2 files changed, 50 insertions, 33 deletions

diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index 4200e8fd6757..b7e16de7fa4e 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -9,7 +9,34 @@
 namespace nix {
 
 
-string DrvInfo::queryDrvPath()
+DrvInfo::DrvInfo(EvalState & state, const string & attrPath, Bindings * attrs)
+    : state(&state), attrs(attrs), attrPath(attrPath)
+{
+}
+
+
+string DrvInfo::queryName() const
+{
+    if (name == "" && attrs) {
+        auto i = attrs->find(state->sName);
+        if (i == attrs->end()) throw TypeError("derivation name missing");
+        name = state->forceStringNoCtx(*i->value);
+    }
+    return name;
+}
+
+
+string DrvInfo::querySystem() const
+{
+    if (system == "" && attrs) {
+        auto i = attrs->find(state->sSystem);
+        system = i == attrs->end() ? "unknown" : state->forceStringNoCtx(*i->value, *i->pos);
+    }
+    return system;
+}
+
+
+string DrvInfo::queryDrvPath() const
 {
     if (drvPath == "" && attrs) {
         Bindings::iterator i = attrs->find(state->sDrvPath);
@@ -20,7 +47,7 @@ string DrvInfo::queryDrvPath()
 }
 
 
-string DrvInfo::queryOutPath()
+string DrvInfo::queryOutPath() const
 {
     if (outPath == "" && attrs) {
         Bindings::iterator i = attrs->find(state->sOutPath);
@@ -76,7 +103,7 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool onlyOutputsToInstall)
 }
 
 
-string DrvInfo::queryOutputName()
+string DrvInfo::queryOutputName() const
 {
     if (outputName == "" && attrs) {
         Bindings::iterator i = attrs->find(state->sOutputName);
@@ -225,17 +252,12 @@ static bool getDerivation(EvalState & state, Value & v,
         if (done.find(v.attrs) != done.end()) return false;
         done.insert(v.attrs);
 
-        Bindings::iterator i = v.attrs->find(state.sName);
-        /* !!! We really would like to have a decent back trace here. */
-        if (i == v.attrs->end()) throw TypeError("derivation name missing");
+        DrvInfo drv(state, attrPath, v.attrs);
 
-        Bindings::iterator i2 = v.attrs->find(state.sSystem);
-
-        DrvInfo drv(state, state.forceStringNoCtx(*i->value), attrPath,
-            i2 == v.attrs->end() ? "unknown" : state.forceStringNoCtx(*i2->value, *i2->pos),
-            v.attrs);
+        drv.queryName();
 
         drvs.push_back(drv);
+
         return false;
 
     } catch (AssertionError & e) {
diff --git a/src/libexpr/get-drvs.hh b/src/libexpr/get-drvs.hh
index 37fcbe829d3c..82fb8a3ac6a8 100644
--- a/src/libexpr/get-drvs.hh
+++ b/src/libexpr/get-drvs.hh
@@ -17,31 +17,32 @@ public:
 private:
     EvalState * state;
 
-    string drvPath;
-    string outPath;
-    string outputName;
+    mutable string name;
+    mutable string system;
+    mutable string drvPath;
+    mutable string outPath;
+    mutable string outputName;
     Outputs outputs;
 
-    bool failed; // set if we get an AssertionError
+    bool failed = false; // set if we get an AssertionError
 
-    Bindings * attrs, * meta;
+    Bindings * attrs = nullptr, * meta = nullptr;
 
     Bindings * getMeta();
 
     bool checkMeta(Value & v);
 
 public:
-    string name;
     string attrPath; /* path towards the derivation */
-    string system;
 
-    DrvInfo(EvalState & state) : state(&state), failed(false), attrs(0), meta(0) { };
-    DrvInfo(EvalState & state, const string & name, const string & attrPath, const string & system, Bindings * attrs)
-        : state(&state), failed(false), attrs(attrs), meta(0), name(name), attrPath(attrPath), system(system) { };
+    DrvInfo(EvalState & state) : state(&state) { };
+    DrvInfo(EvalState & state, const string & attrPath, Bindings * attrs);
 
-    string queryDrvPath();
-    string queryOutPath();
-    string queryOutputName();
+    string queryName() const;
+    string querySystem() const;
+    string queryDrvPath() const;
+    string queryOutPath() const;
+    string queryOutputName() const;
     /** Return the list of outputs. The "outputs to install" are determined by `mesa.outputsToInstall`. */
     Outputs queryOutputs(bool onlyOutputsToInstall = false);
 
@@ -58,15 +59,9 @@ public:
     MetaValue queryMetaInfo(EvalState & state, const string & name) const;
     */
 
-    void setDrvPath(const string & s)
-    {
-        drvPath = s;
-    }
-
-    void setOutPath(const string & s)
-    {
-        outPath = s;
-    }
+    void setName(const string & s) { name = s; }
+    void setDrvPath(const string & s) { drvPath = s; }
+    void setOutPath(const string & s) { outPath = s; }
 
     void setFailed() { failed = true; };
     bool hasFailed() { return failed; };