always re-eval cached failures

This is terrible UX, and frankly an eval failure should be a cache
invalidation anyway.

This removes the CachedEvalError type entirely.

Fixes #223.

Change-Id: I91f8003eabd0ea45003024e96d1de3c7ae8e49d8

doc/manual/rl-next/no-cache-eval-errors.md

@@ -0,0 +1,8 @@
+---
+synopsis: re-evaluate cached evaluation errors
+cls: 771
+---
+
+"cached failure of [expr]" errors have been removed: expressions already in the
+eval cache as a failure will now simply be re-evaluated, removing the need to
+set `--no-eval-cache` or similar to see the error.

src/libexpr/eval-cache.cc

@@ -469,7 +469,7 @@ Suggestions AttrCursor::getSuggestionsForAttr(Symbol name)
     return Suggestions::bestMatches(strAttrNames, root->state.symbols[name]);
 }
 
-std::shared_ptr<AttrCursor> AttrCursor::maybeGetAttr(Symbol name, bool forceErrors)
+std::shared_ptr<AttrCursor> AttrCursor::maybeGetAttr(Symbol name)
 {
     if (root->db) {
         if (!cachedValue)
@@ -487,10 +487,7 @@ std::shared_ptr<AttrCursor> AttrCursor::maybeGetAttr(Symbol name, bool forceErro
                     if (std::get_if<missing_t>(&attr->second))
                         return nullptr;
                     else if (std::get_if<failed_t>(&attr->second)) {
-                        if (forceErrors)
+                        debug("reevaluating failed cached attribute '%s'", getAttrPathStr(name));
-                            debug("reevaluating failed cached attribute '%s'", getAttrPathStr(name));
-                        else
-                            throw CachedEvalError(root->state, "cached failure of attribute '%s'", getAttrPathStr(name));
                     } else
                         return std::make_shared<AttrCursor>(root,
                             std::make_pair(shared_from_this(), name), nullptr, std::move(attr));
@@ -536,9 +533,9 @@ std::shared_ptr<AttrCursor> AttrCursor::maybeGetAttr(std::string_view name)
     return maybeGetAttr(root->state.symbols.create(name));
 }
 
-ref<AttrCursor> AttrCursor::getAttr(Symbol name, bool forceErrors)
+ref<AttrCursor> AttrCursor::getAttr(Symbol name)
 {
-    auto p = maybeGetAttr(name, forceErrors);
+    auto p = maybeGetAttr(name);
     if (!p)
         throw Error("attribute '%s' does not exist", getAttrPathStr(name));
     return ref(p);
@@ -549,11 +546,11 @@ ref<AttrCursor> AttrCursor::getAttr(std::string_view name)
     return getAttr(root->state.symbols.create(name));
 }
 
-OrSuggestions<ref<AttrCursor>> AttrCursor::findAlongAttrPath(const std::vector<Symbol> & attrPath, bool force)
+OrSuggestions<ref<AttrCursor>> AttrCursor::findAlongAttrPath(const std::vector<Symbol> & attrPath)
 {
     auto res = shared_from_this();
     for (auto & attr : attrPath) {
-        auto child = res->maybeGetAttr(attr, force);
+        auto child = res->maybeGetAttr(attr);
         if (!child) {
             auto suggestions = res->getSuggestionsForAttr(attr);
             return OrSuggestions<ref<AttrCursor>>::failed(suggestions);
@@ -750,7 +747,7 @@ bool AttrCursor::isDerivation()
 
 StorePath AttrCursor::forceDerivation()
 {
-    auto aDrvPath = getAttr(root->state.sDrvPath, true);
+    auto aDrvPath = getAttr(root->state.sDrvPath);
     auto drvPath = root->state.store->parseStorePath(aDrvPath->getString());
     if (!root->state.store->isValidPath(drvPath) && !settings.readOnlyMode) {
         /* The eval cache contains 'drvPath', but the actual path has

src/libexpr/eval-cache.hh

@@ -100,11 +100,11 @@ public:
 
     Suggestions getSuggestionsForAttr(Symbol name);
 
-    std::shared_ptr<AttrCursor> maybeGetAttr(Symbol name, bool forceErrors = false);
+    std::shared_ptr<AttrCursor> maybeGetAttr(Symbol name);
 
     std::shared_ptr<AttrCursor> maybeGetAttr(std::string_view name);
 
-    ref<AttrCursor> getAttr(Symbol name, bool forceErrors = false);
+    ref<AttrCursor> getAttr(Symbol name);
 
     ref<AttrCursor> getAttr(std::string_view name);
 
@@ -112,7 +112,7 @@ public:
      * 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);
+    OrSuggestions<ref<AttrCursor>> findAlongAttrPath(const std::vector<Symbol> & attrPath);
 
     std::string getString();
 

src/libexpr/eval-error.cc

@@ -99,7 +99,6 @@ template class EvalErrorBuilder<TypeError>;
 template class EvalErrorBuilder<UndefinedVarError>;
 template class EvalErrorBuilder<MissingArgumentError>;
 template class EvalErrorBuilder<InfiniteRecursionError>;
-template class EvalErrorBuilder<CachedEvalError>;
 template class EvalErrorBuilder<InvalidPathError>;
 
 }

src/libexpr/eval-error.hh

@@ -44,7 +44,6 @@ MakeError(TypeError, EvalError);
 MakeError(UndefinedVarError, EvalError);
 MakeError(MissingArgumentError, EvalError);
 MakeError(RestrictedPathError, Error);
-MakeError(CachedEvalError, EvalError);
 MakeError(InfiniteRecursionError, EvalError);
 
 struct InvalidPathError : public EvalError