Merge pull request #9920 from 9999years/forbid-nested-debuggers

Forbid nested debuggers

doc/manual/rl-next/forbid-nested-debuggers.md

@@ -0,0 +1,32 @@
+---
+synopsis: Nested debuggers are no longer supported
+prs: 9920
+---
+
+Previously, evaluating an expression that throws an error in the debugger would
+enter a second, nested debugger:
+
+```
+nix-repl> builtins.throw "what"
+error: what
+
+
+Starting REPL to allow you to inspect the current state of the evaluator.
+
+Welcome to Nix 2.18.1. Type :? for help.
+
+nix-repl>
+```
+
+Now, it just prints the error message like `nix repl`:
+
+```
+nix-repl> builtins.throw "what"
+error:
+       … while calling the 'throw' builtin
+         at «string»:1:1:
+            1| builtins.throw "what"
+             | ^
+
+       error: what
+```

src/libcmd/repl.cc

@@ -336,13 +336,7 @@ ReplExitStatus NixRepl::mainLoop()
               printMsg(lvlError, e.msg());
             }
         } catch (EvalError & e) {
-            // in debugger mode, an EvalError should trigger another repl session.
+            printMsg(lvlError, e.msg());
-            // 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());
         } catch (Error & e) {
             printMsg(lvlError, e.msg());
         } catch (Interrupted & e) {

src/libexpr/eval.cc

@@ -762,10 +762,24 @@ std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const Stati
     return vm;
 }
 
+/**
+ * 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;
+    }
+};
+
 void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr & expr)
 {
-    // double check we've got the debugRepl function pointer.
+    // Make sure we have a debugger to run and we're not already in a debugger.
-    if (!debugRepl)
+    if (!debugRepl || inDebugger)
         return;
 
     auto dts =
@@ -792,6 +806,7 @@ 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);
         auto exitStatus = (debugRepl)(ref<EvalState>(shared_from_this()), *vm);
         switch (exitStatus) {
             case ReplExitStatus::QuitAll:

src/libexpr/eval.hh

@@ -153,6 +153,7 @@ struct DebugTrace {
     bool isError;
 };
 
+
 class EvalState : public std::enable_shared_from_this<EvalState>
 {
 public:
@@ -222,6 +223,7 @@ public:
      */
     ReplExitStatus (* debugRepl)(ref<EvalState> es, const ValMap & extraEnv);
     bool debugStop;
+    bool inDebugger = false;
     int trylevel;
     std::list<DebugTrace> debugTraces;
     std::map<const Expr*, const std::shared_ptr<const StaticEnv>> exprEnvs;

src/libutil/fmt.hh

@@ -8,7 +8,6 @@
 
 namespace nix {
 
-namespace {
 /**
  * A helper for writing `boost::format` expressions.
  *
@@ -35,14 +34,13 @@ inline void formatHelper(F & f, const T & x, const Args & ... args)
 /**
  * Set the correct exceptions for `fmt`.
  */
-void setExceptions(boost::format & fmt)
+inline void setExceptions(boost::format & fmt)
 {
     fmt.exceptions(
         boost::io::all_error_bits ^
         boost::io::too_many_args_bit ^
         boost::io::too_few_args_bit);
 }
-}
 
 /**
  * A helper for writing a `boost::format` expression to a string.