Add a config switch to fully disable hot-reload

[barosiak]

Adds a config switch that unconditionally disables metadata updater.

Fixes #78540

[Copilot]

Pull request overview

This PR adds a configuration switch to fully disable hot-reload by introducing a new "none" value for the DOTNET_MODIFIABLE_ASSEMBLIES environment variable. This addresses issue #78540 where hot-reload couldn't be disabled when it was causing bugs, particularly when a debugger was attached.

Changes:

• Introduced a tri-state enum (UNSET, NONE, DEBUG) to distinguish between "not configured", "explicitly disabled", and "explicitly enabled"
• Updated both Mono and CoreCLR implementations to recognize the "none" value
• Added test coverage for the new disable functionality

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/mono/mono/metadata/metadata-update.h	Updated enum to add UNSET/NONE/DEBUG states with new numbering
src/mono/mono/component/hot_reload.c	Added "none" value handling and updated logic (contains critical bug)
src/mono/mono/component/hot_reload-stub.c	Updated stub to use UNSET instead of NONE for uninitialized state
src/mono/System.Private.CoreLib/src/System/Reflection/Metadata/MetadataUpdater.cs	Minor whitespace fix
src/libraries/System.Runtime.Loader/tests/ApplyUpdateUtil.cs	Added constant for disabled value
src/libraries/System.Runtime.Loader/tests/ApplyUpdateTest.cs	Added test for disable functionality
src/coreclr/vm/eeconfig.h	Updated enum definition and method signatures for new tri-state
src/coreclr/vm/eeconfig.cpp	Added parsing logic for "none" value
src/coreclr/vm/ceeload.cpp	Updated condition to use new enum
src/coreclr/vm/assemblynative.cpp	Updated logic to check for explicit disable

[jkotas]

Could you please check that this env variable can be used to fully disable hot reload under VS? (#78218 (comment) is the motivating feedback.)

[jkotas]

cc @tmat

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.

[steveisok]

Are you thinking Devs will be more confused than that, or that this scenario would happen often enough that we'd try to get explicit VS support for it? We've gone over 3 years since the bug to add the switch was filed suggesting these issues aren't very common.

I don't think there's a high bar. It probably would have been clearer if I had said I don't think what @barosiak posted in VS was a big deal. It would require someone in VS to make changes.

[barosiak]

I think launchSettings.json would be the recommend place to apply this workaround. Can this be fixed?

It works, I was setting it incorrectly.

To sum up, there are two main ways to disable hot-reload when debugging in VS.

Using UI:

1. Right click on the project in the solution explorer, open Properties
2. Scroll down to the section Debug and click on Open debug launch profiles UI
3. Add DOTNET_MODIFIABLE_ASSEMBLIES environment variable set to none
4. Uncheck Enable Hot Reload

It should look like this:

Using launchSettings.json file:

1. Create a file Properties\launchSettings.json (relative to your project folder) with contents shown below, or add "hotReloadEnabled": false and "DOTNET_MODIFIABLE_ASSEMBLIES": "none" to an existing file.

{
  "profiles": {
    "ConsoleApp": {
      "commandName": "Project",
      "hotReloadEnabled": false,
      "environmentVariables": {
        "DOTNET_MODIFIABLE_ASSEMBLIES": "none"
      }
    }
  }
}

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/mono/mono/component/hot_reload.c:361

• The documentation comment should be updated to reflect the new "none" value. It currently states "all other values are ignored and metadata updates are disabled" but "none" is now a recognized value that explicitly disables metadata updates. Consider updating to: ""debug" means debuggable assemblies are modifiable, "none" explicitly disables metadata updates, and all other values are ignored and metadata updates are disabled."

hot_reload_update_enabled (int *modifiable_assemblies_out)
{
	static gboolean inited = FALSE;
	static int modifiable = MONO_MODIFIABLE_ASSM_UNSET;

	gboolean result = FALSE;
	if (!inited) {
		modifiable = hot_reload_update_enabled_slow_check (NULL);
		inited = TRUE;
		result = (modifiable == MONO_MODIFIABLE_ASSM_DEBUG);
		if (result) {
			mono_trace (G_LOG_LEVEL_DEBUG, MONO_TRACE_METADATA_UPDATE, "Metadata update enabled for debuggable assemblies");
		}
	}
	if (modifiable_assemblies_out)
		*modifiable_assemblies_out = modifiable;
	return result;
}

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Comments suppressed due to low confidence (1)

src/mono/mono/component/hot_reload.c:360

• hot_reload_update_enabled returns result, but result is only computed on the first call (inside if (!inited)). After initialization, subsequent calls will always return FALSE even if modifiable is MONO_MODIFIABLE_ASSM_DEBUG, which can incorrectly disable hot-reload behavior (e.g., callers like mono_metadata_update_enabled/inlining checks). Compute the return value from the cached modifiable on every call (or cache a separate static boolean).

hot_reload_update_enabled (int *modifiable_assemblies_out)
{
	static gboolean inited = FALSE;
	static int modifiable = MONO_MODIFIABLE_ASSM_UNSET;

	gboolean result = FALSE;
	if (!inited) {
		modifiable = hot_reload_update_enabled_slow_check (NULL);
		inited = TRUE;
		result = (modifiable == MONO_MODIFIABLE_ASSM_DEBUG);
		if (result) {
			mono_trace (G_LOG_LEVEL_DEBUG, MONO_TRACE_METADATA_UPDATE, "Metadata update enabled for debuggable assemblies");
		}
	}
	if (modifiable_assemblies_out)
		*modifiable_assemblies_out = modifiable;
	return result;

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated no new comments.