Add a ca-derivations required machine feature

Make ca-derivations require a `ca-derivations` machine feature, and
ca-aware builders expose it.

That way, a network of builders can mix ca-aware and non-ca-aware
machines, and the scheduler will send them in the right place.

src/libstore/parsed-derivations.cc

@@ -91,6 +91,8 @@ StringSet ParsedDerivation::getRequiredSystemFeatures() const
     StringSet res;
     for (auto & i : getStringsAttr("requiredSystemFeatures").value_or(Strings()))
         res.insert(i);
+    if (!derivationHasKnownOutputPaths(drv.type()))
+        res.insert("ca-derivations");
     return res;
 }
 

src/libstore/store-api.cc

@@ -337,6 +337,13 @@ ValidPathInfo Store::addToStoreSlow(std::string_view name, const Path & srcPath,
     return info;
 }
 
+StringSet StoreConfig::getDefaultSystemFeatures()
+{
+    auto res = settings.systemFeatures.get();
+    if (settings.isExperimentalFeatureEnabled("ca-derivations"))
+        res.insert("ca-derivations");
+    return res;
+}
 
 Store::Store(const Params & params)
     : StoreConfig(params)

src/libstore/store-api.hh

@@ -180,6 +180,8 @@ struct StoreConfig : public Config
 
     StoreConfig() = delete;
 
+    StringSet getDefaultSystemFeatures();
+
     virtual ~StoreConfig() { }
 
     virtual const std::string name() = 0;
@@ -196,7 +198,7 @@ struct StoreConfig : public Config
 
     Setting<bool> wantMassQuery{this, false, "want-mass-query", "whether this substituter can be queried efficiently for path validity"};
 
-    Setting<StringSet> systemFeatures{this, settings.systemFeatures,
+    Setting<StringSet> systemFeatures{this, getDefaultSystemFeatures(),
         "system-features",
         "Optional features that the system this store builds on implements (like \"kvm\")."};
 

tests/build-remote-content-addressed-floating.sh

@@ -4,4 +4,6 @@ file=build-hook-ca-floating.nix
 
 sed -i 's/experimental-features .*/& ca-derivations/' "$NIX_CONF_DIR"/nix.conf
 
+CONTENT_ADDRESSED=true
+
 source build-remote.sh

tests/build-remote.sh

@@ -6,12 +6,17 @@ unset NIX_STATE_DIR
 
 function join_by { local d=$1; shift; echo -n "$1"; shift; printf "%s" "${@/#/$d}"; }
 
+EXTRA_SYSTEM_FEATURES=()
+if [[ -n "$CONTENT_ADDRESSED" ]]; then
+    EXTRA_SYSTEM_FEATURES=("ca-derivations")
+fi
+
 builders=(
   # system-features will automatically be added to the outer URL, but not inner
   # remote-store URL.
-  "ssh://localhost?remote-store=$TEST_ROOT/machine1?system-features=foo - - 1 1 foo"
-  "$TEST_ROOT/machine2 - - 1 1 bar"
-  "ssh-ng://localhost?remote-store=$TEST_ROOT/machine3?system-features=baz - - 1 1 baz"
+  "ssh://localhost?remote-store=$TEST_ROOT/machine1?system-features=$(join_by "%20" foo ${EXTRA_SYSTEM_FEATURES[@]}) - - 1 1 $(join_by "," foo ${EXTRA_SYSTEM_FEATURES[@]})"
+  "$TEST_ROOT/machine2 - - 1 1 $(join_by "," bar ${EXTRA_SYSTEM_FEATURES[@]})"
+  "ssh-ng://localhost?remote-store=$TEST_ROOT/machine3?system-features=$(join_by "%20" baz ${EXTRA_SYSTEM_FEATURES[@]}) - - 1 1 $(join_by "," baz ${EXTRA_SYSTEM_FEATURES[@]})"
 )
 
 chmod -R +w $TEST_ROOT/machine* || true