Fix hang in V8 debug builds

This is a partial cherry-pick of upstream commit v8/v8@8953e49478.

Shows up in debug builds because of a buggy sanity check when computing
relationships between command line flags.

Example of the hang:

    #include "v8.h"
    #include "libplatform/libplatform.h"
    int main(void) {
      // works: v8::V8::SetFlagsFromString("--gc-global --noincremental-marking");
      v8::V8::SetFlagsFromString("--gc-global");
      v8::V8::SetFlagsFromString("--noincremental-marking");
      v8::V8::InitializePlatform(v8::platform::NewDefaultPlatform().release());
      v8::V8::Initialize(); // hangs in ComputeFlagListHash when defined(DEBUG)
    }

Author: bnoordhuis

libexec/extract-node

@@ -32,3 +32,4 @@ cd "${src}/node-v${version}"
 patch -p1 < "${top}"/patch/v8-std-is-trivially-destructible.patch
 patch -p1 < "${top}"/patch/v8-disable-madv-dontfork.patch
 patch -p1 < "${top}"/patch/v8-disable-pkey.patch
+patch -p1 < "${top}"/patch/v8-debug-flags-hang.patch

patch/v8-debug-flags-hang.patch

@@ -0,0 +1,26 @@
+diff --git a/deps/v8/src/flags/flags-impl.h b/deps/v8/src/flags/flags-impl.h
+index d2d440c026..111b9b5b9c 100644
+--- a/deps/v8/src/flags/flags-impl.h
++++ b/deps/v8/src/flags/flags-impl.h
+@@ -5,6 +5,8 @@
+ #ifndef V8_FLAGS_FLAGS_IMPL_H_
+ #define V8_FLAGS_FLAGS_IMPL_H_
+ 
++#include <unordered_set>
++
+ #include "src/base/macros.h"
+ #include "src/base/optional.h"
+ #include "src/base/vector.h"
+@@ -91,9 +93,12 @@ struct Flag {
+ #ifdef DEBUG
+   bool ImpliedBy(const void* ptr) const {
+     const Flag* current = this->implied_by_ptr_;
++    std::unordered_set<const Flag*> visited_flags;
+     while (current != nullptr) {
++      visited_flags.insert(current);
+       if (current->PointsTo(ptr)) return true;
+       current = current->implied_by_ptr_;
++      if (visited_flags.contains(current)) break;
+     }
+     return false;
+   }