refactor(3p/nix/libexpr): Remove the nix::Symbol default constructor

Having a default constructor for this causes a variety of annoying
situations across the codebase in which this is initialised to an
unexpected value, leading to constant guarding against those
conditions.

It turns out there's actually no intrinsic reason that this default
constructor needs to exist. The biggest one was addressed in CL/1138
and this commit cleans up the remaining bits.

Change-Id: I4a847f50bc90e72f028598196592a7d8730a4e01
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1139
Reviewed-by: isomer <isomer@tvl.fyi>
Tested-by: BuildkiteCI

third_party/nix/src/libexpr/attr-set.cc

@@ -59,7 +59,7 @@ Bindings::iterator Bindings::end() { return attributes_.end(); }
 
 void Bindings::merge(const Bindings& other) {
   for (auto& [key, value] : other.attributes_) {
-    this->attributes_[key] = value;
+    this->attributes_.insert_or_assign(key, value);
   }
 }
 

third_party/nix/src/libexpr/attr-set.hh

@@ -20,7 +20,6 @@ struct Attr {
   Pos* pos;      // TODO(tazjin): Who owns this?
   Attr(Symbol name, Value* value, Pos* pos = &noPos)
       : name(name), value(value), pos(pos){};
-  Attr() : pos(&noPos){};
 };
 
 // Convenience alias for the backing map, with the garbage-collecting

third_party/nix/src/libexpr/eval.cc

@@ -6,6 +6,7 @@
 #include <fstream>
 #include <iostream>
 #include <new>
+#include <optional>
 #include <variant>
 
 #include <absl/strings/match.h>
@@ -320,6 +321,7 @@ EvalState::EvalState(const Strings& _searchPath, const ref<Store>& store)
       sOutputHash(symbols.Create("outputHash")),
       sOutputHashAlgo(symbols.Create("outputHashAlgo")),
       sOutputHashMode(symbols.Create("outputHashMode")),
+      sDerivationNix(std::nullopt),
       repair(NoRepair),
       store(store),
       baseEnv(allocEnv(128)),
@@ -664,9 +666,9 @@ static inline void mkThunk(Value& v, Env& env, Expr* expr) {
 void EvalState::mkThunk_(Value& v, Expr* expr) { mkThunk(v, baseEnv, expr); }
 
 void EvalState::mkPos(Value& v, Pos* pos) {
-  if ((pos != nullptr) && pos->file.set()) {
+  if ((pos != nullptr) && pos->file.has_value() && pos->file.value().set()) {
     mkAttrs(v, 3);
-    mkString(*allocAttr(v, sFile), pos->file);
+    mkString(*allocAttr(v, sFile), pos->file.value());
     mkInt(*allocAttr(v, sLine), pos->line);
     mkInt(*allocAttr(v, sColumn), pos->column);
   } else {
@@ -847,7 +849,6 @@ void ExprAttrs::eval(EvalState& state, Env& env, Value& value) {
                      nameSym, i.pos, *j->second.pos);
     }
 
-    i.valueExpr->setName(nameSym);
     value.attrs->push_back(
         Attr(nameSym, i.valueExpr->maybeThunk(state, *dynamicEnv), &i.pos));
   }
@@ -936,7 +937,13 @@ void ExprSelect::eval(EvalState& state, Env& env, Value& v) {
     state.forceValue(*vAttrs, (pos2 != nullptr ? *pos2 : this->pos));
 
