From e5eebda19475ab4f25346128e5428c27e526c7ce Mon Sep 17 00:00:00 2001
From: Ben Burdette <bburdette@gmail.com>
Date: Thu, 23 Dec 2021 13:36:39 -0700
Subject: [PATCH] DebugTrace

---
 src/libcmd/command.cc |  2 +-
 src/libcmd/command.hh |  4 +---
 src/libcmd/repl.cc    | 11 +++++------
 src/libexpr/eval.cc   | 22 +++++++++++++++-------
 src/libexpr/eval.hh   |  8 +++++++-
 5 files changed, 29 insertions(+), 18 deletions(-)

diff --git a/src/libcmd/command.cc b/src/libcmd/command.cc
index 0ada5fa3c..6c0f84c4b 100644
--- a/src/libcmd/command.cc
+++ b/src/libcmd/command.cc
@@ -84,7 +84,7 @@ ref<EvalState> EvalCommand::getEvalState()
                 if (expr.staticenv)
                 {
                     auto vm = mapStaticEnvBindings(*expr.staticenv.get(), env);
-                    runRepl(evalState, ref<const Error>(&error), *vm);
+                    runRepl(evalState,  &error, *vm);
                 }
             };
     }
diff --git a/src/libcmd/command.hh b/src/libcmd/command.hh
index e27ee2e9e..e2c72256e 100644
--- a/src/libcmd/command.hh
+++ b/src/libcmd/command.hh
@@ -314,8 +314,6 @@ void printClosureDiff(
 
 void runRepl(
     ref<EvalState> evalState,
-    std::optional<ref<const Error>> debugError,
+    const Error *debugError,
     const std::map<std::string, Value *> & extraEnv);
-
-
 }
diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index 3289aea3e..6859e5c07 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -50,7 +50,7 @@ struct NixRepl
     ref<EvalState> state;
     Bindings * autoArgs;
 
-    std::optional<ref<const Error>> debugError;
+    const Error *debugError;
 
     Strings loadedFiles;
 
@@ -470,13 +470,12 @@ bool NixRepl::processLine(string line)
 
         }
         else if (arg == "error") {
-          if (this->debugError.has_value()) {
-            // TODO user --show-trace setting?
-            showErrorInfo(std::cout, (*debugError)->info(), true);
+          if (this->debugError) {
+              showErrorInfo(std::cout, debugError->info(), true);
           }
           else
           {
-            notice("error information not available");
+              notice("error information not available");
           }
         }
     }
@@ -893,7 +892,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
 
 void runRepl(
     ref<EvalState> evalState,
-    std::optional<ref<const Error>> debugError,
+    const Error *debugError,
     const std::map<std::string, Value *> & extraEnv)
 {
     auto repl = std::make_unique<NixRepl>(evalState);
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index d7deb550e..b8d060276 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -40,7 +40,7 @@ std::function<void(const Error & error, const Env & env, const Expr & expr)> deb
 
 class DebugTraceStacker {
   public:
-   DebugTraceStacker(EvalState &evalState, Trace t)
+   DebugTraceStacker(EvalState &evalState, DebugTrace t)
     :evalState(evalState), trace(t)
     {
         evalState.debugTraces.push_front(t);
@@ -50,7 +50,7 @@ class DebugTraceStacker {
      evalState.debugTraces.pop_front();
    }
    EvalState &evalState;
-   Trace trace;
+   DebugTrace trace;
 };
 
 static char * dupString(const char * s)
@@ -911,12 +911,14 @@ LocalNoInline(void addErrorTrace(Error & e, const Pos & pos, const char * s, con
 }
 
 LocalNoInline(std::unique_ptr<DebugTraceStacker>
-  makeDebugTraceStacker(EvalState &state, std::optional<ErrPos> pos, const char * s, const string & s2))
+  makeDebugTraceStacker(EvalState &state, Expr &expr, std::optional<ErrPos> pos, const char * s, const string & s2))
 {
   return std::unique_ptr<DebugTraceStacker>(
       new DebugTraceStacker(
           state,
-          Trace {.pos = pos,
+          DebugTrace 
+                {.pos = pos,
+                 .expr = expr,
                  .hint = hintfmt(s, s2)
                 }));
 }
@@ -1143,6 +1145,7 @@ void EvalState::cacheFile(
             debuggerHook ?
                 makeDebugTraceStacker(
                     *this,
+                    *e,
                     (e->getPos() ? std::optional(ErrPos(*e->getPos())) : std::nullopt),
                     "while evaluating the file '%1%':", resolvedPath)
                 : std::unique_ptr<DebugTraceStacker>();
@@ -1375,6 +1378,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
           debuggerHook ?
               makeDebugTraceStacker(
                   state,
+                  *this,
                   *pos2,
                   "while evaluating the attribute '%1%'",
                   showAttrPath(state, env, attrPath))
@@ -1524,7 +1528,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
             try {
                 std::unique_ptr<DebugTraceStacker> dts =
                     debuggerHook ?
-                        makeDebugTraceStacker(*this, lambda.pos,
+                        makeDebugTraceStacker(*this, *lambda.body, lambda.pos,
                                            "while evaluating %s",
                                            (lambda.name.set()
                                                ? "'" + (string) lambda.name + "'"
@@ -1925,10 +1929,14 @@ void EvalState::forceValueDeep(Value & v)
         if (v.type() == nAttrs) {
             for (auto & i : *v.attrs)
                 try {
+
                     std::unique_ptr<DebugTraceStacker> dts =
                       debuggerHook ?
-                          makeDebugTraceStacker(*this, *i.pos,
-                                            "while evaluating the attribute '%1%'", i.name)
+                          // if the value is a thunk, we're evaling.  otherwise no trace necessary.
+                          (i.value->isThunk() ? 
+                              makeDebugTraceStacker(*this, *v.thunk.expr, *i.pos,
+                                                "while evaluating the attribute '%1%'", i.name)
+                              : std::unique_ptr<DebugTraceStacker>())
                       : std::unique_ptr<DebugTraceStacker>();
 
                     recurse(*i.value);
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index c3717b3c2..c7a19e100 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -74,6 +74,12 @@ struct RegexCache;
 
 std::shared_ptr<RegexCache> makeRegexCache();
 
+struct DebugTrace {
+    std::optional<ErrPos> pos;
+    Expr &expr;
+    hintformat hint;
+};
+
 
 class EvalState
 {
@@ -109,7 +115,7 @@ public:
     RootValue vCallFlake = nullptr;
     RootValue vImportedDrvToDerivation = nullptr;
 
-    std::list<Trace> debugTraces;
+    std::list<DebugTrace> debugTraces;
 
 private:
     SrcToStore srcToStore;