From c119ab9db0edf65379593883c5ed5253549ebbd0 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Mon, 4 Nov 2019 13:48:58 +0100
Subject: [PATCH] Enable recursive Nix using a feature

Derivations that want to use recursion should now set

  requiredSystemFeatures = [ "recursive-nix" ];

to make the daemon socket appear.

Also, Nix should be configured with "experimental-features =
recursive-nix".
---
 src/libstore/build.cc   | 5 ++++-
 src/libstore/globals.cc | 2 +-
 tests/recursive.sh      | 4 +++-
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index e8b23d396..e77512ca4 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2248,7 +2248,8 @@ void DerivationGoal::startBuilder()
 
     /* Fire up a Nix daemon to process recursive Nix calls from the
        builder. */
-    startDaemon();
+    if (parsedDrv->getRequiredSystemFeatures().count("recursive-nix"))
+        startDaemon();
 
     /* Run the builder. */
     printMsg(lvlChatty, format("executing builder '%1%'") % drv->builder);
@@ -2832,6 +2833,8 @@ struct RestrictedStore : public LocalFSStore
 
 void DerivationGoal::startDaemon()
 {
+    settings.requireExperimentalFeature("recursive-nix");
+
     Store::Params params;
     params["path-info-cache-size"] = "0";
     params["store"] = worker.store.storeDir;
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 249c36673..042aaea95 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -95,7 +95,7 @@ StringSet Settings::getDefaultSystemFeatures()
     /* For backwards compatibility, accept some "features" that are
        used in Nixpkgs to route builds to certain machines but don't
        actually require anything special on the machines. */
-    StringSet features{"nixos-test", "benchmark", "big-parallel"};
+    StringSet features{"nixos-test", "benchmark", "big-parallel", "recursive-nix"};
 
     #if __linux__
     if (access("/dev/kvm", R_OK | W_OK) == 0)
diff --git a/tests/recursive.sh b/tests/recursive.sh
index 25204caf1..62896c015 100644
--- a/tests/recursive.sh
+++ b/tests/recursive.sh
@@ -4,7 +4,7 @@ clearStore
 
 export unreachable=$(nix add-to-store ./recursive.sh)
 
-nix build -o $TEST_ROOT/result -L '(
+nix --experimental-features 'nix-command recursive-nix' build -o $TEST_ROOT/result -L '(
   with import ./config.nix;
   with import <nix/config.nix>;
   mkDerivation {
@@ -15,6 +15,8 @@ nix build -o $TEST_ROOT/result -L '(
     # Note: this is a string without context.
     unreachable = builtins.getEnv "unreachable";
 
+    requiredSystemFeatures = [ "recursive-nix" ];
+
     buildCommand = '\'\''
       mkdir $out
       PATH=${nixBinDir}:$PATH