   } catch (Error& e) {
-    if ((pos2 != nullptr) && pos2->file != state.sDerivationNix) {
+    // This code relies on 'sDerivationNix' being correcty mutated at
+    // some prior point (it would previously otherwise have been a
+    // nullptr).
+    //
+    // We haven't seen this fail, so for now the contained value is
+    // just accessed at the risk of potentially crashing.
+    if ((pos2 != nullptr) && pos2->file != state.sDerivationNix.value()) {
       addErrorPrefix(e, "while evaluating the attribute '%1%' at %2%:\n",
                      showAttrPath(state, env, attrPath), *pos2);
     }
@@ -1792,8 +1799,8 @@ void EvalState::printStats() {
         auto list = topObj.list("functions");
         for (auto& i : functionCalls) {
           auto obj = list.object();
-          if (i.first->name.set()) {
-            obj.attr("name", (const std::string&)i.first->name);
+          if (i.first->name.has_value()) {
+            obj.attr("name", (const std::string&)i.first->name.value());
           } else {
             obj.attr("name", nullptr);
           }

third_party/nix/src/libexpr/eval.hh

@@ -62,10 +62,13 @@ class EvalState {
   SymbolTable symbols;
 
   const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue, sSystem,
-      sOverrides, sOutputs, sOutputName, sIgnoreNulls, sFile, sLine, sColumn,
-      sFunctor, sToString, sRight, sWrong, sStructuredAttrs, sBuilder, sArgs,
-      sOutputHash, sOutputHashAlgo, sOutputHashMode;
-  Symbol sDerivationNix;
+      sOutputs, sOutputName, sIgnoreNulls, sFile, sLine, sColumn, sFunctor,
+      sToString, sRight, sWrong, sStructuredAttrs, sBuilder, sArgs, sOutputHash,
+      sOutputHashAlgo, sOutputHashMode;
+
+  // Symbol representing the path to the built-in 'derivation.nix'
+  // file, set during primops initialisation.
+  std::optional<Symbol> sDerivationNix;
 
   /* If set, force copying files to the Nix store even if they
      already exist there. */

third_party/nix/src/libexpr/nixexpr.cc

@@ -187,11 +187,11 @@ void ExprConcatStrings::show(std::ostream& str) const {
 void ExprPos::show(std::ostream& str) const { str << "__curPos"; }
 
 std::ostream& operator<<(std::ostream& str, const Pos& pos) {
-  if (!pos) {
+  if (!pos || !pos.file.has_value()) {
     str << "undefined position";
   } else {
     str << (format(ANSI_BOLD "%1%" ANSI_NORMAL ":%2%:%3%") %
-            (std::string)pos.file % pos.line % pos.column)
+            (std::string)pos.file.value() % pos.line % pos.column)
                .str();
   }
   return str;
@@ -401,17 +401,12 @@ void ExprConcatStrings::bindVars(const StaticEnv& env) {
 void ExprPos::bindVars(const StaticEnv& env) {}
 
 /* Storing function names. */
-
-void Expr::setName(Symbol& name) {}
-
-void ExprLambda::setName(Symbol& name) {
-  this->name = name;
-  body->setName(name);
-}
+void ExprLambda::setName(Symbol& name) { this->name = name; }
 
 std::string ExprLambda::showNamePos() const {
   return (format("%1% at %2%") %
-          (name.set() ? "'" + (std::string)name + "'" : "anonymous function") %
+          (name.has_value() ? "'" + (std::string)name.value() + "'"
+                            : "anonymous function") %
           pos)
       .str();
 }

third_party/nix/src/libexpr/nixexpr.hh

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <map>
+#include <optional>
 #include <variant>
 
 #include "libexpr/symbol-table.hh"
@@ -21,20 +22,28 @@ MakeError(RestrictedPathError, Error);
 /* Position objects. */
 
 struct Pos {
-  Symbol file;
+  std::optional<Symbol> file;
   unsigned int line, column;
-  Pos() : line(0), column(0){};
-  Pos(const Symbol& file, unsigned int line, unsigned int column)
+  Pos(const std::optional<Symbol>& file, unsigned int line, unsigned int column)
       : file(file), line(line), column(column){};
+
+  // TODO(tazjin): remove this - empty pos is never useful
+  Pos() : file(std::nullopt), line(0), column(0){};
+
   operator bool() const { return line != 0; }
+
   bool operator<(const Pos& p2) const {
+    if (!file.has_value()) {
+      return true;
+    }
+
     if (!line) {
       return p2.line;
     }
     if (!p2.line) {
       return false;
     }
-    int d = ((std::string)file).compare((std::string)p2.file);
+    int d = ((std::string)file.value()).compare((std::string)p2.file.value());
     if (d < 0) {
       return true;
     }
@@ -75,7 +84,6 @@ struct Expr {
   virtual void bindVars(const StaticEnv& env);
   virtual void eval(EvalState& state, Env& env, Value& v);
   virtual Value* maybeThunk(EvalState& state, Env& env);
-  virtual void setName(Symbol& name);
 };
 
