nix.conf: add build-hook-use-substitutes

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

@@ -302,6 +302,20 @@ flag, e.g. <literal>--option gc-keep-outputs false</literal>.</para>
   </varlistentry>
 
 
+  <varlistentry><term><literal>build-hook-use-substitutes</literal></term>
+
+    <listitem><para>If set to <literal>true</literal>, Nix will instruct
+    remote build hooks to use their own binary substitutes if available. In
+    practical terms, this means that remote hosts will fetch as many build
+    dependencies as possible from their own substitutes (e.g, from
+    <literal>cache.nixos.org</literal>), instead of waiting for this host to
+    upload them all. This can drastically reduce build times if the network
+    connection between this computer and the remote build host is slow. Defaults
+    to <literal>false</literal>.</para></listitem>
+
+  </varlistentry>
+
+
   <varlistentry><term><literal>build-fallback</literal></term>
 
     <listitem><para>If set to <literal>true</literal>, Nix will fall

src/build-remote/build-remote.cc

@@ -268,7 +268,8 @@ int main (int argc, char * * argv)
         }
         alarm(0);
         signal(SIGALRM, old);
-        copyPaths(store, ref<Store>(sshStore), inputs);
+        copyPaths(store, ref<Store>(sshStore), inputs,
+                  settings.buildHookUseSubstitutes);
         uploadLock = -1;
 
         cerr << "building ‘" << drvPath << "’ on ‘" << hostName << "’\n";
@@ -277,7 +278,8 @@ int main (int argc, char * * argv)
         std::remove_if(outputs.begin(), outputs.end(), [=](const Path & path) { return store->isValidPath(path); });
         if (!outputs.empty()) {
             setenv("NIX_HELD_LOCKS", concatStringsSep(" ", outputs).c_str(), 1); /* FIXME: ugly */
-            copyPaths(ref<Store>(sshStore), store, outputs);
+            copyPaths(ref<Store>(sshStore), store, outputs,
+                      settings.buildHookUseSubstitutes);
         }
         return;
     });

src/libstore/globals.cc

@@ -59,6 +59,7 @@ Settings::Settings()
     lockCPU = getEnv("NIX_AFFINITY_HACK", "1") == "1";
     showTrace = false;
     enableImportNative = false;
+    buildHookUseSubstitutes = false;
 }
 
 
@@ -183,6 +184,7 @@ void Settings::update()
     _get(enableImportNative, "allow-unsafe-native-code-during-evaluation");
     _get(useCaseHack, "use-case-hack");
     _get(preBuildHook, "pre-build-hook");
+    _get(buildHookUseSubstitutes, "build-hook-use-substitutes");
     _get(keepGoing, "keep-going");
     _get(keepFailed, "keep-failed");
 }

src/libstore/globals.hh

@@ -193,6 +193,9 @@ struct Settings {
        build settings */
     Path preBuildHook;
 
+    /* Whether the build hook should be instructed to use substitutes. */
+    bool buildHookUseSubstitutes;
+
 private:
     SettingsMap settings, overrides;