nix-instantiate: Fix read-only evaluation

src/libexpr/eval.cc

@@ -138,6 +138,7 @@ EvalState::EvalState()
     , sName(symbols.create("name"))
     , sSystem(symbols.create("system"))
     , sOverrides(symbols.create("__overrides"))
+    , sOutputName(symbols.create("outputName"))
     , baseEnv(allocEnv(128))
     , baseEnvDispl(0)
     , staticBaseEnv(false, 0)

src/libexpr/eval.hh

@@ -93,7 +93,7 @@ public:
     SymbolTable symbols;
 
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName,
-        sSystem, sOverrides;
+        sSystem, sOverrides, sOutputName;
 
     /* If set, force copying files to the Nix store even if they
        already exist there. */

src/libexpr/get-drvs.cc

@@ -28,6 +28,17 @@ string DrvInfo::queryOutPath(EvalState & state) const
 }
 
 
+string DrvInfo::queryOutputName(EvalState & state) const
+{
+    if (outputName == "" && attrs) {
+        Bindings::iterator i = attrs->find(state.sOutputName);
+        PathSet context;
+        (string &) outputName = i != attrs->end() ? state.coerceToString(*i->value, context) : "";
+    }
+    return outputName;
+}
+
+
 MetaInfo DrvInfo::queryMetaInfo(EvalState & state) const
 {
     if (metaInfoRead) return meta;

src/libexpr/get-drvs.hh

@@ -28,6 +28,7 @@ struct DrvInfo
 private:
     string drvPath;
     string outPath;
+    string outputName;
 
     bool metaInfoRead;
     MetaInfo meta;
@@ -46,6 +47,7 @@ public:
 
     string queryDrvPath(EvalState & state) const;
     string queryOutPath(EvalState & state) const;
+    string queryOutputName(EvalState & state) const;
     MetaInfo queryMetaInfo(EvalState & state) const;
     MetaValue queryMetaInfo(EvalState & state, const string & name) const;
 

src/nix-instantiate/nix-instantiate.cc

@@ -62,16 +62,9 @@ void processExpr(EvalState & state, const Strings & attrPaths,
                     Path drvPath = i->queryDrvPath(state);
 
                     /* What output do we want? */
-                    Path outPath = i->queryOutPath(state);
+                    string outputName = i->queryOutputName(state);
-                    Derivation drv = derivationFromPath(*store, drvPath);
-                    string outputName;
-                    foreach (DerivationOutputs::iterator, i, drv.outputs)
-                        if (i->second.path == outPath) {
-                            outputName = i->first;
-                            break;
-                        }
                     if (outputName == "")
-                        throw Error(format("derivation `%1%' does not have an output `%2%'") % drvPath % outPath);
+                        throw Error(format("derivation `%1%' lacks an `outputName' attribute ") % drvPath);
 
                     if (gcRoot == "")
                         printGCWarning();