From aab0116a9c10a681512a28a101c20893088e8def Mon Sep 17 00:00:00 2001
From: Rebecca Turner <rbt@sent.as>
Date: Fri, 2 Feb 2024 20:07:42 -0800
Subject: [PATCH] Forbid nested debuggers

---
 src/libcmd/repl.cc  |  8 +-------
 src/libexpr/eval.cc |  5 +++--
 src/libexpr/eval.hh | 19 +++++++++++++++++++
 3 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index c9df1c257568..7223350f29d1 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -304,13 +304,7 @@ void NixRepl::mainLoop()
               printMsg(lvlError, e.msg());
             }
         } catch (EvalError & e) {
-            // in debugger mode, an EvalError should trigger another repl session.
-            // when that session returns the exception will land here.  No need to show it again;
-            // show the error for this repl session instead.
-            if (state->debugRepl && !state->debugTraces.empty())
-                showDebugTrace(std::cout, state->positions, state->debugTraces.front());
-            else
-                printMsg(lvlError, e.msg());
+            printMsg(lvlError, e.msg());
         } catch (Error & e) {
             printMsg(lvlError, e.msg());
         } catch (Interrupted & e) {
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 5bc62589c8fe..a8f6728f64d3 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -764,8 +764,8 @@ std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const Stati
 
 void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr & expr)
 {
-    // double check we've got the debugRepl function pointer.
-    if (!debugRepl)
+    // Make sure we have a debugger to run and we're not already in a debugger.
+    if (!debugRepl || inDebugger)
         return;
 
     auto dts =
@@ -792,6 +792,7 @@ void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr &
     auto se = getStaticEnv(expr);
     if (se) {
         auto vm = mapStaticEnvBindings(symbols, *se.get(), env);
+        DebuggerGuard _guard(inDebugger);
         (debugRepl)(ref<EvalState>(shared_from_this()), *vm);
     }
 }
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 3c7c5da2741c..08d53403d94d 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -152,6 +152,24 @@ struct DebugTrace {
     bool isError;
 };
 
+
+namespace {
+/**
+ * Sets `inDebugger` to true on construction and false on destruction.
+ */
+class DebuggerGuard {
+    bool & inDebugger;
+public:
+    DebuggerGuard(bool & inDebugger) : inDebugger(inDebugger) {
+        inDebugger = true;
+    }
+    ~DebuggerGuard() {
+        inDebugger = false;
+    }
+};
+}
+
+
 class EvalState : public std::enable_shared_from_this<EvalState>
 {
 public:
@@ -222,6 +240,7 @@ public:
     void (* debugRepl)(ref<EvalState> es, const ValMap & extraEnv);
     bool debugStop;
     bool debugQuit;
+    bool inDebugger = false;
     int trylevel;
     std::list<DebugTrace> debugTraces;
     std::map<const Expr*, const std::shared_ptr<const StaticEnv>> exprEnvs;