Add scanner for Cython dependencies (#19722)

* add scanner for cython dependencies

* cleanup and comments

* comment about relative imports

site_scons/site_tools/cython.py

@@ -1,18 +1,52 @@
+import re
 import SCons
 from SCons.Action import Action
+from SCons.Scanner import Scanner
 
+pyx_from_import_re = re.compile(r'^from\s+(\S+)\s+cimport', re.M)
+pyx_import_re = re.compile(r'^cimport\s+(\S+)', re.M)
+cdef_import_re = re.compile(r'^cdef extern from\s+.(\S+).:', re.M)
+
+
+def pyx_scan(node, env, path, arg=None):
+  contents = node.get_text_contents()
+
+  # from <module> cimport ...
+  matches = pyx_from_import_re.findall(contents)
+  # cimport <module>
+  matches += pyx_import_re.findall(contents)
+
+  # Modules can be either .pxd or .pyx files
+  files = [m.replace('.', '/') + '.pxd' for m in matches]
+  files += [m.replace('.', '/') + '.pyx' for m in matches]
+
+  # cdef extern from <file>
+  files += cdef_import_re.findall(contents)
+
+  # Handle relative imports
+  cur_dir = str(node.get_dir())
+  files = [cur_dir + f if f.startswith('/') else f for f in files]
+
+  # Filter out non-existing files (probably system imports)
+  files = [f for f in files if env.File(f).exists()]
+  return env.File(files)
+
+
+pyxscanner = Scanner(function=pyx_scan, skeys=['.pyx', '.pxd'], recursive=True)
 cythonAction = Action("$CYTHONCOM")
 
+
 def create_builder(env):
   try:
     cython = env['BUILDERS']['Cython']
   except KeyError:
     cython = SCons.Builder.Builder(
-               action = cythonAction,
-               emitter = {},
-               suffix = cython_suffix_emitter,
-               single_source = 1
-             )
+      action=cythonAction,
+      emitter={},
+      suffix=cython_suffix_emitter,
+      single_source=1
+    )
+    env.Append(SCANNERS=pyxscanner)
     env['BUILDERS']['Cython'] = cython
   return cython