From af775bdcf97bd065505af2ea5f6e292166f05d32 Mon Sep 17 00:00:00 2001
From: regnat <rg@regnat.ovh>
Date: Wed, 9 Jun 2021 18:39:55 +0200
Subject: [PATCH] Make lazyGetAttrField return something more informative

---
 src/libcmd/installables.cc |  2 +-
 src/libexpr/attr-path.cc   |  2 +-
 src/libexpr/eval.cc        | 16 +++++++++-------
 src/libexpr/eval.hh        |  9 ++++++++-
 src/nix/search.cc          |  3 ++-
 5 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 4206c70ce..e5f479974 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -438,7 +438,7 @@ Value * InstallableFlake::getFlakeOutputs(EvalState & state, const flake::Locked
 
     auto vRes = state.allocValue();
     auto gotField = state.lazyGetAttrField(*vFlake, {state.symbols.create("outputs")}, noPos, *vRes);
-    assert(gotField);
+    assert(gotField != EvalState::LazyValueType::Missing);
     return vRes;
 }
 
diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index defecb008..ffa887d33 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -68,7 +68,7 @@ std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attr
                 throw Error("empty attribute name in selection path '%1%'", attrPath);
 
             auto v2 = state.allocValue();
-            auto gotField = state.lazyGetAttrField(*v, {state.symbols.create(attr)}, pos, *v2);
+            auto gotField = state.lazyGetAttrField(*v, {state.symbols.create(attr)}, pos, *v2) != EvalState::LazyValueType::Missing;
             if (!gotField)
                 throw AttrPathNotFound("attribute '%1%' in selection path '%2%' not found", attr, attrPath);
             v = v2;
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 48752a925..c659b513a 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1189,7 +1189,7 @@ void EvalState::updateCacheStats(ValueCache::CacheResult cacheResult)
     };
 }
 
-bool EvalState::lazyGetAttrField(Value & attrs, const std::vector<Symbol> & selector, const Pos & pos, Value & dest)
+EvalState::LazyValueType EvalState::lazyGetAttrField(Value & attrs, const std::vector<Symbol> & selector, const Pos & pos, Value & dest)
 {
     auto eval_cache = attrs.getEvalCache();
     if (eval_cache.isEmpty()) {
@@ -1201,8 +1201,8 @@ bool EvalState::lazyGetAttrField(Value & attrs, const std::vector<Symbol> & sele
     switch (cacheResult.returnCode) {
         case ValueCache::CacheHit:
             if (cacheResult.lastQueriedSymbolIfMissing)
-                return false;
-            return true;
+                return LazyValueType::Missing;
+            return LazyValueType::PlainValue;
         case ValueCache::Forward: {
             auto recordAsVar = new ExprCastedVar(&attrs);
             auto accessExpr = new ExprSelect(pos, recordAsVar, selector);
@@ -1214,11 +1214,13 @@ bool EvalState::lazyGetAttrField(Value & attrs, const std::vector<Symbol> & sele
                 thunk,
                 new ValueCache(resultingCursor)
             );
-            return true;
+            return LazyValueType::DelayedAttrSet;
                                   }
         default:
-            return getAttrField(attrs, selector, pos, dest);
-            ;
+            if(getAttrField(attrs, selector, pos, dest))
+                return LazyValueType::PlainValue;
+            else
+                return LazyValueType::Missing;
     }
 
 }
@@ -1308,7 +1310,7 @@ std::vector<Attr> EvalState::getFields(Value & attrs, const Pos & pos)
         for (auto & attrName : *attrNames) {
             auto newValue = allocValue();
             try {
-            if (lazyGetAttrField(attrs, {attrName}, pos, *newValue)) {
+            if (lazyGetAttrField(attrs, {attrName}, pos, *newValue) != LazyValueType::Missing) {
                 res.push_back(Attr(attrName, newValue));
             } else {
                 everythingCached = false;
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index f421e8dd2..73ef7f598 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -356,9 +356,16 @@ private:
 public:
 
     bool getAttrField(Value & attrs, const std::vector<Symbol> & selector, const Pos & pos, Value & dest);
+
+    enum struct LazyValueType {
+        PlainValue,
+        DelayedAttrSet,
+        Missing,
+    };
+
     // Similar to `getAttrField`, but if the cache says that the result is an
     // attribute set, just return a thunk to it rather than forcing it.
-    bool lazyGetAttrField(Value & attrs, const std::vector<Symbol> & selector, const Pos & pos, Value & dest);
+    LazyValueType lazyGetAttrField(Value & attrs, const std::vector<Symbol> & selector, const Pos & pos, Value & dest);
 
     // Similar to `getAttrField`, but throws an `Error` if the field can’t be
     // found
diff --git a/src/nix/search.cc b/src/nix/search.cc
index 0b8655117..b865073ad 100644
--- a/src/nix/search.cc
+++ b/src/nix/search.cc
@@ -154,7 +154,8 @@ struct CmdSearch : InstallableCommand, MixJSON
                     recurse();
 
                 else if (attrPath[0] == "legacyPackages" && attrPath.size() > 2) {
-                    auto hasRecurse = state->getAttrField(current, {state->sRecurseForDerivations}, noPos, *vTmp);
+                    auto recurseFieldInfo = state->lazyGetAttrField(current, {state->sRecurseForDerivations}, noPos, *vTmp);
+                    auto hasRecurse = recurseFieldInfo == EvalState::LazyValueType::PlainValue;
                     if (hasRecurse && state->forceBool(*vTmp, noPos))
                         recurse();
                 }