refactor(3p/nix): Only initialise garbage-collector where needed

Only libexpr depends on the garbage collector, specifically only
instantiations of EvalState actually require the GC to be initialised.

Rather than always starting it for the whole program, even if it is
not needed, this change moves the GC initialisation into libexpr,
guarded by absl::call_once.

This should make it possible to run the nix daemon without the garbage
collector interfering, granted that things are correcty separated and
the daemon does not actually invoke the evaluator.

Based on my investigation so far, the daemon logic itself does not
require libexpr to be present at all - so I think it is safe - but the
current monobinary might have some tricks up its sleeve that will
cause problems for us. We can deal with those if they arise.

Relates to https://b.tvl.fyi/issues/30

Change-Id: I61c745f96420c02e089bd3c362ac3ccb117d3073
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1584
Tested-by: BuildkiteCI
Reviewed-by: edef <edef@edef.eu>
Reviewed-by: kanepyork <rikingcoding@gmail.com>

third_party/nix/src/libexpr/eval.cc

@@ -12,6 +12,7 @@
 
 #define GC_INCLUDE_NEW
 
+#include <absl/base/call_once.h>
 #include <absl/container/flat_hash_set.h>
 #include <absl/strings/match.h>
 #include <gc/gc.h>
@@ -34,6 +35,65 @@
 #include "libutil/visitor.hh"
 
 namespace nix {
+namespace {
+
+// Called when the Boehm GC runs out of memory.
+static void* BoehmOomHandler(size_t requested) {
+  /* Convert this to a proper C++ exception. */
+  LOG(FATAL) << "Garbage collector ran out of memory; requested " << requested
+             << " bytes";
+  throw std::bad_alloc();
+}
+
+void ConfigureBoehmGc() {
+  /* Don't look for interior pointers. This reduces the odds of
+     misdetection a bit. */
+  GC_set_all_interior_pointers(0);
+
+  /* We don't have any roots in data segments, so don't scan from
+     there. */
+  GC_set_no_dls(1);
+
+  GC_INIT();
+
+  GC_set_oom_fn(BoehmOomHandler);
+
+  /* Set the initial heap size to something fairly big (25% of
+     physical RAM, up to a maximum of 384 MiB) so that in most cases
+     we don't need to garbage collect at all.  (Collection has a
+     fairly significant overhead.)  The heap size can be overridden
+     through libgc's GC_INITIAL_HEAP_SIZE environment variable.  We
+     should probably also provide a nix.conf setting for this.  Note
+     that GC_expand_hp() causes a lot of virtual, but not physical
+     (resident) memory to be allocated.  This might be a problem on
+     systems that don't overcommit. */
+  if (getenv("GC_INITIAL_HEAP_SIZE") == nullptr) {
+    size_t size = 32 * 1024 * 1024;
+#if HAVE_SYSCONF && defined(_SC_PAGESIZE) && defined(_SC_PHYS_PAGES)
+    size_t maxSize = 384 * 1024 * 1024;
+    long pageSize = sysconf(_SC_PAGESIZE);
+    long pages = sysconf(_SC_PHYS_PAGES);
+    if (pageSize != -1) {
+      size = (pageSize * pages) / 4;
+    }  // 25% of RAM
+    if (size > maxSize) {
+      size = maxSize;
+    }
+#endif
+    DLOG(INFO) << "setting initial heap size to " << size << " bytes";
+    GC_expand_hp(size);
+  }
+}
+
+}  // namespace
+
+namespace expr {
+
+absl::once_flag gc_flag;
+
+void InitGC() { absl::call_once(gc_flag, &ConfigureBoehmGc); }
+
+}  // namespace expr
 
 static char* dupString(const char* s) {
   char* t;
@@ -187,14 +247,6 @@ std::string showType(const Value& v) {
   abort();
 }
 
-#if HAVE_BOEHMGC
-/* Called when the Boehm GC runs out of memory. */
-static void* oomHandler(size_t requested) {
-  /* Convert this to a proper C++ exception. */
-  throw std::bad_alloc();
-}
-#endif
-
 static Symbol getName(const AttrName& name, EvalState& state, Env& env) {
   return std::visit(
       util::overloaded{[&](const Symbol& name) -> Symbol { return name; },
@@ -207,59 +259,6 @@ static Symbol getName(const AttrName& name, EvalState& state, Env& env) {
       name);
 }
 
