diff --git a/doc/manual/src/glossary.md b/doc/manual/src/glossary.md
index 55ad9e1c2..4c9b2c52e 100644
--- a/doc/manual/src/glossary.md
+++ b/doc/manual/src/glossary.md
@@ -71,10 +71,9 @@
   [`__contentAddressed`](./language/advanced-attributes.md#adv-attr-__contentAddressed)
   attribute set to `true`.
 
-- [fixed-output derivation]{#gloss-fixed-output-derivation}
+- [fixed-output derivation]{#gloss-fixed-output-derivation} (FOD)
 
-  A derivation which includes the
-  [`outputHash`](./language/advanced-attributes.md#adv-attr-outputHash) attribute.
+  A [derivation] where a cryptographic hash of the [output] is determined in advance using the [`outputHash`](./language/advanced-attributes.md#adv-attr-outputHash) attribute, and where the [`builder`](@docroot@/language/derivations.md#attr-builder) executable has access to the network.
 
 - [store]{#gloss-store}
 
diff --git a/doc/manual/src/language/advanced-attributes.md b/doc/manual/src/language/advanced-attributes.md
index 113062db1..70ec06f57 100644
--- a/doc/manual/src/language/advanced-attributes.md
+++ b/doc/manual/src/language/advanced-attributes.md
@@ -120,12 +120,11 @@ Derivations can declare some infrequently used optional attributes.
     configuration setting.
 
   - [`outputHash`]{#adv-attr-outputHash}; [`outputHashAlgo`]{#adv-attr-outputHashAlgo}; [`outputHashMode`]{#adv-attr-outputHashMode}\
-    These attributes declare that the derivation is a so-called
-    *fixed-output derivation*, which means that a cryptographic hash of
-    the output is already known in advance. When the build of a
-    fixed-output derivation finishes, Nix computes the cryptographic
-    hash of the output and compares it to the hash declared with these
-    attributes. If there is a mismatch, the build fails.
+    These attributes declare that the derivation is a so-called *fixed-output derivation* (FOD), which means that a cryptographic hash of the output is already known in advance.
+
+    As opposed to regular derivations, the [`builder`] executable of a fixed-output derivation has access to the network.
+    Nix computes a cryptographic hash of its output and compares that to the hash declared with these attributes.
+    If there is a mismatch, the derivation fails.
 
     The rationale for fixed-output derivations is derivations such as
     those produced by the `fetchurl` function. This function downloads a
@@ -279,7 +278,9 @@ Derivations can declare some infrequently used optional attributes.
 
     > **Note**
     >
-    > If set to `false`, the [`builder`](./derivations.md#attr-builder) should be able to run on the system type specified in the [`system` attribute](./derivations.md#attr-system), since the derivation cannot be substituted.
+    > If set to `false`, the [`builder`] should be able to run on the system type specified in the [`system` attribute](./derivations.md#attr-system), since the derivation cannot be substituted.
+
+    [`builder`]: ./derivations.md#attr-builder
 
   - [`__structuredAttrs`]{#adv-attr-structuredAttrs}\
     If the special attribute `__structuredAttrs` is set to `true`, the other derivation
diff --git a/src/libexpr/eval-cache.hh b/src/libexpr/eval-cache.hh
index cac985829..b1911e3a4 100644
--- a/src/libexpr/eval-cache.hh
+++ b/src/libexpr/eval-cache.hh
@@ -31,7 +31,7 @@ struct CachedEvalError : EvalError
 class EvalCache : public std::enable_shared_from_this<EvalCache>
 {
     friend class AttrCursor;
-    friend class CachedEvalError;
+    friend struct CachedEvalError;
 
     std::shared_ptr<AttrDb> db;
     EvalState & state;
@@ -87,7 +87,7 @@ typedef std::variant<
 class AttrCursor : public std::enable_shared_from_this<AttrCursor>
 {
     friend class EvalCache;
-    friend class CachedEvalError;
+    friend struct CachedEvalError;
 
     ref<EvalCache> root;
     typedef std::optional<std::pair<std::shared_ptr<AttrCursor>, Symbol>> Parent;
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 3524c8b1e..f441977a5 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -354,7 +354,7 @@ void initGC()
     // TODO: Remove __APPLE__ condition.
     //       Comment suggests an implementation that works on darwin and windows
     //       https://github.com/ivmai/bdwgc/issues/362#issuecomment-1936672196
-    #if GC_VERSION_MAJOR >= 8 && GC_VERSION_MINOR >= 4 && !defined(__APPLE__)
+    #if GC_VERSION_MAJOR >= 8 && GC_VERSION_MINOR >= 2 && GC_VERSION_MICRO >= 4 && !defined(__APPLE__)
     GC_set_sp_corrector(&fixupBoehmStackPointer);
 
     if (!GC_get_sp_corrector()) {
@@ -365,7 +365,7 @@ void initGC()
         };
     }
     #else
-    #warning "BoehmGC version does not support GC while coroutine exists. GC will be disabled inside coroutines. Consider updating bwd-gc to 8.4 or later."
+    #warning "BoehmGC version does not support GC while coroutine exists. GC will be disabled inside coroutines. Consider updating bdw-gc to 8.2.4 or later."
     #endif
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 22d7f188f..9177b0a2f 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -4062,17 +4062,23 @@ static RegisterPrimOp primop_convertHash({
 
 struct RegexCache
 {
-    // TODO use C++20 transparent comparison when available
-    std::unordered_map<std::string_view, std::regex> cache;
-    std::list<std::string> keys;
+    struct State
+    {
+        // TODO use C++20 transparent comparison when available
+        std::unordered_map<std::string_view, std::regex> cache;
+        std::list<std::string> keys;
+    };
+
+    Sync<State> state_;
 
     std::regex get(std::string_view re)
     {
-        auto it = cache.find(re);
-        if (it != cache.end())
+        auto state(state_.lock());
+        auto it = state->cache.find(re);
+        if (it != state->cache.end())
             return it->second;
-        keys.emplace_back(re);
-        return cache.emplace(keys.back(), std::regex(keys.back(), std::regex::extended)).first->second;
+        state->keys.emplace_back(re);
+        return state->cache.emplace(state->keys.back(), std::regex(state->keys.back(), std::regex::extended)).first->second;
     }
 };
 
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index b601604dc..77df08edd 100644
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -477,9 +477,7 @@ static void main_nix_build(int argc, char * * argv)
         // Set the environment.
         auto env = getEnv();
 
-        auto tmp = getEnvNonEmpty("TMPDIR");
-        if (!tmp)
-            tmp = getEnvNonEmpty("XDG_RUNTIME_DIR").value_or("/tmp");
+        auto tmp = getEnvNonEmpty("TMPDIR").value_or("/tmp");
 
         if (pure) {
             decltype(env) newEnv;
@@ -491,7 +489,7 @@ static void main_nix_build(int argc, char * * argv)
             env["__ETC_PROFILE_SOURCED"] = "1";
         }
 
-        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = *tmp;
+        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmp;
         env["NIX_STORE"] = store->storeDir;
         env["NIX_BUILD_CORES"] = std::to_string(settings.buildCores);