Make __file__ work in config file (#23973)

As of today folks who want to find stuff relative to the config file
itself need to do `os.getenv('EM_CONFIG')` which is pretty hacky.

Author: sbc100

ChangeLog.md

@@ -24,6 +24,8 @@ See docs/process.md for more on how version tagging works.
   source map : use `-sSOURCE_MAP_PREFIXES=["<old>=<new>"]` with `-gsource-map`.
   Alternatively, you can now embed the sources content into the source map file
   using `-gsource-map=inline`. (#23741)
+- The python `__file__` builtin now works in the emscripten config file.
+  (#23973)
 
 4.0.5 - 03/12/25
 ----------------

test/test_other.py

@@ -845,6 +845,17 @@ def test_em_config_missing_arg(self):
     out = self.expect_fail([EMCC, '--em-config'])
     self.assertContained('error: --em-config must be followed by a filename', out)
 
+  def test_em_config_filename(self):
+    create_file('myconfig.py', f'''
+      LLVM_ROOT = '{config.LLVM_ROOT}'
+      BINARYEN_ROOT = '{config.BINARYEN_ROOT}'
+      CACHE = '{os.path.abspath("cache")}'
+      print("filename", __file__)
+    ''')
+    proc = self.run_process([EMAR, '--em-config', 'myconfig.py', '--version'], stdout=PIPE, stderr=PIPE)
+    self.assertContained('LLVM', proc.stdout)
+    self.assertContained('filename myconfig.py', proc.stdout)
+
   def test_emsize(self):
     # test binaryen generated by running:
     #   emcc test/hello_world.c -Oz --closure 1 -o test/other/test_emsize.js

tools/config.py

@@ -97,7 +97,7 @@ def parse_config_file():
 
   Also check EM_<KEY> environment variables to override specific config keys.
   """
-  config = {}
+  config = {'__file__': EM_CONFIG}
   config_text = utils.read_file(EM_CONFIG)
   try:
     exec(config_text, config)