Fix an issue if user passes a relative path to Python (#26799)

Fix an issue if user passes a relative path to Python, and some system
cache libraries need rebuilding. Emcc tool would spawn sub-emcc tools to
rebuild the cache, with different CWD, resulting in the relative path
lookups pointing to the wrong relative directories. To fix the issue,
normalize the relative tool paths first.

<img width="2380" height="992" alt="image"
src="https://github.com/user-attachments/assets/e22b71c0-b73e-411c-9ef5-f51a890f01aa"
/>

Author: juj

test/test_sanity.py

@@ -847,3 +847,26 @@ def test_bootstrap_without_em_config(self):
 
     # Running bootstrap.py should not fail
     self.run_process([utils.exe_path_from_root('bootstrap')], env=env)
+
+  # Verify that if user specifies a relative path to Python executable, then
+  # Emscripten is still able to build.
+  def test_emcc_with_relative_python_path(self):
+    restore_and_set_up()
+    # Clear the cache, since rebuilding the cache has been observed to fail
+    # if Python path is specified as relative.
+    self.clear_cache()
+
+    try:
+      relative_python = os.path.relpath(os.environ.get('EMSDK_PYTHON'), os.getcwd())
+    except ValueError:
+      self.skipTest('Python and Emscripten are located on different drives, cannot run this test.')
+
+    relative_python_escaped = relative_python.replace("\\", "\\\\")
+    add_to_config(f'PYTHON = "{relative_python_escaped}"')
+
+    env = os.environ.copy()
+    env['EMSDK_PYTHON'] = relative_python
+
+    output = self.do([EMCC, test_file('hello_world.c')], env=env)
+    self.assertNotContained('error', output)
+    self.assertExists('a.out.js')

tools/config.py

@@ -74,6 +74,20 @@ def normalize_config_settings():
     PORTS = os.path.join(CACHE, 'ports')
 
 
+def normalize_relative_python_path():
+  # User may have specified the EMSDK_PYTHON environment variable to point to
+  # the Python interpreter, e.g.
+  #
+  #  EMSDK_PYTHON=../../path/to/python emcc test/hello_world.c
+  #
+  # As part of its operation, emcc may spawn sub-emcc tasks when building
+  # libraries to cache. These sub-emcc tasks will run in a different CWD, so
+  # reinitialize EMSDK_PYTHON here so that sub-tool spawns will use the same
+  # Python interpreter as the parent.
+  if os.environ.get('EMSDK_PYTHON'):
+    os.environ['EMSDK_PYTHON'] = sys.executable
+
+
 def set_config_from_tool_location(config_key, tool_binary, f):
   val = globals()[config_key]
   if val is None:
@@ -168,6 +182,7 @@ def read_config():
   set_config_from_tool_location('BINARYEN_ROOT', 'wasm-opt', lambda x: os.path.dirname(os.path.dirname(x)))
 
   normalize_config_settings()
+  normalize_relative_python_path()
 
 
 def generate_config(path):