_DEBUG is not defined when compiling Godot in debug mode on Windows

[Zylann]

Godot 3.2 master

I am writing a module making use of STL because Godot containers are not fit for some use cases, and I realized that even in debug mode, they don't do any boundary checks. It would have been useful to have when I ran into heap corruption...

Then I realized it's because _DEBUG, a macro usually active on Windows debug builds to enable standard lib checks, was not defined. Is there a reason for that?

https://docs.microsoft.com/en-us/visualstudio/debugger/enabling-debug-features-in-visual-cpp-d-debug?view=vs-2019

I tried to enable it in my module, but then linking with the rest of Godot fails.

if env['target'] == 'debug':
	if env.msvc:
		env_mymodule.Append(CXXFLAGS=['/D_DEBUG'])

modules.windows.tools.64.lib(register_types.windows.tools.64.obj):-1: error: LNK2038: mismatch detected for '_ITERATOR_DEBUG_LEVEL': value '2' doesn't match value '0' in crash_handler_windows.windows.tools.64.obj

I verified with this

#ifdef _DEBUG
	print_line("HELLO DEBUG");
#endif

EDIT: this can be fixed by modifying platform/windows/detect.py:

@@ -338,11 +338,11 @@ def configure_msvc(env, vcvars_msvc_config):
         env.AppendUnique(CPPDEFINES=["WINDOWS_SUBSYSTEM_CONSOLE"])
 
     ## Compile/link flags
 
     if env["use_static_cpp"]:
-        env.AppendUnique(CCFLAGS=["/MT"])
+        env.AppendUnique(CCFLAGS=["/MTd"])
     else:
         env.AppendUnique(CCFLAGS=["/MD"])

(of course we need to do this only in dev builds)
/MTd both defines _DEBUG and uses the debug runtime library, as _DEBUG alone won't work. Unfortunately, due to a bug in the debug runtime, any thread_local defined in global space holding a standard library container will cause an error when a debugger is attached.
See:
baldurk/renderdoc#1743
https://developercommunity.visualstudio.com/t/race-condition-on-g-tss-mutex-with-static-crt/672664
An easy workaround is to wrap these thread_local in a function, which will make them created on-demand instead of application start.
The only place I had to fix in Godot is the thirdparty oidn, which defines a thread_local in oidn/core/device.cpp.

[akien-mga]

Feel free to make a PR, including a patch for oidn if it's still needed.

This should be dependent on the optimize flag like godotengine/godot-cpp#932, e.g.:

diff --git a/platform/windows/detect.py b/platform/windows/detect.py
index 705e83dace..c9030fc982 100644
--- a/platform/windows/detect.py
+++ b/platform/windows/detect.py
@@ -339,10 +339,11 @@ def configure_msvc(env, vcvars_msvc_config):
 
     ## Compile/link flags
 
+    runtime_suffix = "d" if env["optimize"] in ["debug", "none"] else ""
     if env["use_static_cpp"]:
-        env.AppendUnique(CCFLAGS=["/MT"])
+        env.AppendUnique(CCFLAGS=["/MT" + runtime_suffix])
     else:
-        env.AppendUnique(CCFLAGS=["/MD"])
+        env.AppendUnique(CCFLAGS=["/MD" + runtime_suffix])
 
     if env["arch"] == "x86_32":
         env["x86_libtheora_opt_vc"] = True

Or another option which may work around the /MTd bug:

diff --git a/platform/windows/detect.py b/platform/windows/detect.py
index 705e83dace..2386ad159c 100644
--- a/platform/windows/detect.py
+++ b/platform/windows/detect.py
@@ -339,10 +339,14 @@ def configure_msvc(env, vcvars_msvc_config):
 
     ## Compile/link flags
 
-    if env["use_static_cpp"]:
-        env.AppendUnique(CCFLAGS=["/MT"])
+    if env["optimize"] in ["debug", "none"]:
+        # Always use dynamic runtime, debug static CRT is breaks thread_local.
+        env.AppendUnique(CCFLAGS=["/MDd"])
     else:
-        env.AppendUnique(CCFLAGS=["/MD"])
+        if env["use_static_cpp"]:
+            env.AppendUnique(CCFLAGS=["/MT"])
+        else:
+            env.AppendUnique(CCFLAGS=["/MD"])
 
     if env["arch"] == "x86_32":
         env["x86_libtheora_opt_vc"] = True

I guess I'll PR that for testing.