From 408a7bfac1f4282ff6647696dfbc7988eed3a2ca Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Mon, 26 Nov 2012 17:39:09 +0100
Subject: nix-instantiate: Fix read-only evaluation

---
 src/libexpr/eval.cc                    |  1 +
 src/libexpr/eval.hh                    |  2 +-
 src/libexpr/get-drvs.cc                | 11 +++++++++++
 src/libexpr/get-drvs.hh                |  2 ++
 src/nix-instantiate/nix-instantiate.cc | 11 ++---------
 5 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index b176bbc82497..31aaad5485f0 100644
--- a/src/libexpr/eval.cc
+++ b/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)
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index a3c55a38860f..432a0bad1c6c 100644
--- a/src/libexpr/eval.hh
+++ b/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. */
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index f9e7dc6dbe3e..2ee55bdcaea0 100644
--- a/src/libexpr/get-drvs.cc
+++ b/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;
diff --git a/src/libexpr/get-drvs.hh b/src/libexpr/get-drvs.hh
index 8159417a0131..f84636a1ab3d 100644
--- a/src/libexpr/get-drvs.hh
+++ b/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;
 
diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
index a5053c32376d..53cd711895ff 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/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);
-                    Derivation drv = derivationFromPath(*store, drvPath);
-                    string outputName;
-                    foreach (DerivationOutputs::iterator, i, drv.outputs)
-                        if (i->second.path == outPath) {
-                            outputName = i->first;
-                            break;
-                        }
+                    string outputName = i->queryOutputName(state);
                     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();
-- 
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