Make CinderX work with optimized Python builds (again)

Summary:
D62604340 introduced a step to the build where `sed` is used to edit `site.py` to *enable* CinderX in the 3.12 distribution as needed. Unfortunately this doesn't work in optimized builds. Those builds "deep-freeze" a number of startup Python libraries including `site.py` in a state *before* the edit is made.

To fix this, move the CinderX initialization code into a Python module that's not in the frozen libraries, and only include this in the distribution if CinderX is desired. During Python startup, if the `init_cinderx` does not exist then proceed without it leaving the set of imported modules unchanged. This resolves the issue D62604340 was trying to address.

Reviewed By: itamaro

Differential Revision: D65584596

fbshipit-source-id: 212dbe22a93fa44d33750166b80d03bffa559502

Lib/init_cinderx.py

@@ -0,0 +1,21 @@
+# This file is deliberately not included in the set of deep-frozen bootstrap
+# libraries so it can easily be excluded from the distribution to avoid
+# attempting to initialize CinderX. This is desirable because this code imports
+# importlib.util and some tests are surprised when they find this unexpectedly
+# loaded as part of bootstrap.
+
+import importlib.util
+
+# `cinderx` is the Python module in fbcode/cinderx/PythonLib which wraps the
+# `_cinderx` C++ extension in fbcode/cinderx/_cinderx.cpp.  Both need to be
+# available to load CinderX.
+if not (
+    importlib.util.find_spec("cinderx") is None
+    or importlib.util.find_spec("_cinderx") is None
+):
+    try:
+        import cinderx
+
+        cinderx.init()
+    except Exception as e:
+        raise RuntimeError("Failed to initialize CinderX module") from e

Lib/site.py

@@ -600,25 +600,6 @@ def execusercustomize():
                 (err.__class__.__name__, err))
 
 
-def init_cinder():
-    import importlib.util
-
-    # `cinderx` is the Python module in fbcode/cinderx/PythonLib which wraps the
-    # `_cinderx` C++ extension in fbcode/cinderx/_cinderx.cpp.  Both need to be
-    # available to load CinderX.
-    if (
-        importlib.util.find_spec("cinderx") is None
-        or importlib.util.find_spec("_cinderx") is None
-    ):
-        return
-    try:
-        import cinderx
-
-        cinderx.init()
-    except Exception as e:
-        raise RuntimeError("Failed to initialize CinderX module") from e
-
-
 def main():
     """Add standard site-specific directories to the module search path.
 
@@ -644,10 +625,15 @@ def main():
     sethelper()
     if not sys.flags.isolated:
         enablerlcompleter()
-    # Do not attempt to initialize cinder *by default*, as it's not relevant
-    # for regular MetaPython users (and cinderx is not even available).
-    # This gets enabled at build-time when cinderx is enabled (see D62604340).
-    # init_cinder()
+    # If we've been built with CinderX support there will be an init_cinderx.py
+    # included in the default module search path. In this case we import it
+    # which will have the side-effect of initializing CinderX. If this module
+    # is missing then we weren't build with CinderX so ignoring the import
+    # error is fine.
+    try:
+        import init_cinderx
+    except ImportError:
+        pass
     execsitecustomize()
     if ENABLE_USER_SITE:
         execusercustomize()