Make system-features a store setting

This seems more correct. It also means one can specify the features a
store should support with --store and remote-store=..., which is useful.
I use this to clean up the build remotes test.

src/libstore/build.cc

@@ -1475,7 +1475,7 @@ void DerivationGoal::tryToBuild()
     /* Don't do a remote build if the derivation has the attribute
        `preferLocalBuild' set.  Also, check and repair modes are only
        supported for local builds. */
-    bool buildLocally = buildMode != bmNormal || parsedDrv->willBuildLocally();
+    bool buildLocally = buildMode != bmNormal || parsedDrv->willBuildLocally(worker.store);
 
     /* Is the build hook willing to accept this job? */
     if (!buildLocally) {
@@ -1964,13 +1964,13 @@ void linkOrCopy(const Path & from, const Path & to)
 void DerivationGoal::startBuilder()
 {
     /* Right platform? */
-    if (!parsedDrv->canBuildLocally())
+    if (!parsedDrv->canBuildLocally(worker.store))
         throw Error("a '%s' with features {%s} is required to build '%s', but I am a '%s' with features {%s}",
             drv->platform,
             concatStringsSep(", ", parsedDrv->getRequiredSystemFeatures()),
             worker.store.printStorePath(drvPath),
             settings.thisSystem,
-            concatStringsSep<StringSet>(", ", settings.systemFeatures));
+            concatStringsSep<StringSet>(", ", worker.store.systemFeatures));
 
     if (drv->isBuiltin())
         preloadNSS();
@@ -3179,7 +3179,7 @@ void DerivationGoal::runChild()
                 createDirs(chrootRootDir + "/dev/shm");
                 createDirs(chrootRootDir + "/dev/pts");
                 ss.push_back("/dev/full");
-                if (settings.systemFeatures.get().count("kvm") && pathExists("/dev/kvm"))
+                if (worker.store.systemFeatures.get().count("kvm") && pathExists("/dev/kvm"))
                     ss.push_back("/dev/kvm");
                 ss.push_back("/dev/null");
                 ss.push_back("/dev/random");

src/libstore/machines.cc

@@ -1,6 +1,7 @@
 #include "machines.hh"
 #include "util.hh"
 #include "globals.hh"
+#include "store-api.hh"
 
 #include <algorithm>
 
@@ -48,6 +49,29 @@ bool Machine::mandatoryMet(const std::set<string> & features) const {
         });
 }
 
+ref<Store> Machine::openStore() const {
+    Store::Params storeParams;
+    if (hasPrefix(storeUri, "ssh://")) {
+        storeParams["max-connections"] = "1";
+        storeParams["log-fd"] = "4";
+        if (sshKey != "")
+            storeParams["ssh-key"] = sshKey;
+    }
+    {
+        auto & fs = storeParams["system-features"];
+        auto append = [&](auto feats) {
+            for (auto & f : feats) {
+                if (fs.size() > 0) fs += ' ';
+                fs += f;
+            }
+        };
+        append(supportedFeatures);
+        append(mandatoryFeatures);
+    }
+
+    return nix::openStore(storeUri, storeParams);
+}
+
 void parseMachines(const std::string & s, Machines & machines)
 {
     for (auto line : tokenizeString<std::vector<string>>(s, "\n;")) {

src/libstore/machines.hh

@@ -4,6 +4,8 @@
 
 namespace nix {
 
+class Store;
+
 struct Machine {
 
     const string storeUri;
@@ -28,6 +30,8 @@ struct Machine {
         decltype(supportedFeatures) supportedFeatures,
         decltype(mandatoryFeatures) mandatoryFeatures,
         decltype(sshPublicHostKey) sshPublicHostKey);
+
+    ref<Store> openStore() const;
 };
 
 typedef std::vector<Machine> Machines;

src/libstore/parsed-derivations.cc

@@ -94,7 +94,7 @@ StringSet ParsedDerivation::getRequiredSystemFeatures() const
     return res;
 }
 
-bool ParsedDerivation::canBuildLocally() const
+bool ParsedDerivation::canBuildLocally(Store & localStore) const
 {
     if (drv.platform != settings.thisSystem.get()
         && !settings.extraPlatforms.get().count(drv.platform)
@@ -102,14 +102,14 @@ bool ParsedDerivation::canBuildLocally() const
         return false;
 
     for (auto & feature : getRequiredSystemFeatures())
-        if (!settings.systemFeatures.get().count(feature)) return false;
+        if (!localStore.systemFeatures.get().count(feature)) return false;
 
     return true;
 }
 
-bool ParsedDerivation::willBuildLocally() const
+bool ParsedDerivation::willBuildLocally(Store & localStore) const
 {
-    return getBoolAttr("preferLocalBuild") && canBuildLocally();
+    return getBoolAttr("preferLocalBuild") && canBuildLocally(localStore);
 }
 
 bool ParsedDerivation::substitutesAllowed() const

src/libstore/parsed-derivations.hh

@@ -29,9 +29,9 @@ public:
 
     StringSet getRequiredSystemFeatures() const;
 
-    bool canBuildLocally() const;
+    bool canBuildLocally(Store & localStore) const;
 
-    bool willBuildLocally() const;
+    bool willBuildLocally(Store & localStore) const;
 
     bool substitutesAllowed() const;
 };

src/libstore/store-api.hh

@@ -164,6 +164,10 @@ public:
 
     Setting<bool> wantMassQuery{this, false, "want-mass-query", "whether this substituter can be queried efficiently for path validity"};
 
+    Setting<StringSet> systemFeatures{this, settings.systemFeatures,
+        "system-features",
+        "Optional features that the system this store builds on implements (like \"kvm\")."};
+
 protected:
 
     struct PathInfoCacheValue {