Make InstallableFlake::toValue() and toDerivation() behave consistently

In particular, this means that 'nix eval` (which uses toValue()) no
longer auto-calls functions or functors (because
AttrCursor::findAlongAttrPath() doesn't).

Fixes #6152.

Also use ref<> in a few places, and don't return attrpaths from
getCursor() because cursors already have a getAttrPath() method.

src/libcmd/installables.cc

@@ -334,16 +334,16 @@ DerivedPath Installable::toDerivedPath()
     return std::move(buildables[0]);
 }
 
-std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
+std::vector<ref<eval_cache::AttrCursor>>
 Installable::getCursors(EvalState & state)
 {
     auto evalCache =
         std::make_shared<nix::eval_cache::EvalCache>(std::nullopt, state,
             [&]() { return toValue(state).first; });
-    return {{evalCache->getRoot(), ""}};
+    return {evalCache->getRoot()};
 }
 
-std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>
+ref<eval_cache::AttrCursor>
 Installable::getCursor(EvalState & state)
 {
     auto cursors = getCursors(state);
@@ -566,27 +566,9 @@ InstallableFlake::InstallableFlake(
 
 std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableFlake::toDerivation()
 {
-    auto lockedFlake = getLockedFlake();
+    auto attr = getCursor(*state);
 
-    auto cache = openEvalCache(*state, lockedFlake);
+    auto attrPath = attr->getAttrPathStr();
-    auto root = cache->getRoot();
-
-    Suggestions suggestions;
-
-    for (auto & attrPath : getActualAttrPaths()) {
-        debug("trying flake output attribute '%s'", attrPath);
-
-        auto attrOrSuggestions = root->findAlongAttrPath(
-            parseAttrPath(*state, attrPath),
-            true
-        );
-
-        if (!attrOrSuggestions) {
-            suggestions += attrOrSuggestions.getSuggestions();
-            continue;
-        }
-
-        auto attr = *attrOrSuggestions;
 
     if (!attr->isDerivation())
         throw Error("flake output attribute '%s' is not a derivation", attrPath);
@@ -598,11 +580,7 @@ std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableF
         attr->getAttr(state->sOutputName)->getString()
     };
 
-        return {attrPath, lockedFlake->flake.lockedRef, std::move(drvInfo)};
+    return {attrPath, getLockedFlake()->flake.lockedRef, std::move(drvInfo)};
-    }
-
-    throw Error(suggestions, "flake '%s' does not provide attribute %s",
-        flakeRef, showAttrPaths(getActualAttrPaths()));
 }
 
 std::vector<InstallableValue::DerivationInfo> InstallableFlake::toDerivations()
@@ -614,33 +592,10 @@ std::vector<InstallableValue::DerivationInfo> InstallableFlake::toDerivations()
 
 std::pair<Value *, Pos> InstallableFlake::toValue(EvalState & state)
 {
-    auto lockedFlake = getLockedFlake();
+    return {&getCursor(state)->forceValue(), noPos};
-
-    auto vOutputs = getFlakeOutputs(state, *lockedFlake);
-
-    auto emptyArgs = state.allocBindings(0);
-
-    Suggestions suggestions;
-
-    for (auto & attrPath : getActualAttrPaths()) {
-        try {
-            auto [v, pos] = findAlongAttrPath(state, attrPath, *emptyArgs, *vOutputs);
-            state.forceValue(*v, pos);
-            return {v, pos};
-        } catch (AttrPathNotFound & e) {
-            suggestions += e.info().suggestions;
-        }
 }
 
-    throw Error(
+std::vector<ref<eval_cache::AttrCursor>>
-        suggestions,
-        "flake '%s' does not provide attribute %s",
-        flakeRef,
-        showAttrPaths(getActualAttrPaths())
-    );
-}
-
-std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
 InstallableFlake::getCursors(EvalState & state)
 {
     auto evalCache = openEvalCache(state,
@@ -648,21 +603,55 @@ InstallableFlake::getCursors(EvalState & state)
 
     auto root = evalCache->getRoot();
 
-    std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>> res;
+    std::vector<ref<eval_cache::AttrCursor>> res;
 
     for (auto & attrPath : getActualAttrPaths()) {
         auto attr = root->findAlongAttrPath(parseAttrPath(state, attrPath));
-        if (attr) res.push_back({*attr, attrPath});
+        if (attr) res.push_back(ref(*attr));
     }
 
