From f3dc7b55b78a731346e10902397fb3434c2eca22 Mon Sep 17 00:00:00 2001
From: Laurens Valk <laurens@pybricks.com>
Date: Thu, 19 Mar 2026 09:11:02 +0100
Subject: [PATCH] sagas/mpy: Use legacy `__main__` on old firmware.

Needed for backwards compatibility. Older firmware always look for __main__.

See https://github.com/pybricks/support/issues/2364
---
 src/hub/reducers.test.ts | 21 +++++++++++++++++++++
 src/hub/reducers.ts      | 13 +++++++++++++
 src/mpy/sagas.ts         | 18 ++++++++++++------
 3 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/src/hub/reducers.test.ts b/src/hub/reducers.test.ts
index 87b081a2..b65c2d3d 100644
--- a/src/hub/reducers.test.ts
+++ b/src/hub/reducers.test.ts
@@ -58,6 +58,7 @@ test('initial state', () => {
           "runtime": "hub.runtime.disconnected",
           "selectedSlot": 0,
           "useLegacyDownload": false,
+          "useLegacyMainModule": false,
           "useLegacyStartUserProgram": false,
           "useLegacyStdio": false,
         }
@@ -544,3 +545,23 @@ describe('useLegacyStartUserProgram', () => {
         ).toBeFalsy();
     });
 });
+
+describe('useLegacyMainModule', () => {
+    test('Pybricks Profile < v1.5.0', () => {
+        expect(
+            reducers(
+                { useLegacyMainModule: false } as State,
+                bleDIServiceDidReceiveSoftwareRevision('1.4.0'),
+            ).useLegacyMainModule,
+        ).toBeTruthy();
+    });
+
+    test('Pybricks Profile >= v1.5.0', () => {
+        expect(
+            reducers(
+                { useLegacyMainModule: true } as State,
+                bleDIServiceDidReceiveSoftwareRevision('1.5.0'),
+            ).useLegacyMainModule,
+        ).toBeFalsy();
+    });
+});
diff --git a/src/hub/reducers.ts b/src/hub/reducers.ts
index c4809250..242c2958 100644
--- a/src/hub/reducers.ts
+++ b/src/hub/reducers.ts
@@ -381,6 +381,18 @@ const useLegacyStartUserProgram: Reducer<boolean> = (state = false, action) => {
     return state;
 };
 
+/**
+ * When true, use the legacy `__main__` module name instead of the actual file name.
+ */
+const useLegacyMainModule: Reducer<boolean> = (state = false, action) => {
+    if (bleDIServiceDidReceiveSoftwareRevision.matches(action)) {
+        // Behavior changed starting with Pybricks Profile v1.5.0.
+        return !semver.satisfies(action.version, '^1.5.0');
+    }
+
+    return state;
+};
+
 /*
  * Returns number of available slots or 0 for slots not supported.
  */
@@ -418,6 +430,7 @@ export default combineReducers({
     useLegacyDownload,
     useLegacyStdio,
     useLegacyStartUserProgram,
+    useLegacyMainModule,
     numOfSlots,
     selectedSlot,
 });
diff --git a/src/mpy/sagas.ts b/src/mpy/sagas.ts
index fe574b5f..b8c284a1 100644
--- a/src/mpy/sagas.ts
+++ b/src/mpy/sagas.ts
@@ -104,8 +104,8 @@ function* handleCompile(action: ReturnType<typeof compile>): Generator {
 /**
  * Compiles code into the Pybricks multi-mpy6 file format.
  *
- * This includes a __main__ module which is the file currently open in the
- * editor and any imported modules that can be found in the user file system.
+ * This includes the file currently open in the editor and any imported modules
+ * that can be found in the user file system.
  */
 function* handleCompileMulti6(): Generator {
     // REVISIT: should we be getting the active file here or have it as an
@@ -128,16 +128,22 @@ function* handleCompileMulti6(): Generator {
         return;
     }
 
-    const mainPy = yield* editorGetValue();
+    const useLegacyMainModule = yield* select(
+        (s: RootState) => s.hub.useLegacyMainModule,
+    );
+
+    const mainPyContents = yield* editorGetValue();
     const mainPyPath = metadata.path ?? '__main__.py';
-    const mainPyName = mainPyPath.replace(/\.[^.]+$/, '');
+    const mainPyName = useLegacyMainModule
+        ? '__main__'
+        : mainPyPath.replace(/\.[^.]+$/, '');
 
     const pyFiles = new Map<string, FileContents>([
-        [mainPyName, { path: mainPyPath, contents: mainPy }],
+        [mainPyName, { path: mainPyPath, contents: mainPyContents }],
     ]);
 
     const checkedModules = new Set<string>([mainPyName]);
-    const uncheckedScripts = new Array<string>(mainPy);
+    const uncheckedScripts = new Array<string>(mainPyContents);
 
     for (;;) {
         // parse all unchecked scripts to find imported modules that haven't