Factor out GC initialization code

This is not really part of the evaluator: it is just an integration
between Boehm GC and Boost coroutines usable for any purpose. The
evaluator (merely) optionally uses it.

src/libexpr/eval-gc.cc

@@ -0,0 +1,226 @@
+#include "error.hh"
+#include "environment-variables.hh"
+#include "serialise.hh"
+#include "eval-gc.hh"
+
+#if HAVE_BOEHMGC
+
+#  include <pthread.h>
+#  if __FreeBSD__
+#    include <pthread_np.h>
+#  endif
+
+#  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>
+#  include <boost/context/stack_context.hpp>
+
+#endif
+
+namespace nix {
+
+#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();
+}
+
+class BoehmGCStackAllocator : public StackAllocator
+{
+    boost::coroutines2::protected_fixedsize_stack stack{
+        // We allocate 8 MB, the default max stack size on NixOS.
+        // A smaller stack might be quicker to allocate but reduces the stack
+        // depth available for source filter expressions etc.
+        std::max(boost::context::stack_traits::default_size(), static_cast<std::size_t>(8 * 1024 * 1024))};
+
+    // This is specific to boost::coroutines2::protected_fixedsize_stack.
+    // The stack protection page is included in sctx.size, so we have to
+    // subtract one page size from the stack size.
+    std::size_t pfss_usable_stack_size(boost::context::stack_context & sctx)
+    {
+        return sctx.size - boost::context::stack_traits::page_size();
+    }
+
+public:
+    boost::context::stack_context allocate() override
+    {
+        auto sctx = stack.allocate();
+
+        // Stacks generally start at a high address and grow to lower addresses.
+        // Architectures that do the opposite are rare; in fact so rare that
+        // boost_routine does not implement it.
+        // So we subtract the stack size.
+        GC_add_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
+        return sctx;
+    }
+
+    void deallocate(boost::context::stack_context sctx) override
+    {
+        GC_remove_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
+        stack.deallocate(sctx);
+    }
+};
+
+static BoehmGCStackAllocator boehmGCStackAllocator;
+
+/**
+ * When a thread goes into a coroutine, we lose its original sp until
+ * control flow returns to the thread.
+ * While in the coroutine, the sp points outside the thread stack,
+ * so we can detect this and push the entire thread stack instead,
+ * as an approximation.
+ * The coroutine's stack is covered by `BoehmGCStackAllocator`.
+ * This is not an optimal solution, because the garbage is scanned when a
+ * coroutine is active, for both the coroutine and the original thread stack.
+ * However, the implementation is quite lean, and usually we don't have active
+ * coroutines during evaluation, so this is acceptable.
+ */
+void fixupBoehmStackPointer(void ** sp_ptr, void * _pthread_id)
+{
+    void *& sp = *sp_ptr;
+    auto pthread_id = reinterpret_cast<pthread_t>(_pthread_id);
+    pthread_attr_t pattr;
+    size_t osStackSize;
+    void * osStackLow;
+    void * osStackBase;
+
+#  ifdef __APPLE__
+    osStackSize = pthread_get_stacksize_np(pthread_id);
+    osStackLow = pthread_get_stackaddr_np(pthread_id);
+#  else
+    if (pthread_attr_init(&pattr)) {
+        throw Error("fixupBoehmStackPointer: pthread_attr_init failed");
+    }
+#    ifdef HAVE_PTHREAD_GETATTR_NP
+    if (pthread_getattr_np(pthread_id, &pattr)) {
+        throw Error("fixupBoehmStackPointer: pthread_getattr_np failed");
+    }
+#    elif HAVE_PTHREAD_ATTR_GET_NP
+    if (!pthread_attr_init(&pattr)) {
+        throw Error("fixupBoehmStackPointer: pthread_attr_init failed");
+    }
+    if (!pthread_attr_get_np(pthread_id, &pattr)) {
+        throw Error("fixupBoehmStackPointer: pthread_attr_get_np failed");
+    }
+#    else
+#      error "Need one of `pthread_attr_get_np` or `pthread_getattr_np`"
+#    endif
+    if (pthread_attr_getstack(&pattr, &osStackLow, &osStackSize)) {
+        throw Error("fixupBoehmStackPointer: pthread_attr_getstack failed");
+    }
+    if (pthread_attr_destroy(&pattr)) {
+        throw Error("fixupBoehmStackPointer: pthread_attr_destroy failed");
+    }
+#  endif
+    osStackBase = (char *) osStackLow + osStackSize;
+    // NOTE: We assume the stack grows down, as it does on all architectures we support.
+    //       Architectures that grow the stack up are rare.
+    if (sp >= osStackBase || sp < osStackLow) { // lo is outside the os stack
+        sp = osStackBase;
+    }
+}
+
+/* Disable GC while this object lives. Used by CoroutineContext.
+ *
+ * Boehm keeps a count of GC_disable() and GC_enable() calls,
+ * and only enables GC when the count matches.
+ */
+class BoehmDisableGC
+{
+public:
+    BoehmDisableGC()
+    {
+        GC_disable();
+    };
+    ~BoehmDisableGC()
+    {
+        GC_enable();
+    };
+};
+
+static inline void initGCReal()
+{
+    /* 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);
+
+    StackAllocator::defaultAllocator = &boehmGCStackAllocator;
+
+// 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 >= 2 && GC_VERSION_MICRO >= 4 && !defined(__APPLE__)
+    GC_set_sp_corrector(&fixupBoehmStackPointer);
+
+    if (!GC_get_sp_corrector()) {
+        printTalkative("BoehmGC on this platform does not support sp_corrector; will disable GC inside coroutines");
+        /* Used to disable GC when entering coroutines on macOS */
+        create_coro_gc_hook = []() -> std::shared_ptr<void> { return std::make_shared<BoehmDisableGC>(); };
+    }
+#  else
+#    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
+
+    /* 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")) {
+        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
+        debug("setting initial heap size to %1% bytes", size);
+        GC_expand_hp(size);
+    }
+}
+
+#endif
+
+static bool gcInitialised = false;
+
+void initGC()
+{
+    if (gcInitialised)
+        return;
+
+#if HAVE_BOEHMGC
+    initGCReal();
+#endif
+
+    gcInitialised = true;
+}
+
+void assertGCInitialized()
+{
+    assert(gcInitialised);
+}
+
+}