     return res;
 }
 
+ref<eval_cache::AttrCursor> InstallableFlake::getCursor(EvalState & state)
+{
+    auto lockedFlake = getLockedFlake();
+
+    auto cache = openEvalCache(state, lockedFlake);
+    auto root = cache->getRoot();
+
+    Suggestions suggestions;
+
+    auto attrPaths = getActualAttrPaths();
+
+    for (auto & attrPath : attrPaths) {
+        debug("trying flake output attribute '%s'", attrPath);
+
+        auto attrOrSuggestions = root->findAlongAttrPath(
+            parseAttrPath(state, attrPath),
+            true
+        );
+
+        if (!attrOrSuggestions) {
+            suggestions += attrOrSuggestions.getSuggestions();
+            continue;
+        }
+
+        return *attrOrSuggestions;
+    }
+
+    throw Error(
+        suggestions,
+        "flake '%s' does not provide attribute %s",
+        flakeRef,
+        showAttrPaths(attrPaths));
+}
+
 std::shared_ptr<flake::LockedFlake> InstallableFlake::getLockedFlake() const
 {
+    if (!_lockedFlake) {
         flake::LockFlags lockFlagsApplyConfig = lockFlags;
         lockFlagsApplyConfig.applyNixConfig = true;
-    if (!_lockedFlake) {
         _lockedFlake = std::make_shared<flake::LockedFlake>(lockFlake(*state, flakeRef, lockFlagsApplyConfig));
     }
     return _lockedFlake;

src/libcmd/installables.hh

@@ -80,10 +80,10 @@ struct Installable
         return {};
     }
 
-    virtual std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
+    virtual std::vector<ref<eval_cache::AttrCursor>>
     getCursors(EvalState & state);
 
-    std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>
+    virtual ref<eval_cache::AttrCursor>
     getCursor(EvalState & state);
 
     virtual FlakeRef nixpkgsFlakeRef() const
@@ -180,9 +180,15 @@ struct InstallableFlake : InstallableValue
 
     std::pair<Value *, Pos> toValue(EvalState & state) override;
 
-    std::vector<std::pair<std::shared_ptr<eval_cache::AttrCursor>, std::string>>
+    /* Get a cursor to every attrpath in getActualAttrPaths() that
+       exists. */
+    std::vector<ref<eval_cache::AttrCursor>>
     getCursors(EvalState & state) override;
 
+    /* Get a cursor to the first attrpath in getActualAttrPaths() that
+       exists, or throw an exception with suggestions if none exists. */
+    ref<eval_cache::AttrCursor> getCursor(EvalState & state) override;
+
     std::shared_ptr<flake::LockedFlake> getLockedFlake() const;
 
     FlakeRef nixpkgsFlakeRef() const override;

src/libexpr/eval-cache.cc

@@ -306,9 +306,9 @@ Value * EvalCache::getRootValue()
     return *value;
 }
 
-std::shared_ptr<AttrCursor> EvalCache::getRoot()
+ref<AttrCursor> EvalCache::getRoot()
 {
-    return std::make_shared<AttrCursor>(ref(shared_from_this()), std::nullopt);
+    return make_ref<AttrCursor>(ref(shared_from_this()), std::nullopt);
 }
 
 AttrCursor::AttrCursor(

src/libexpr/eval-cache.hh

@@ -33,7 +33,7 @@ public:
         EvalState & state,
         RootLoader rootLoader);
 
-    std::shared_ptr<AttrCursor> getRoot();
+    ref<AttrCursor> getRoot();
 };
 
 enum AttrType {
@@ -104,6 +104,8 @@ public:
 
     ref<AttrCursor> getAttr(std::string_view name);
 
+    /* Get an attribute along a chain of attrsets. Note that this does
+       not auto-call functors or functions. */
     OrSuggestions<ref<AttrCursor>> findAlongAttrPath(const std::vector<Symbol> & attrPath, bool force = false);
 
     std::string getString();

src/nix/app.cc

@@ -61,7 +61,7 @@ std::string resolveString(Store & store, const std::string & toResolve, const Bu
 
 UnresolvedApp Installable::toApp(EvalState & state)
 {
-    auto [cursor, attrPath] = getCursor(state);
+    auto cursor = getCursor(state);
 
     auto type = cursor->getAttr("type")->getString();
 
@@ -101,7 +101,7 @@ UnresolvedApp Installable::toApp(EvalState & state)
     }
 
     else
-        throw Error("attribute '%s' has unsupported type '%s'", attrPath, type);
+        throw Error("attribute '%s' has unsupported type '%s'", cursor->getAttrPathStr(), type);
 }
 
 // FIXME: move to libcmd

src/nix/flake.cc

@@ -705,7 +705,7 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
             defaultTemplateAttrPathsPrefixes,
             lockFlags);
 
-        auto [cursor, attrPath] = installable.getCursor(*evalState);
+        auto cursor = installable.getCursor(*evalState);
 
         auto templateDirAttr = cursor->getAttr("path");
         auto templateDir = templateDirAttr->getString();

src/nix/search.cc

@@ -165,8 +165,8 @@ struct CmdSearch : InstallableCommand, MixJSON
             }
         };
 
-        for (auto & [cursor, prefix] : installable->getCursors(*state))
+        for (auto & cursor : installable->getCursors(*state))
-            visit(*cursor, parseAttrPath(*state, prefix), true);
+            visit(*cursor, cursor->getAttrPath(), true);
 
         if (!json && !results)
             throw Error("no results for the given search term(s)!");