-static bool gcInitialised = false;
-
-void initGC() {
-  if (gcInitialised) {
-    return;
-  }
-
-#if HAVE_BOEHMGC
-  /* Initialise the Boehm garbage collector. */
-
-  /* Don't look for interior pointers. This reduces the odds of
-     misdetection a bit. */
-  GC_set_all_interior_pointers(0);
-
-  /* We don't have any roots in data segments, so don't scan from
-     there. */
-  GC_set_no_dls(1);
-
-  GC_INIT();
-
-  GC_set_oom_fn(oomHandler);
-
-  /* Set the initial heap size to something fairly big (25% of
-     physical RAM, up to a maximum of 384 MiB) so that in most cases
-     we don't need to garbage collect at all.  (Collection has a
-     fairly significant overhead.)  The heap size can be overridden
-     through libgc's GC_INITIAL_HEAP_SIZE environment variable.  We
-     should probably also provide a nix.conf setting for this.  Note
-     that GC_expand_hp() causes a lot of virtual, but not physical
-     (resident) memory to be allocated.  This might be a problem on
-     systems that don't overcommit. */
-  if (getenv("GC_INITIAL_HEAP_SIZE") == nullptr) {
-    size_t size = 32 * 1024 * 1024;
-#if HAVE_SYSCONF && defined(_SC_PAGESIZE) && defined(_SC_PHYS_PAGES)
-    size_t maxSize = 384 * 1024 * 1024;
-    long pageSize = sysconf(_SC_PAGESIZE);
-    long pages = sysconf(_SC_PHYS_PAGES);
-    if (pageSize != -1) {
-      size = (pageSize * pages) / 4;
-    }  // 25% of RAM
-    if (size > maxSize) {
-      size = maxSize;
-    }
-#endif
-    DLOG(INFO) << "setting initial heap size to " << size << " bytes";
-    GC_expand_hp(size);
-  }
-
-#endif
-
-  gcInitialised = true;
-}
-
 /* Very hacky way to parse $NIX_PATH, which is colon-separated, but
    can contain URLs (e.g. "nixpkgs=https://bla...:foo=https://"). */
 static Strings parseNixPath(const std::string& s) {
@@ -334,9 +333,9 @@ EvalState::EvalState(const Strings& _searchPath, const ref<Store>& store)
       store(store),
       baseEnv(allocEnv(128)),
       staticBaseEnv(false, nullptr) {
-  countCalls = getEnv("NIX_COUNT_CALLS", "0") != "0";
+  expr::InitGC();
 
-  assert(gcInitialised);
+  countCalls = getEnv("NIX_COUNT_CALLS", "0") != "0";
 
   /* Initialise the Nix expression search path. */
   if (!evalSettings.pureEval) {

third_party/nix/src/libexpr/eval.hh

@@ -16,6 +16,13 @@
 #include "libutil/hash.hh"
 
 namespace nix {
+namespace expr {
+
+// Initialise the Boehm GC once per program instance. This should be
+// called in places that require the garbage collector.
+void InitGC();
+
+}  // namespace expr
 
 class Store;
 class EvalState;
@@ -59,9 +66,6 @@ std::ostream& operator<<(std::ostream& str, const Value& v);
 typedef std::pair<std::string, std::string> SearchPathElem;
 typedef std::list<SearchPathElem> SearchPath;
 
-/* Initialise the Boehm GC, if applicable. */
-void initGC();
-
 typedef std::map<Path, Expr*, std::less<Path>,
                  traceable_allocator<std::pair<const Path, Expr*>>>
     FileParseCache;

third_party/nix/src/nix/main.cc

@@ -126,7 +126,6 @@ void mainWrapped(int argc, char** argv) {
   }
 
   initNix();
-  initGC();
 
   programPath = argv[0];
   std::string programName = baseNameOf(programPath);

third_party/nix/src/tests/attr-set.cc

@@ -115,7 +115,7 @@ class AttrSetTest : public ::testing::Test {
  protected:
   EvalState* eval_state_;
   void SetUp() override {
-    nix::initGC();
+    nix::expr::InitGC();
     auto store = std::make_shared<DummyStore>();
     eval_state_ = new EvalState({"."}, ref<Store>(store));
     symbol_table = &eval_state_->symbols;

third_party/nix/src/tests/language-tests.cc

@@ -111,7 +111,7 @@ class NixEnvironment : public testing::Environment {
  public:
   void SetUp() override {
     google::InitGoogleLogging("--logtostderr=false");
-    nix::initGC();
+    nix::expr::InitGC();
   }
 };
 

third_party/nix/src/tests/value-to-json.cc

@@ -12,7 +12,7 @@
 
 class ValueTest : public ::testing::Test {
  protected:
-  static void SetUpTestCase() { nix::initGC(); }
+  static void SetUpTestCase() { nix::expr::InitGC(); }
 
   static void TearDownTestCase() {}
 };