refactor(3p/nix/libexpr): Make nix::AttrName a std::variant

nix:AttrName was one of the few classes that relied on the default
constructor of nix::Symbol (which I am trying to remove in a separate
change).

The class essentially represents the name of an attribute in a set,
which is either just a string expression or a dynamically evaluated
expression (e.g. string interpolation).

Previously it would be constructed by only setting one of the fields
and defaulting the other, now it is an explicit std::variant.

Note that there are several code paths where not all eventualities are
handled and this code is bug-for-bug compatible with those, except
that unknown conditions (which should never work) are now throwing
instead of silently doing ... something.

The language tests pass with this change, and the depot derivations
that I tested with evaluated successfully.

Change-Id: Icf1ee60a5f8308f4ab18a82749e00cf37a938a8f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1138
Reviewed-by: edef <edef@edef.eu>
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

third_party/nix/src/libexpr/eval.cc

@@ -6,6 +6,7 @@
 #include <fstream>
 #include <iostream>
 #include <new>
+#include <variant>
 
 #include <absl/strings/match.h>
 #include <gc/gc.h>
@@ -24,6 +25,7 @@
 #include "libutil/hash.hh"
 #include "libutil/json.hh"
 #include "libutil/util.hh"
+#include "libutil/visitor.hh"
 
 namespace nix {
 
@@ -185,13 +187,15 @@ static void* oomHandler(size_t requested) {
 #endif
 
 static Symbol getName(const AttrName& name, EvalState& state, Env& env) {
-  if (name.symbol.set()) {
-    return name.symbol;
-  }
+  return std::visit(
+      util::overloaded{[&](const Symbol& name) -> Symbol { return name; },
+                       [&](Expr* expr) -> Symbol {
                          Value nameValue;
-  name.expr->eval(state, env, nameValue);
+                         expr->eval(state, env, nameValue);
                          state.forceStringNoCtx(nameValue);
                          return state.symbols.Create(nameValue.string.s);
+                       }},
+      name);
 }
 
 static bool gcInitialised = false;
@@ -890,12 +894,7 @@ static std::string showAttrPath(EvalState& state, Env& env,
     } else {
       first = false;
     }
-    try {
     out << getName(i, state, env);
-    } catch (Error& e) {
-      assert(!i.symbol.set());
-      out << "\"${" << *i.expr << "}\"";
-    }
   }
   return out.str();
 }

third_party/nix/src/libexpr/nixexpr.cc

@@ -1,9 +1,11 @@
 #include "libexpr/nixexpr.hh"
 
 #include <cstdlib>
+#include <variant>
 
 #include "libstore/derivations.hh"
 #include "libutil/util.hh"
+#include "libutil/visitor.hh"
 
 namespace nix {
 
@@ -198,17 +200,17 @@ std::ostream& operator<<(std::ostream& str, const Pos& pos) {
 std::string showAttrPath(const AttrPath& attrPath) {
   std::ostringstream out;
   bool first = true;
-  for (auto& i : attrPath) {
+  for (auto& attr : attrPath) {
     if (!first) {
       out << '.';
     } else {
       first = false;
     }
-    if (i.symbol.set()) {
-      out << i.symbol;
-    } else {
-      out << "\"${" << *i.expr << "}\"";
-    }
+
+    std::visit(util::overloaded{
+                   [&](const Symbol& sym) { out << sym; },
+                   [&](const Expr* expr) { out << "\"${" << *expr << "}\""; }},
+               attr);
   }
   return out.str();
 }
@@ -268,8 +270,8 @@ void ExprSelect::bindVars(const StaticEnv& env) {
     def->bindVars(env);
   }
   for (auto& i : attrPath) {
-    if (!i.symbol.set()) {
-      i.expr->bindVars(env);
+    if (auto* expr = std::get_if<Expr*>(&i)) {
+      (*expr)->bindVars(env);
     }
   }
 }
@@ -277,8 +279,8 @@ void ExprSelect::bindVars(const StaticEnv& env) {
 void ExprOpHasAttr::bindVars(const StaticEnv& env) {
   e->bindVars(env);
   for (auto& i : attrPath) {
-    if (!i.symbol.set()) {
-      i.expr->bindVars(env);
+    if (auto* expr = std::get_if<Expr*>(&i)) {
+      (*expr)->bindVars(env);
     }
   }
 }

