From f34b52b52163ec0db60dca6907f309acc45e2205 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Wed, 1 May 2024 18:47:19 +0200
Subject: [PATCH] libexpr: Add missing GC root for baseEnv

This missing GC root wasn't much of a problem before, because the
heap would end up with a reference to the `baseEnv` pretty soon,
but when unit testing, the construction of `EvalState` doesn't
necessarily happen well before GC runs for the first time.

Found while unit testing the Rust bindings that currently reside
at https://github.com/nixops4/nixops4/tree/main/rust
---
 src/libexpr/eval.cc | 8 +++++++-
 src/libexpr/eval.hh | 5 +++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 35ccca79a..aa058b04f 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -50,6 +50,7 @@
 
 #include <gc/gc.h>
 #include <gc/gc_cpp.h>
+#include <gc/gc_allocator.h>
 
 #include <boost/coroutine2/coroutine.hpp>
 #include <boost/coroutine2/protected_fixedsize_stack.hpp>
@@ -342,6 +343,8 @@ void initGC()
     gcInitialised = true;
 }
 
+static constexpr size_t BASE_ENV_SIZE = 128;
+
 EvalState::EvalState(
     const LookupPath & _lookupPath,
     ref<Store> store,
@@ -424,8 +427,11 @@ EvalState::EvalState(
 #if HAVE_BOEHMGC
     , valueAllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
     , env1AllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
+    , baseEnvP(std::allocate_shared<Env *>(traceable_allocator<Env *>(), &allocEnv(BASE_ENV_SIZE)))
+    , baseEnv(**baseEnvP)
+#else
+    , baseEnv(allocEnv(BASE_ENV_SIZE))
 #endif
-    , baseEnv(allocEnv(128))
     , staticBaseEnv{std::make_shared<StaticEnv>(nullptr, nullptr)}
 {
     corepkgsFS->setPathDisplay("<nix", ">");
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 508361301..ae8b9dd04 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -547,6 +547,11 @@ public:
      */
     SingleDerivedPath coerceToSingleDerivedPath(const PosIdx pos, Value & v, std::string_view errorCtx);
 
+#if HAVE_BOEHMGC
+    /** A GC root for the baseEnv reference. */
+    std::shared_ptr<Env *> baseEnvP;
+#endif
+
 public:
 
     /**