void queryMissing(StoreAPI & store, const PathSet & targets,
     PathSet & willBuild, PathSet & willSubstitute, PathSet & unknown,
     unsigned long long & downloadSize, unsigned long long & narSize);
 
+bool willBuildLocally(Derivation & drv);
+
 
 }