AttrCursor: Remove forceErrors

src/libexpr/eval-cache.cc

@@ -7,6 +7,25 @@
 
 namespace nix::eval_cache {
 
+CachedEvalError::CachedEvalError(ref<AttrCursor> cursor, Symbol attr)
+ : EvalError(cursor->root->state, "cached failure of attribute '%s'", cursor->getAttrPathStr(attr))
+ , cursor(cursor), attr(attr)
+{ }
+
+void CachedEvalError::force()
+{
+ auto & v = cursor->forceValue();
+
+ if (v.type() == nAttrs) {
+ auto a = v.attrs()->get(this->attr);
+
+ state.forceValue(*a->value, a->pos);
+ }
+
+ // Shouldn't happen.
+ throw EvalError(state, "evaluation of cached failed attribute '%s' unexpectedly succeeded", cursor->getAttrPathStr(attr));
+}
+
 static const char * schema = R"sql(
 create table if not exists Attributes (
  parent integer not null,
@@ -470,7 +489,7 @@ 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)
@@ -487,12 +506,9 @@ std::shared_ptr<AttrCursor> AttrCursor::maybeGetAttr(Symbol name, bool forceErro
  if (attr) {
  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));
- else
- throw CachedEvalError(root->state, "cached failure of attribute '%s'", getAttrPathStr(name));
- } else
+ else if (std::get_if<failed_t>(&attr->second))
+ throw CachedEvalError(ref(shared_from_this()), name);
+ else
  return std::make_shared<AttrCursor>(root,
  std::make_pair(shared_from_this(), name), nullptr, std::move(attr));
  }
@@ -537,9 +553,9 @@ 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);
@@ -550,11 +566,11 @@ 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);
@@ -751,7 +767,7 @@ 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

@@ -10,14 +10,28 @@
 
 namespace nix::eval_cache {
 
-MakeError(CachedEvalError, EvalError);
-
 struct AttrDb;
 class AttrCursor;
 
+struct CachedEvalError : EvalError
+{
+ const ref<AttrCursor> cursor;
+ const Symbol attr;
+
+ CachedEvalError(ref<AttrCursor> cursor, Symbol attr);
+
+ /**
+ * Evaluate this attribute, which should result in a regular
+ * `EvalError` exception being thrown.
+ */
+ [[noreturn]]
+ void force();
+};
+
 class EvalCache : public std::enable_shared_from_this<EvalCache>
 {
  friend class AttrCursor;
+ friend class CachedEvalError;
 
  std::shared_ptr<AttrDb> db;
  EvalState & state;
@@ -73,6 +87,7 @@ typedef std::variant<
 class AttrCursor : public std::enable_shared_from_this<AttrCursor>
 {
  friend class EvalCache;
+ friend class CachedEvalError;
 
  ref<EvalCache> root;
  typedef std::optional<std::pair<std::shared_ptr<AttrCursor>, Symbol>> Parent;
@@ -102,19 +117,19 @@ 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);
 
  /**
  * 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

@@ -43,7 +43,6 @@ MakeError(Abort, EvalError);
 MakeError(TypeError, EvalError);
 MakeError(UndefinedVarError, EvalError);
 MakeError(MissingArgumentError, EvalError);
-MakeError(CachedEvalError, EvalError);
 MakeError(InfiniteRecursionError, EvalError);
 
 struct InvalidPathError : public EvalError

src/nix/flake.cc

@@ -1176,7 +1176,7 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
  // If we don't recognize it, it's probably content
  return true;
  } catch (EvalError & e) {
- // Some attrs may contain errors, eg. legacyPackages of
+ // Some attrs may contain errors, e.g. legacyPackages of
  // nixpkgs. We still want to recurse into it, instead of
  // skipping it at all.
  return true;

src/nix/main.cc

@@ -18,6 +18,7 @@
 #include "terminal.hh"
 #include "users.hh"
 #include "network-proxy.hh"
+#include "eval-cache.hh"
 
 #include <sys/types.h>
 #include <regex>
@@ -515,7 +516,15 @@ void mainWrapped(int argc, char * * argv)
  if (args.command->second->forceImpureByDefault() && !evalSettings.pureEval.overridden) {
  evalSettings.pureEval = false;
  }
- args.command->second->run();
+
+ try {
+ args.command->second->run();
+ } catch (eval_cache::CachedEvalError & e) {
+ /* Evaluate the original attribute that resulted in this
+ cached error so that we can show the original error to the
+ user. */
+ e.force();
+ }
 }
 
 }

tests/functional/flakes/eval-cache.sh

@@ -0,0 +1,22 @@
+source ./common.sh
+
+requireGit
+
+flake1Dir="$TEST_ROOT/eval-cache-flake"
+
+createGitRepo "$flake1Dir" ""
+
+cat >"$flake1Dir/flake.nix" <<EOF
+{
+ description = "Fnord";
+ outputs = { self }: {
+ foo.bar = throw "breaks";
+ };
+}
+EOF
+
+git -C "$flake1Dir" add flake.nix
+git -C "$flake1Dir" commit -m "Init"
+
+expect 1 nix build "$flake1Dir#foo.bar" 2>&1 | grepQuiet 'error: breaks'
+expect 1 nix build "$flake1Dir#foo.bar" 2>&1 | grepQuiet 'error: breaks'

tests/functional/local.mk

@@ -16,6 +16,7 @@ nix_tests = \
  flakes/build-paths.sh \
  flakes/flake-in-submodule.sh \
  flakes/prefetch.sh \
+ flakes/eval-cache.sh \
  gc.sh \
  nix-collect-garbage-d.sh \
  remote-store.sh \