 std::ostream& operator<<(std::ostream& str, const Expr& e);
@@ -219,8 +227,9 @@ struct Formals {
 };
 
 struct ExprLambda : Expr {
+ public:
   Pos pos;
-  Symbol name;
+  std::optional<Symbol> name;
   Symbol arg;
   bool matchAttrs;
   Formals* formals;
@@ -233,10 +242,11 @@ struct ExprLambda : Expr {
         formals(formals),
         body(body) {
     if (!arg.empty() && formals &&
-        formals->argNames.find(arg) != formals->argNames.end())
+        formals->argNames.find(arg) != formals->argNames.end()) {
       throw ParseError(
           format("duplicate formal function argument '%1%' at %2%") % arg %
           pos);
+    }
   };
   void setName(Symbol& name);
   std::string showNamePos() const;

third_party/nix/src/libexpr/parser.y

@@ -16,6 +16,7 @@
 #ifndef BISON_HEADER
 #define BISON_HEADER
 
+#include <optional>
 #include <variant>
 #include "libutil/util.hh"
 #include "libexpr/nixexpr.hh"
@@ -30,7 +31,7 @@ namespace nix {
         SymbolTable & symbols;
         Expr* result;
         Path basePath;
-        Symbol path;
+        std::optional<Symbol> path;
         std::string error;
         Symbol sLetBody;
         ParseData(EvalState & state)
@@ -139,7 +140,6 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
         } else {
             // This attr path is not defined. Let's create it.
             attrs->attrs[*sym] = ExprAttrs::AttrDef(e, pos);
-            e->setName(*sym);
         }
     } else {
         // Same caveat as the identical line above.
@@ -502,9 +502,10 @@ attrpath
       if (str) {
           $$->push_back(AttrName(str->s));
           delete str;
-      } else
+      } else {
           $$->push_back(AttrName($3));
       }
+    }
   | attr { $$ = new std::vector<AttrName>; $$->push_back(AttrName(data->symbols.Create($1))); }
   | string_attr
     { $$ = new std::vector<AttrName>;

third_party/nix/src/libexpr/primops.cc

@@ -234,7 +234,7 @@ void prim_exec(EvalState& state, const Pos& pos, Value** args, Value& v) {
   auto output = runProgram(program, true, commandArgs);
   Expr* parsed;
   try {
-    parsed = state.parseExprFromString(output, pos.file);
+    parsed = state.parseExprFromString(output, pos.file.value());
   } catch (Error& e) {
     e.addPrefix(format("While parsing the output from '%1%', at %2%\n") %
                 program % pos);

third_party/nix/src/libexpr/symbol-table.hh

@@ -13,8 +13,6 @@ class Symbol {
   friend class SymbolTable;
 
  public:
-  Symbol() : s(0){};
-
   bool operator==(const Symbol& s2) const { return s == s2.s; }
 
   bool operator!=(const Symbol& s2) const { return s != s2.s; }

third_party/nix/src/libexpr/value-to-xml.cc

@@ -20,7 +20,7 @@ static void printValueAsXML(EvalState& state, bool strict, bool location,
                             PathSet& drvsSeen);
 
 static void posToXML(XMLAttrs& xmlAttrs, const Pos& pos) {
-  xmlAttrs["path"] = pos.file;
+  xmlAttrs["path"] = pos.file.value();
   xmlAttrs["line"] = (format("%1%") % pos.line).str();
   xmlAttrs["column"] = (format("%1%") % pos.column).str();
 }