* Support for system-wide distributed / multi-platform builds.

Just set nix.distributedBuilds to true and nix.buildMachines 
  to a list of machines that can perform Nix builds via SSH,
  and local builds will be forwarded appropriately.  So now
  any user can say something like

    nix-build /etc/nixos/nixpkgs/ --arg system '"powerpc-linux"' -A libxml2

  and the build for powerpc-linux will be forwarded to a machine 
  of that type.

svn path=/nixos/trunk/; revision=9696

etc/default.nix

@@ -1,4 +1,6 @@
-{config, pkgs, upstartJobs, systemPath, wrapperDir, defaultShell, extraEtc}:
+{ config, pkgs, upstartJobs, systemPath, wrapperDir
+, defaultShell, extraEtc, nixEnvVars
+}:
 
 let 
 
@@ -106,6 +108,7 @@ import ../helpers/make-etc.nix {
         inherit (pkgs) systemKernel glibc;
         timeZone = config.time.timeZone;
         defaultLocale = config.i18n.defaultLocale;
+        inherit nixEnvVars;
       };
       target = "profile";
     }
@@ -213,6 +216,16 @@ UseSTARTTLS=${if config.networking.defaultMailServer.useSTARTTLS then "YES" else
     ]
   )
 
+  # List of machines for distributed Nix builds in the format expected
+  # by build-remote.pl.
+  ++ optional config.nix.distributedBuilds {
+    source = pkgs.writeText "nix.machines"
+      (pkgs.lib.concatStrings (map (machine:
+        "${machine.sshUser}@${machine.hostName} ${machine.system} ${machine.sshKey} ${toString machine.maxJobs}\n"
+      ) config.nix.buildMachines));
+    target = "nix.machines";
+  }
+    
   # Additional /etc files declared by Upstart jobs.
   ++ extraEtc;
   

etc/profile.sh

@@ -1,6 +1,5 @@
 export PATH=@wrapperDir@:/var/run/current-system/sw/bin:/var/run/current-system/sw/sbin
 export MODULE_DIR=@systemKernel@/lib/modules
-export NIX_CONF_DIR=/nix/etc/nix
 export NIXPKGS_CONFIG=/nix/etc/config.nix
 export PAGER=less
 export TZ=@timeZone@
@@ -27,6 +26,10 @@ else
 fi
 
 
+# Set up the environment variables for running Nix.
+@nixEnvVars@
+
+
 # Set up the per-user profile.
 NIX_USER_PROFILE_DIR=/nix/var/nix/profiles/per-user/$USER
 mkdir -m 0755 -p $NIX_USER_PROFILE_DIR

system/activate-configuration.sh

@@ -67,6 +67,8 @@ mkdir -m 0755 -p /var/run/console # for pam_console
 touch /var/run/utmp # must exist
 chmod 644 /var/run/utmp
 
+mkdir -m 0755 -p /var/run/nix/current-load # for distributed builds
+
 mkdir -m 0755 -p /var/log
 
 touch /var/log/wtmp # must exist

system/options.nix

@@ -1283,6 +1283,52 @@
       ";
     };
 
+    distributedBuilds = mkOption {
+      default = false;
+      description = "
+        Whether to distribute builds to the machines listed in
+        <option>nix.buildMachines</option>.
+      ";
+    };
+
+    buildMachines = mkOption {
+      example = [
+        { hostName = "voila.labs.cs.uu.nl";
+          sshUser = "nix";
+          sshKey = "/root/.ssh/id_buildfarm";
+          system = "powerpc-darwin";
+          maxJobs = 1;
+        }
+        { hostName = "linux64.example.org";
+          sshUser = "buildfarm";
+          sshKey = "/root/.ssh/id_buildfarm";
+          system = "x86_64-linux";
+          maxJobs = 2;
+        }
+      ];
+      description = "
+        This option lists the machines to be used if distributed
+        builds are enabled (see
+        <option>nix.distributedBuilds</option>).  Nix will perform
+        derivations on those machines via SSh by copying the inputs to
+        the Nix store on the remote machine, starting the build, then
+        copying the output back to the local Nix store.  Each element
+        of the list should be an attribute set containing the
+        machine's host name (<varname>hostname</varname>), the user
+        name to be used for the SSH connection
+        (<varname>sshUser</varname>), the Nix system type
+        (<varname>system</varname>, e.g.,
+        <literal>\"i686-linux\"</literal>), the maximum number of jobs
+        to be run in parallel on that machine
+        (<varname>maxJobs</varname>), and the path to the SSH private
+        key to be used to connect (<varname>sshKey</varname>).  The
+        SSH private key should not have a passphrase, and the
+        corresponding public key should be added to
+        <filename>~<replaceable>sshUser<replaceable>/authorized_keys</filename>
+        on the remote machine.
+      ";
+    };
+
   };
 