third_party/nix/src/libexpr/nixexpr.hh

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <map>
+#include <variant>
 
 #include "libexpr/symbol-table.hh"
 #include "libexpr/value.hh"
@@ -60,12 +61,7 @@ class EvalState;
 struct StaticEnv;
 
 /* An attribute path is a sequence of attribute names. */
-struct AttrName {
-  Symbol symbol;
-  Expr* expr;
-  AttrName(const Symbol& s) : symbol(s){};
-  AttrName(Expr* e) : expr(e){};
-};
+using AttrName = std::variant<Symbol, Expr*>;
 
 typedef std::vector<AttrName> AttrPath;
 

third_party/nix/src/libexpr/parser.y

@@ -16,6 +16,7 @@
 #ifndef BISON_HEADER
 #define BISON_HEADER
 
+#include <variant>
 #include "libutil/util.hh"
 #include "libexpr/nixexpr.hh"
 #include "libexpr/eval.hh"
@@ -84,30 +85,39 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
     // Checking attrPath validity.
     // ===========================
     for (i = attrPath.begin(); i + 1 < attrPath.end(); i++) {
-        if (i->symbol.set()) {
-            ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(i->symbol);
+        if (const auto* sym = std::get_if<Symbol>(&(*i)); sym && sym->set()) {
+            ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(*sym);
             if (j != attrs->attrs.end()) {
                 if (!j->second.inherited) {
                     ExprAttrs * attrs2 = dynamic_cast<ExprAttrs *>(j->second.e);
                     if (!attrs2) { dupAttr(attrPath, pos, j->second.pos); }
                     attrs = attrs2;
-                } else
-                    dupAttr(attrPath, pos, j->second.pos);
                 } else {
-                ExprAttrs * nested = new ExprAttrs;
-                attrs->attrs[i->symbol] = ExprAttrs::AttrDef(nested, pos);
-                attrs = nested;
+                    dupAttr(attrPath, pos, j->second.pos);
                 }
             } else {
                 ExprAttrs* nested = new ExprAttrs;
-            attrs->dynamicAttrs.push_back(ExprAttrs::DynamicAttrDef(i->expr, nested, pos));
+                attrs->attrs[*sym] = ExprAttrs::AttrDef(nested, pos);
+                attrs = nested;
+            }
+        } else {
+          // Yes, this code does not handle all conditions
+          // exhaustively. We use std::get to throw if the condition
+          // that isn't covered happens, which is potentially a
+          // behaviour change from the previous default constructed
+          // Symbol. It should alert us about anything untoward going
+          // on here.
+          auto* expr = std::get<Expr*>(*i);
+
+          ExprAttrs *nested = new ExprAttrs;
+          attrs->dynamicAttrs.push_back(ExprAttrs::DynamicAttrDef(expr, nested, pos));
           attrs = nested;
         }
     }
     // Expr insertion.
     // ==========================
-    if (i->symbol.set()) {
-        ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(i->symbol);
+    if (auto* sym = std::get_if<Symbol>(&(*i)); sym && sym->set()) {
+        ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(*sym);
         if (j != attrs->attrs.end()) {
             // This attr path is already defined. However, if both
             // e and the expr pointed by the attr path are two attribute sets,
@@ -118,8 +128,9 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
             if (jAttrs && ae) {
                 for (auto & ad : ae->attrs) {
                     auto j2 = jAttrs->attrs.find(ad.first);
-                    if (j2 != jAttrs->attrs.end()) // Attr already defined in iAttrs, error.
+                    if (j2 != jAttrs->attrs.end()) {// Attr already defined in iAttrs, error.
                         dupAttr(ad.first, j2->second.pos, ad.second.pos);
+                    }
                     jAttrs->attrs[ad.first] = ad.second;
                 }
             } else {
@@ -127,11 +138,13 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
             }
         } else {
             // This attr path is not defined. Let's create it.
-            attrs->attrs[i->symbol] = ExprAttrs::AttrDef(e, pos);
-            e->setName(i->symbol);
+            attrs->attrs[*sym] = ExprAttrs::AttrDef(e, pos);
+            e->setName(*sym);
         }
     } else {
-        attrs->dynamicAttrs.push_back(ExprAttrs::DynamicAttrDef(i->expr, e, pos));
+        // Same caveat as the identical line above.
+        auto* expr = std::get<Expr*>(*i);
+        attrs->dynamicAttrs.push_back(ExprAttrs::DynamicAttrDef(expr, e, pos));
     }
 }
 
