[test] Improve stdout/stderr capturing for browser tests. NFC (emscripten-core#24174)

Now you just need to edit one line in browser_reporting.js to have
stdout/stderr sent back to the server.

This makes debugging issues, especially in headless mode, much simpler.

Ideally I'd like to make this even easier (e.g. enabled via a command
line flag), but this is a good step in the right direction.

I made this change while trying to invesitgate flakiness in
`test_sdl_mouse`. The ouput from that test with this logging enabled now
looks like this:

```
test_sdl_mouse (test_browser.browser.test_sdl_mouse) ... 127.0.0.1 - - [23/Apr/2025 10:42:49] code 404, message File not found
[client stderr: 'motion: abs:10,20  rel:10,20']
[client stderr: 'button down: 1,1  10,20']
[client stderr: 'button up: 1,0  10,20']
[client stderr: 'motion: abs:30,77  rel:20,57']
ok
[Browser harness server terminated]
```

Author: sbc100

test/browser_reporting.js

@@ -1,11 +1,14 @@
+// Set this to true to have stdout and stderr sent back to the server
+var captureStdoutStderr = false;
+
 var hasModule = typeof Module === 'object' && Module;
 
 var reportingURL = 'http://localhost:8888/';
 
 async function reportResultToServer(result) {
   if (reportResultToServer.reported) {
     // Only report one result per test, even if the test misbehaves and tries to report more.
-    reportErrorToServer(`excessive reported results, sending ${result}, test will fail`);
+    reportStderrToServer(`excessive reported results, sending ${result}, test will fail`);
   }
   reportResultToServer.reported = true;
   if ((typeof ENVIRONMENT_IS_NODE !== 'undefined' && ENVIRONMENT_IS_NODE) || (typeof ENVIRONMENT_IS_AUDIO_WORKLET !== 'undefined' && ENVIRONMENT_IS_AUDIO_WORKLET)) {
@@ -29,14 +32,22 @@ function maybeReportResultToServer(result) {
   }
 }
 
-function reportErrorToServer(message) {
+function reportStderrToServer(message) {
   if (typeof ENVIRONMENT_IS_NODE !== 'undefined' && ENVIRONMENT_IS_NODE) {
     err(message);
   } else {
     fetch(`${reportingURL}?stderr=${encodeURIComponent(message)}`);
   }
 }
 
+function reportStdoutToServer(message) {
+  if (typeof ENVIRONMENT_IS_NODE !== 'undefined' && ENVIRONMENT_IS_NODE) {
+    out(message);
+  } else {
+    fetch(`${reportingURL}?stdout=${encodeURIComponent(message)}`);
+  }
+}
+
 function reportTopLevelError(e) {
   // MINIMAL_RUNTIME doesn't handle exit or call the below onExit handler
   // so we detect the exit by parsing the uncaught exception message.
@@ -89,4 +100,19 @@ if (hasModule) {
     }
     Module['onAbort'].proxy = true;
   }
+
+  if (captureStdoutStderr) {
+    const origPrint = Module['print'];
+    const origPrintErr = Module['printErr'];
+
+    Module['print'] = (...args) => {
+      origPrint?.(...args);
+      reportStdoutToServer(args.join(' '));
+    };
+
+    Module['printErr'] = (...args) => {
+      origPrintErr?.(...args);
+      reportStderrToServer(args.join(' '));
+    };
+  }
 }

test/common.py

@@ -2120,7 +2120,9 @@ def do_POST(self):
         print(f'do_POST: unexpected POST: {urlinfo.query}')
 
     def do_GET(self):
-      if self.path == '/run_harness':
+      info = urlparse(self.path)
+      query = parse_qs(info.query)
+      if info.path == '/run_harness':
         if DEBUG:
           print('[server startup]')
         self.send_response(200)
@@ -2160,18 +2162,17 @@ def do_GET(self):
         self.end_headers()
         self.wfile.write(b'OK')
 
-      elif 'stdout=' in self.path or 'stderr=' in self.path:
-        '''
-          To get logging to the console from browser tests, add this to
-          print/printErr/the exception handler in src/shell.html:
-
-            fetch(encodeURI('http://localhost:8888?stdout=' + text));
-        '''
-        print('[client logging:', unquote_plus(self.path), ']')
+      elif info.path == '/' and 'stdout' in query or 'stderr' in query:
+        # Logging reported by reportStdoutToServer / reportStderrToServer.
+        #
+        # To automatically capture stderr/stdout message from browser tests, modify
+        # `captureStdoutStderr` in `test/browser_reporting.js`.
+        for key, value in query.items():
+          print(f"[client {key}: '{value[0]}']")
         self.send_response(200)
         self.send_header('Content-type', 'text/html')
         self.end_headers()
-      elif self.path == '/check':
+      elif info.path == '/check':
         self.send_response(200)
         self.send_header('Content-type', 'text/html')
         self.end_headers()