 

system/system.nix

@@ -153,24 +153,35 @@ rec {
   };
 
 
+  # Environment variables for running Nix.
+  nixEnvVars =
+    "export NIX_CONF_DIR=/nix/etc/nix\n" +
+    (if config.nix.distributedBuilds then
+      "export NIX_BUILD_HOOK=${nix}/libexec/nix/build-remote.pl\n" +
+      "export NIX_REMOTE_SYSTEMS=/etc/nix.machines\n" +
+      "export NIX_CURRENT_LOAD=/var/run/nix/current-load\n"
+    else "");
+
+              
   # The services (Upstart) configuration for the system.
   upstartJobs = import ../upstart-jobs/default.nix {
-    inherit config pkgs nix modprobe nssModulesPath;
+    inherit config pkgs nix modprobe nssModulesPath nixEnvVars;
   };
 
 
   # The static parts of /etc.
   etc = import ../etc/default.nix {
-    inherit config pkgs upstartJobs systemPath wrapperDir defaultShell;
+    inherit config pkgs upstartJobs systemPath wrapperDir
+      defaultShell nixEnvVars;
     extraEtc = pkgs.lib.concatLists (map (job: job.extraEtc) upstartJobs.jobs);
   };
 
   # Font aggregation
-	fontDir = import ./fontdir.nix {
+  fontDir = import ./fontdir.nix {
-		inherit (pkgs) stdenv;
+    inherit (pkgs) stdenv;
-		inherit pkgs config;
+    inherit pkgs config;
-		inherit (pkgs.xorg) mkfontdir mkfontscale fontalias;
+    inherit (pkgs.xorg) mkfontdir mkfontscale fontalias;
-	};
+  };
 
   # The wrapper setuid programs (since we can't have setuid programs
   # in the Nix store).

upstart-jobs/default.nix

@@ -1,4 +1,4 @@
-{config, pkgs, nix, modprobe, nssModulesPath}:
+{config, pkgs, nix, modprobe, nssModulesPath, nixEnvVars}:
 
 let 
 
@@ -79,8 +79,7 @@ import ../upstart-jobs/gather.nix {
       
     # Nix daemon - required for multi-user Nix.
     (import ../upstart-jobs/nix-daemon.nix {
-      inherit nix;
+      inherit config pkgs nix nixEnvVars;
-      inherit (pkgs) openssl;
     })
 
     # Cron daemon.

upstart-jobs/nix-daemon.nix

@@ -1,4 +1,4 @@
-{nix, openssl}:
+{config, pkgs, nix, nixEnvVars}:
 
 {
   name = "nix-daemon";
@@ -6,11 +6,11 @@
   job = "
     start on startup
     stop on shutdown
-    env NIX_CONF_DIR=/nix/etc/nix
     respawn
     script
-        export PATH=${openssl}/bin:$PATH
+      export PATH=${if config.nix.distributedBuilds then "${pkgs.openssh}/bin:" else ""}${pkgs.openssl}/bin:${nix}/bin:$PATH
-        exec ${nix}/bin/nix-worker --daemon > /dev/null 2>&1
+      ${nixEnvVars}
+      exec ${nix}/bin/nix-worker --daemon > /dev/null 2>&1
     end script
   ";