src/libexpr/eval-gc.hh

@@ -0,0 +1,16 @@
+#pragma once
+///@file
+
+namespace nix {
+
+/**
+ * Initialise the Boehm GC, if applicable.
+ */
+void initGC();
+
+/**
+ * Make sure `initGC` has already been called.
+ */
+void assertGCInitialized();
+
+}

src/libexpr/eval.cc

@@ -45,20 +45,11 @@
 
 #if HAVE_BOEHMGC
 
-#define GC_INCLUDE_NEW
+#  define GC_INCLUDE_NEW
 
-#include <pthread.h>
+#  include <gc/gc.h>
-#if __FreeBSD__
+#  include <gc/gc_cpp.h>
-#  include <pthread_np.h>
+#  include <gc/gc_allocator.h>
-#endif
-
-#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>
-#include <boost/context/stack_context.hpp>
 
 #endif
 
@@ -211,109 +202,6 @@ bool Value::isTrivial() const
 }
 
 
-#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();
-}
-
-class BoehmGCStackAllocator : public StackAllocator {
-    boost::coroutines2::protected_fixedsize_stack stack {
-        // We allocate 8 MB, the default max stack size on NixOS.
-        // A smaller stack might be quicker to allocate but reduces the stack
-        // depth available for source filter expressions etc.
-        std::max(boost::context::stack_traits::default_size(), static_cast<std::size_t>(8 * 1024 * 1024))
-    };
-
-    // This is specific to boost::coroutines2::protected_fixedsize_stack.
-    // The stack protection page is included in sctx.size, so we have to
-    // subtract one page size from the stack size.
-    std::size_t pfss_usable_stack_size(boost::context::stack_context &sctx) {
-        return sctx.size - boost::context::stack_traits::page_size();
-    }
-
-  public:
-    boost::context::stack_context allocate() override {
-        auto sctx = stack.allocate();
-
-        // Stacks generally start at a high address and grow to lower addresses.
-        // Architectures that do the opposite are rare; in fact so rare that
-        // boost_routine does not implement it.
-        // So we subtract the stack size.
-        GC_add_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
-        return sctx;
-    }
-
-    void deallocate(boost::context::stack_context sctx) override {
-        GC_remove_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
-        stack.deallocate(sctx);
-    }
-
-};
-
-static BoehmGCStackAllocator boehmGCStackAllocator;
-
-/**
- * When a thread goes into a coroutine, we lose its original sp until
- * control flow returns to the thread.
- * While in the coroutine, the sp points outside the thread stack,
- * so we can detect this and push the entire thread stack instead,
- * as an approximation.
- * The coroutine's stack is covered by `BoehmGCStackAllocator`.
- * This is not an optimal solution, because the garbage is scanned when a
- * coroutine is active, for both the coroutine and the original thread stack.
- * However, the implementation is quite lean, and usually we don't have active
- * coroutines during evaluation, so this is acceptable.
- */
-void fixupBoehmStackPointer(void ** sp_ptr, void * _pthread_id) {
-    void *& sp = *sp_ptr;
-    auto pthread_id = reinterpret_cast<pthread_t>(_pthread_id);
-    pthread_attr_t pattr;
-    size_t osStackSize;
-    void * osStackLow;
-    void * osStackBase;
-
-#  ifdef __APPLE__
-    osStackSize = pthread_get_stacksize_np(pthread_id);
-    osStackLow = pthread_get_stackaddr_np(pthread_id);
-#  else
-    if (pthread_attr_init(&pattr)) {
-        throw Error("fixupBoehmStackPointer: pthread_attr_init failed");
-    }
-#    ifdef HAVE_PTHREAD_GETATTR_NP
