addErrorContext: Don't force the message inside the exception handler

src/libexpr/primops.cc

@@ -1022,20 +1022,32 @@ static RegisterPrimOp primop_throw(
 
 static void prim_addErrorContext(EvalState & state, const PosIdx pos, Value ** args, Value & v)
 {
+    std::exception_ptr error;
     try {
         state.forceValue(*args[1], pos);
         v = *args[1];
+        return;
+    } catch (Error &) {
+        error = std::current_exception();
+    }
+
+    /* Evaluate the message outside of the catch handler: it may force
+       thunks and thereby suspend the current fiber, which is not
+       allowed while an exception is being handled. */
+    NixStringContext context;
+    auto message = state
+                       .coerceToString(
+                           pos,
+                           *args[0],
+                           context,
+                           "while evaluating the error message passed to builtins.addErrorContext",
+                           false,
+                           false)
+                       .toOwned();
+
+    try {
+        std::rethrow_exception(error);
     } catch (Error & e) {
-        NixStringContext context;
-        auto message = state
-                           .coerceToString(
-                               pos,
-                               *args[0],
-                               context,
-                               "while evaluating the error message passed to builtins.addErrorContext",
-                               false,
-                               false)
-                           .toOwned();
         e.addTrace(nullptr, HintFmt(message), TracePrint::Always);
         throw;
     }

tests/functional/meson.build

@@ -146,6 +146,7 @@ suites = [
       'optimise-store.sh',
       'otel.sh',
       'output-normalization.sh',
+      'parallel-eval.sh',
       'pass-as-file.sh',
       'path-from-hash-part.sh',
       'path-info.sh',

tests/functional/parallel-eval.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+# Tests for the multi-threaded evaluator (`eval-cores`).
+
+source common.sh
+
+# `builtins.addErrorContext` evaluates its message after the body has
+# thrown. If the message depends on a thunk that another fiber is
+# evaluating, the current fiber has to wait for it. That must not
+# happen inside the exception handler (which the fiber cannot suspend
+# in), so the message is evaluated after leaving the handler.
+cat > "$TEST_ROOT/add-error-context.nix" <<'NIX'
+let
+  big = builtins.foldl' (a: b: a + b) 0 (builtins.genList (x: x) 3000000);
+in
+{
+  a = big;
+  b = builtins.addErrorContext "context mentioning ${toString big}" (throw "boom");
+}
+NIX
+
+expectStderr 1 nix eval --eval-cores 4 --json --file "$TEST_ROOT/add-error-context.nix" | grepQuiet "context mentioning 4499998500000"