builtin:{unpack-channel,buildenv}: Get output path from the derivation

Similar to 1ee42c5b88, get the "out"
path from the derivation (and complain if it doesn't exist), rather
than getting it from the environment.

src/libstore/build/local-derivation-goal.cc

@@ -2130,16 +2130,17 @@ void LocalDerivationGoal::runChild()
             try {
                 logger = makeJSONLogger(*logger);
 
-                BasicDerivation & drv2(*drv);
-                for (auto & e : drv2.env)
-                    e.second = rewriteStrings(e.second, inputRewrites);
+                std::map<std::string, Path> outputs;
+                for (auto & e : drv->outputs)
+                    outputs.insert_or_assign(e.first,
+                        worker.store.printStorePath(scratchOutputs.at(e.first)));
 
                 if (drv->builder == "builtin:fetchurl")
-                    builtinFetchurl(drv2, netrcData);
+                    builtinFetchurl(*drv, outputs, netrcData);
                 else if (drv->builder == "builtin:buildenv")
-                    builtinBuildenv(drv2);
+                    builtinBuildenv(*drv, outputs);
                 else if (drv->builder == "builtin:unpack-channel")
-                    builtinUnpackChannel(drv2);
+                    builtinUnpackChannel(*drv, outputs);
                 else
                     throw Error("unsupported builtin builder '%1%'", drv->builder.substr(8));
                 _exit(0);

src/libstore/build/local-derivation-goal.hh

@@ -106,7 +106,7 @@ struct LocalDerivationGoal : public DerivationGoal
     RedirectedOutputs redirectedOutputs;
 
     /**
-     * The outputs paths used during the build.
+     * The output paths used during the build.
      *
      * - Input-addressed derivations or fixed content-addressed outputs are
      *   sometimes built when some of their outputs already exist, and can not

src/libstore/builtins.hh

@@ -6,7 +6,13 @@
 namespace nix {
 
 // TODO: make pluggable.
-void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData);
-void builtinUnpackChannel(const BasicDerivation & drv);
+void builtinFetchurl(
+    const BasicDerivation & drv,
+    const std::map<std::string, Path> & outputs,
+    const std::string & netrcData);
+
+void builtinUnpackChannel(
+    const BasicDerivation & drv,
+    const std::map<std::string, Path> & outputs);
 
 }

src/libstore/builtins/buildenv.cc

@@ -161,7 +161,9 @@ void buildProfile(const Path & out, Packages && pkgs)
     debug("created %d symlinks in user environment", state.symlinks);
 }
 
-void builtinBuildenv(const BasicDerivation & drv)
+void builtinBuildenv(
+    const BasicDerivation & drv,
+    const std::map<std::string, Path> & outputs)
 {
     auto getAttr = [&](const std::string & name) {
         auto i = drv.env.find(name);
@@ -169,7 +171,7 @@ void builtinBuildenv(const BasicDerivation & drv)
         return i->second;
     };
 
-    Path out = getAttr("out");
+    auto out = outputs.at("out");
     createDirs(out);
 
     /* Convert the stuff we get from the environment back into a

src/libstore/builtins/buildenv.hh

@@ -45,6 +45,8 @@ typedef std::vector<Package> Packages;
 
 void buildProfile(const Path & out, Packages && pkgs);
 
-void builtinBuildenv(const BasicDerivation & drv);
+void builtinBuildenv(
+    const BasicDerivation & drv,
+    const std::map<std::string, Path> & outputs);
 
 }

src/libstore/builtins/fetchurl.cc

@@ -6,7 +6,10 @@
 
 namespace nix {
 
-void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
+void builtinFetchurl(
+    const BasicDerivation & drv,
+    const std::map<std::string, Path> & outputs,
+    const std::string & netrcData)
 {
     /* Make the host's netrc data available. Too bad curl requires
        this to be stored in a file. It would be nice if we could just
@@ -24,14 +27,8 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
     if (!dof)
         throw Error("'builtin:fetchurl' must be a fixed-output derivation");
 
-    auto getAttr = [&](const std::string & name) {
-        auto i = drv.env.find(name);
-        if (i == drv.env.end()) throw Error("attribute '%s' missing", name);
-        return i->second;
-    };
-
-    Path storePath = getAttr("out");
-    auto mainUrl = getAttr("url");
+    auto storePath = outputs.at("out");
+    auto mainUrl = drv.env.at("url");
     bool unpack = getOr(drv.env, "unpack", "") == "1";
 
     /* Note: have to use a fresh fileTransfer here because we're in

src/libstore/builtins/unpack-channel.cc

@@ -3,7 +3,9 @@
 
 namespace nix {
 
-void builtinUnpackChannel(const BasicDerivation & drv)
+void builtinUnpackChannel(
+    const BasicDerivation & drv,
+    const std::map<std::string, Path> & outputs)
 {
     auto getAttr = [&](const std::string & name) {
         auto i = drv.env.find(name);
@@ -11,7 +13,7 @@ void builtinUnpackChannel(const BasicDerivation & drv)
         return i->second;
     };
 
-    Path out = getAttr("out");
+    auto out = outputs.at("out");
     auto channelName = getAttr("channelName");
     auto src = getAttr("src");