-    if (pthread_getattr_np(pthread_id, &pattr)) {
-        throw Error("fixupBoehmStackPointer: pthread_getattr_np failed");
-    }
-#    elif HAVE_PTHREAD_ATTR_GET_NP
-    if (!pthread_attr_init(&pattr)) {
-        throw Error("fixupBoehmStackPointer: pthread_attr_init failed");
-    }
-    if (!pthread_attr_get_np(pthread_id, &pattr)) {
-        throw Error("fixupBoehmStackPointer: pthread_attr_get_np failed");
-    }
-#    else
-#      error "Need one of `pthread_attr_get_np` or `pthread_getattr_np`"
-#    endif
-    if (pthread_attr_getstack(&pattr, &osStackLow, &osStackSize)) {
-        throw Error("fixupBoehmStackPointer: pthread_attr_getstack failed");
-    }
-    if (pthread_attr_destroy(&pattr)) {
-        throw Error("fixupBoehmStackPointer: pthread_attr_destroy failed");
-    }
-#  endif
-    osStackBase = (char *)osStackLow + osStackSize;
-    // NOTE: We assume the stack grows down, as it does on all architectures we support.
-    //       Architectures that grow the stack up are rare.
-    if (sp >= osStackBase || sp < osStackLow) { // lo is outside the os stack
-        sp = osStackBase;
-    }
-}
-
-#endif
-
-
 static Symbol getName(const AttrName & name, EvalState & state, Env & env)
 {
     if (name.symbol) {
@@ -326,92 +214,6 @@ static Symbol getName(const AttrName & name, EvalState & state, Env & env)
     }
 }
 
-#if HAVE_BOEHMGC
-/* Disable GC while this object lives. Used by CoroutineContext.
- *
- * Boehm keeps a count of GC_disable() and GC_enable() calls,
- * and only enables GC when the count matches.
- */
-class BoehmDisableGC {
-public:
-    BoehmDisableGC() {
-        GC_disable();
-    };
-    ~BoehmDisableGC() {
-        GC_enable();
-    };
-};
-#endif
-
-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);
-
-    StackAllocator::defaultAllocator = &boehmGCStackAllocator;
-
-    // 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 >= 2 && GC_VERSION_MICRO >= 4 && !defined(__APPLE__)
-    GC_set_sp_corrector(&fixupBoehmStackPointer);
-
-    if (!GC_get_sp_corrector()) {
-        printTalkative("BoehmGC on this platform does not support sp_corrector; will disable GC inside coroutines");
-        /* Used to disable GC when entering coroutines on macOS */
-        create_coro_gc_hook = []() -> std::shared_ptr<void> {
-            return std::make_shared<BoehmDisableGC>();
-        };
-    }
-    #else
-    #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
-
-
-    /* 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")) {
-        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
-        debug("setting initial heap size to %1% bytes", size);
-        GC_expand_hp(size);
-    }
-
-#endif
-
-    gcInitialised = true;
-}
-
 static constexpr size_t BASE_ENV_SIZE = 128;
 
 EvalState::EvalState(
@@ -508,7 +310,7 @@ EvalState::EvalState(
 
     countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
 
-    assert(gcInitialised);
+    assertGCInitialized();
 
     static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
 

src/libexpr/eval.hh

@@ -3,6 +3,7 @@
 
 #include "attr-set.hh"
 #include "eval-error.hh"
+#include "eval-gc.hh"
 #include "types.hh"
 #include "value.hh"
 #include "nixexpr.hh"
@@ -146,12 +147,6 @@ std::string printValue(EvalState & state, Value & v);
 std::ostream & operator << (std::ostream & os, const ValueType t);
 
 
-/**
- * Initialise the Boehm GC, if applicable.
- */
-void initGC();
-
-
 struct RegexCache;
 
 std::shared_ptr<RegexCache> makeRegexCache();