Remove $NIX_BUILD_HOOK and $NIX_CURRENT_LOAD

This is to simplify remote build configuration. These environment
variables predate nix.conf.

The build hook now has a sensible default (namely build-remote).

The current load is kept in the Nix state directory now.

6 files changed, 12 insertions, 156 deletions

diff --git a/doc/manual/advanced-topics/distributed-builds.xml b/doc/manual/advanced-topics/distributed-builds.xml
index d5bc1c592553..1957e1105e68 100644
--- a/doc/manual/advanced-topics/distributed-builds.xml
+++ b/doc/manual/advanced-topics/distributed-builds.xml
@@ -22,10 +22,7 @@ will call whenever it wants to build a derivation.  The build hook
 will perform it in the usual way if possible, or it can accept it, in
 which case it is responsible for somehow getting the inputs of the
 build to another machine, doing the build there, and getting the
-results back.  The details of the build hook protocol are described in
-the documentation of the <link
-linkend="envar-build-hook"><envar>NIX_BUILD_HOOK</envar>
-variable</link>.</para>
+results back.</para>
 
 <example xml:id='ex-remote-systems'><title>Remote machine configuration:
 <filename>remote-systems.conf</filename></title>
@@ -103,14 +100,6 @@ requiredSystemFeatures = [ "kvm" ];
 
 </orderedlist>
 
-You should also set up the environment variable
-<envar>NIX_CURRENT_LOAD</envar> to point at a directory (e.g.,
-<filename>/var/run/nix/current-load</filename>) that
-<filename>build-remote</filename> uses to remember how many builds
-it is currently executing remotely.  It doesn't look at the actual
-load on the remote machine, so if you have multiple instances of Nix
-running, they should use the same <envar>NIX_CURRENT_LOAD</envar>
-file.  Maybe in the future <filename>build-remote</filename> will
-look at the actual remote load.</para>
+</para>
 
 </chapter>
diff --git a/doc/manual/command-ref/env-common.xml b/doc/manual/command-ref/env-common.xml
index c757cb17bd10..a83aeaf2e575 100644
--- a/doc/manual/command-ref/env-common.xml
+++ b/doc/manual/command-ref/env-common.xml
@@ -148,139 +148,6 @@ $ mount -o bind /mnt/otherdisk/nix /nix</screen>
 </varlistentry>
 
 
-<varlistentry xml:id="envar-build-hook"><term><envar>NIX_BUILD_HOOK</envar></term>
-
-  <listitem>
-
-  <para>Specifies the location of the <emphasis>build hook</emphasis>,
-  which is a program (typically some script) that Nix will call
-  whenever it wants to build a derivation.  This is used to implement
-  distributed builds<phrase condition="manual"> (see <xref
-  linkend="chap-distributed-builds" />)</phrase>.</para>
-
-  <!--
-  The protocol by
-  which the calling Nix process and the build hook communicate is as
-  follows.
-
-  <para>The build hook is called with the following command-line
-  arguments:
-
-  <orderedlist>
-
-    <listitem><para>A boolean value <literal>0</literal> or
-    <literal>1</literal> specifying whether Nix can locally execute
-    more builds, as per the <link
-    linkend="opt-max-jobs"><option>- -max-jobs</option> option</link>.
-    The purpose of this argument is to allow the hook to not have to
-    maintain bookkeeping for the local machine.</para></listitem>
-
-    <listitem><para>The Nix platform identifier for the local machine
-    (e.g., <literal>i686-linux</literal>).</para></listitem>
-
-    <listitem><para>The Nix platform identifier for the derivation,
-    i.e., its <link linkend="attr-system"><varname>system</varname>
-    attribute</link>.</para></listitem>
-
-    <listitem><para>The store path of the derivation.</para></listitem>
-
-  </orderedlist>
-
-  </para>
-
-  <para>On the basis of this information, and whatever persistent
-  state the build hook keeps about other machines and their current
-  load, it has to decide what to do with the build.  It should print
-  out on standard error one of the following responses (terminated by
-  a newline, <literal>"\n"</literal>):
-
-  <variablelist>
-
-    <varlistentry><term><literal># decline</literal></term>
-
-      <listitem><para>The build hook is not willing or able to perform
-      the build; the calling Nix process should do the build itself,
-      if possible.</para></listitem>
-
-    </varlistentry>
-
-    <varlistentry><term><literal># postpone</literal></term>
-
-      <listitem><para>The build hook cannot perform the build now, but
-      can do so in the future (e.g., because all available build slots
-      on remote machines are in use).  The calling Nix process should
-      postpone this build until at least one currently running build
-      has terminated.</para></listitem>
-
-    </varlistentry>
-
-    <varlistentry><term><literal># accept</literal></term>
-
-      <listitem><para>The build hook has accepted the
-      build.</para></listitem>
-
-    </varlistentry>
-
-  </variablelist>
-
-  </para>
-
-  <para>After sending <literal># accept</literal>, the hook should
-  read one line from standard input, which will be the string
-  <literal>okay</literal>.  It can then proceed with the build.
-  Before sending <literal>okay</literal>, Nix will store in the hook’s
-  current directory a number of text files that contain information
-  about the derivation:
-
-  <variablelist>
-
-    <varlistentry><term><filename>inputs</filename></term>
-
-      <listitem><para>The set of store paths that are inputs to the
-      build process (one per line).  These have to be copied
-      <emphasis>to</emphasis> the remote machine (in addition to the
-      store derivation itself).</para></listitem>
-
-    </varlistentry>
-
-    <varlistentry><term><filename>outputs</filename></term>
-
-      <listitem><para>The set of store paths that are outputs of the
-      derivation (one per line).  These have to be copied
-      <emphasis>from</emphasis> the remote machine if the build
-      succeeds.</para></listitem>
-
-    </varlistentry>
-
-    <varlistentry><term><filename>references</filename></term>
-
-      <listitem><para>The reference graph of the inputs, in the format
-      accepted by the command <command>nix-store
-      - -register-validity</command>.  It is necessary to run this
-      command on the remote machine after copying the inputs to inform
-      Nix on the remote machine that the inputs are valid
-      paths.</para></listitem>
-
-    </varlistentry>
-
-  </variablelist>
-
-  </para>
-
-  <para>The hook should copy the inputs to the remote machine,
-  register the validity of the inputs, perform the remote build, and
-  copy the outputs back to the local machine.  An exit code other than
-  <literal>0</literal> indicates that the hook has failed.  An exit
-  code equal to 100 means that the remote build failed (as opposed to,
-  e.g., a network error).</para>
-  -->
-
-  </listitem>
-
-
-</varlistentry>
-
-
 <varlistentry xml:id="envar-remote"><term><envar>NIX_REMOTE</envar></term>
 
