nix.conf: builders-use-substitutes

Fixes #937

3 files changed, 23 insertions, 2 deletions

diff --git a/doc/manual/command-ref/conf-file.xml b/doc/manual/command-ref/conf-file.xml
index 87f05fb1c704..2ddca991f19c 100644
--- a/doc/manual/command-ref/conf-file.xml
+++ b/doc/manual/command-ref/conf-file.xml
@@ -321,6 +321,20 @@ false</literal>.</para>
   </varlistentry>
 
 
+  <varlistentry><term><literal>builders-use-substitutes</literal></term>
+
+    <listitem><para>If set to <literal>true</literal>, Nix will instruct
+    remote build machines 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>fallback</literal></term>
 
     <listitem><para>If set to <literal>true</literal>, Nix will fall
diff --git a/src/build-remote/build-remote.cc b/src/build-remote/build-remote.cc
index 445006b327fd..df579729af29 100644
--- a/src/build-remote/build-remote.cc
+++ b/src/build-remote/build-remote.cc
@@ -218,9 +218,11 @@ connected:
             signal(SIGALRM, old);
         }
 
+        auto substitute = settings.buildersUseSubstitutes ? Substitute : NoSubstitute;
+
         {
             Activity act(*logger, lvlTalkative, actUnknown, fmt("copying dependencies to '%s'", storeUri));
-            copyPaths(store, ref<Store>(sshStore), inputs, NoRepair, NoCheckSigs);
+            copyPaths(store, ref<Store>(sshStore), inputs, NoRepair, NoCheckSigs, substitute);
         }
 
         uploadLock = -1;
@@ -240,7 +242,7 @@ connected:
         if (!missing.empty()) {
             Activity act(*logger, lvlTalkative, actUnknown, fmt("copying outputs from '%s'", storeUri));
             setenv("NIX_HELD_LOCKS", concatStringsSep(" ", missing).c_str(), 1); /* FIXME: ugly */
-            copyPaths(ref<Store>(sshStore), store, missing, NoRepair, NoCheckSigs);
+            copyPaths(ref<Store>(sshStore), store, missing, NoRepair, NoCheckSigs, substitute);
         }
 
         return;
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index ae4b78a018e0..af72f7b1e35d 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -138,6 +138,11 @@ public:
     Setting<std::string> builders{this, "@" + nixConfDir + "/machines", "builders",
         "A semicolon-separated list of build machines, in the format of nix.machines."};
 
+    Setting<bool> buildersUseSubstitutes{this, false, "builders-use-substitutes",
+        "Whether build machines should use their own substitutes for obtaining "
+        "build dependencies if possible, rather than waiting for this host to "
+        "upload them."};
+
     Setting<off_t> reservedSize{this, 8 * 1024 * 1024, "gc-reserved-space",
         "Amount of reserved disk space for the garbage collector."};