Fix an issue if user passes a relative path to Python

test/test_sanity.py

@@ -846,3 +846,26 @@ 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):