   <listitem><para>This variable should be set to
diff --git a/src/build-remote/build-remote.cc b/src/build-remote/build-remote.cc
index 1ee8a625b6bc..c41383bcf27d 100644
--- a/src/build-remote/build-remote.cc
+++ b/src/build-remote/build-remote.cc
@@ -140,7 +140,9 @@ int main (int argc, char * * argv)
         settings.buildTimeout = std::stoll(argv[3]);
         verbosity = (Verbosity) std::stoll(argv[4]);
 
-        currentLoad = getEnv("NIX_CURRENT_LOAD", "/run/nix/current-load");
+        /* It would be more appropriate to use $XDG_RUNTIME_DIR, since
+           that gets cleared on reboot, but it wouldn't work on OS X. */
+        currentLoad = settings.nixStateDir + "/current-load";
 
         std::shared_ptr<Store> sshStore;
         AutoCloseFD bestSlotLock;
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index a0efd880400c..9b3f799b9f2f 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -583,11 +583,7 @@ struct HookInstance
 
 HookInstance::HookInstance()
 {
-    debug("starting build hook");
-
-    Path buildHook = getEnv("NIX_BUILD_HOOK");
-    if (string(buildHook, 0, 1) != "/") buildHook = settings.nixLibexecDir + "/nix/" + buildHook;
-    buildHook = canonPath(buildHook);
+    debug("starting build hook ‘%s’", settings.buildHook);
 
     /* Create a pipe to get the output of the child. */
     fromHook.create();
@@ -621,9 +617,9 @@ HookInstance::HookInstance()
             std::to_string(verbosity)
         };
 
-        execv(buildHook.c_str(), stringsToCharPtrs(args).data());
+        execv(settings.buildHook.get().c_str(), stringsToCharPtrs(args).data());
 
-        throw SysError(format("executing ‘%1%’") % buildHook);
+        throw SysError("executing ‘%s’", settings.buildHook);
     });
 
     pid.setSeparatePG(true);
@@ -1569,7 +1565,7 @@ void DerivationGoal::buildDone()
 
 HookReply DerivationGoal::tryBuildHook()
 {
-    if (!settings.useBuildHook || getEnv("NIX_BUILD_HOOK") == "" || !useDerivation) return rpDecline;
+    if (!settings.useBuildHook || !useDerivation) return rpDecline;
 
     if (!worker.hook)
         worker.hook = std::make_unique<HookInstance>();
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index b4f44de2e65d..25cc3e068ee7 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -127,6 +127,9 @@ public:
     Setting<bool> useBuildHook{this, true, "remote-builds",
         "Whether to use build hooks (for distributed builds)."};
 
+    PathSetting buildHook{this, true, nixLibexecDir + "/nix/build-remote", "build-hook",
+        "The path of the helper program that executes builds to remote machines."};
+
     Setting<off_t> reservedSize{this, 8 * 1024 * 1024, "gc-reserved-space",
         "Amount of reserved disk space for the garbage collector."};
 
diff --git a/tests/remote-builds.nix b/tests/remote-builds.nix
index 63aaa4d88f56..39bd090e43e7 100644
--- a/tests/remote-builds.nix
+++ b/tests/remote-builds.nix
@@ -43,7 +43,6 @@ in
         { config, pkgs, ... }:
         { nix.maxJobs = 0; # force remote building
           nix.distributedBuilds = true;
-          nix.envVars = pkgs.lib.mkAfter { NIX_BUILD_HOOK = "${nix}/libexec/nix/build-remote"; };
           nix.buildMachines =
             [ { hostName = "slave1";
                 sshUser = "root";