[test] Manually restore changed flags in SaveFlags

SaveFlags previously worked by re-setting the flags using the command
line. Unfortunately, this could reset flags being used by concurrent
processes, which would cause TSAN issues.

Now, SaveFlags stores a copy of the state of all flags on creation, and
only resets changed flags in its destructor. It does this by (ab)using
the flag-definitions.h pseudo-header, adding a new mode to that header
which applies an includer-defined macro to each flag definition.

Change-Id: I4c156ecb36b4b7c05402138088266465d31e33b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1530809Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60350}

src/flag-definitions.h

@@ -72,6 +72,11 @@
 #define DEFINE_NEG_VALUE_IMPLICATION(whenflag, thenflag, value) \
   if (!FLAG_##whenflag) FLAG_##thenflag = value;
 
+// We apply a generic macro to the flags.
+#elif defined(FLAG_MODE_APPLY)
+
+#define FLAG_FULL FLAG_MODE_APPLY
+
 #else
 #error No mode supplied when including flags.defs
 #endif
@@ -114,6 +119,10 @@ struct MaybeBoolFlag {
   }
   bool has_value;
   bool value;
+
+  bool operator!=(const MaybeBoolFlag& other) const {
+    return has_value != other.has_value || value != other.value;
+  }
 };
 #endif
 
@@ -1558,5 +1567,6 @@ DEFINE_BOOL(lite_mode, V8_LITE_BOOL,
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
+#undef FLAG_MODE_APPLY
 
 #undef COMMA

test/unittests/test-utils.cc

@@ -43,19 +43,22 @@ v8::IsolateWrapper* SharedIsolateHolder::isolate_wrapper_ = nullptr;
 
 namespace internal {
 
-SaveFlags::SaveFlags() { non_default_flags_ = FlagList::argv(); }
+SaveFlags::SaveFlags() {
+  // For each flag, save the current flag value.
+#define FLAG_MODE_APPLY(ftype, ctype, nam, def, cmt) SAVED_##nam = FLAG_##nam;
+#include "src/flag-definitions.h"  // NOLINT
+#undef FLAG_MODE_APPLY
+}
 
 SaveFlags::~SaveFlags() {
-  FlagList::ResetAllFlags();
-  int argc = static_cast<int>(non_default_flags_->size());
-  FlagList::SetFlagsFromCommandLine(
-      &argc, const_cast<char**>(non_default_flags_->data()),
-      false /* remove_flags */);
-  for (auto flag = non_default_flags_->begin();
-       flag != non_default_flags_->end(); ++flag) {
-    delete[] * flag;
+  // For each flag, set back the old flag value if it changed (don't write the
+  // flag if it didn't change, to keep TSAN happy).
+#define FLAG_MODE_APPLY(ftype, ctype, nam, def, cmt) \
+  if (SAVED_##nam != FLAG_##nam) {                   \
+    FLAG_##nam = SAVED_##nam;                        \
   }
-  delete non_default_flags_;
+#include "src/flag-definitions.h"  // NOLINT
+#undef FLAG_MODE_APPLY
 }
 
 }  // namespace internal

test/unittests/test-utils.h

@@ -309,7 +309,9 @@ class SaveFlags {
   ~SaveFlags();
 
  private:
-  std::vector<const char*>* non_default_flags_;
+#define FLAG_MODE_APPLY(ftype, ctype, nam, def, cmt) ctype SAVED_##nam;
+#include "src/flag-definitions.h"  // NOLINT
+#undef FLAG_MODE_APPLY
 
   DISALLOW_COPY_AND_ASSIGN(SaveFlags);
 };