@@ -444,19 +457,23 @@ binds
   | binds INHERIT attrs ';'
     { $$ = $1;
       for (auto & i : *$3) {
-          if ($$->attrs.find(i.symbol) != $$->attrs.end())
-              dupAttr(i.symbol, makeCurPos(@3, data), $$->attrs[i.symbol].pos);
+          auto sym = std::get<Symbol>(i);
+          if ($$->attrs.find(sym) != $$->attrs.end()) {
+              dupAttr(sym, makeCurPos(@3, data), $$->attrs[sym].pos);
+          }
           Pos pos = makeCurPos(@3, data);
-          $$->attrs[i.symbol] = ExprAttrs::AttrDef(new ExprVar(CUR_POS, i.symbol), pos, true);
+          $$->attrs[sym] = ExprAttrs::AttrDef(new ExprVar(CUR_POS, sym), pos, true);
       }
     }
   | binds INHERIT '(' expr ')' attrs ';'
     { $$ = $1;
       /* !!! Should ensure sharing of the expression in $4. */
       for (auto & i : *$6) {
-          if ($$->attrs.find(i.symbol) != $$->attrs.end())
-              dupAttr(i.symbol, makeCurPos(@6, data), $$->attrs[i.symbol].pos);
-          $$->attrs[i.symbol] = ExprAttrs::AttrDef(new ExprSelect(CUR_POS, $4, i.symbol), makeCurPos(@6, data));
+          auto sym = std::get<Symbol>(i);
+          if ($$->attrs.find(sym) != $$->attrs.end()) {
+            dupAttr(sym, makeCurPos(@6, data), $$->attrs[sym].pos);
+          }
+          $$->attrs[sym] = ExprAttrs::AttrDef(new ExprSelect(CUR_POS, $4, sym), makeCurPos(@6, data));
       }
     }
   | { $$ = new ExprAttrs; }

third_party/nix/src/libutil/config.cc

@@ -102,8 +102,8 @@ void AbstractConfig::applyConfigFile(const Path& path) {
       }
 
       // TODO(tazjin): absl::string_view after path functions are fixed.
-      std::vector<std::string> tokens =
-          absl::StrSplit(line, absl::ByAnyChar(" \t\n\r"), absl::SkipWhitespace());
+      std::vector<std::string> tokens = absl::StrSplit(
+          line, absl::ByAnyChar(" \t\n\r"), absl::SkipWhitespace());
       if (tokens.empty()) {
         continue;
       }

third_party/nix/src/nix-daemon/nix-daemon.cc

@@ -16,6 +16,7 @@
 #include <unistd.h>
 
 #include "libmain/shared.hh"
+#include "libproto/worker.pb.h"
 #include "libstore/derivations.hh"
 #include "libstore/globals.hh"
 #include "libstore/local-store.hh"
@@ -27,7 +28,6 @@
 #include "libutil/serialise.hh"
 #include "libutil/util.hh"
 #include "nix/legacy.hh"
-#include "libproto/worker.pb.h"
 
 using namespace nix;