[lldb] add fallback for LLDB_PYTHON_RELATIVE_PATH

[lawrence_danna]

Summary

Some pythons are configured to set platlib somewhere outside of their
sys.prefix. It's important that we at least use some reasonable
default for LLDB_PYTHON_RELATIVE_PATH even in that case, because
even if the user overrides it on the cmake invocation, cmake will
still be called without the override in order to build tablegen.

[JDevlieghere]

LGTM

[clayborg]

Yep, fixes it for me! Thanks!!

lldb/bindings/python/get-python-config.py

71

Add newline

[lawrence_danna]

fixed whitespace and commited

lldb/bindings/python/get-python-config.py

if args.variable_name == "LLDB_PYTHON_RELATIVE_PATH":
         # The way we're going to compute this is to take the relative path from
         # PYTHON'S prefix to where python libraries are supposed to be
         # installed.
         #
         # The result is if LLDB and python are give the same prefix, then
         # lldb's python lib will be put in the correct place for python to find it.
         # If not, you'll have to use lldb -P or lldb -print-script-interpreter-info
         # to figure out where it is.
+        try:
             print(relpath_nodots(sysconfig.get_path("platlib"), sys.prefix))
+        except ValueError:
+            # Try to fall back to something reasonable if sysconfig's platlib
+            # is outside of sys.prefix
+            if os.name == 'posix':
+                print('lib/python%d.%d/site-packages' % sys.version_info[:2])
+            elif os.name == 'nt':
+                print('Lib\\site-packages')
+            else:
+                raise
     elif args.variable_name == "LLDB_PYTHON_EXE_RELATIVE_PATH":
         tried = list()
         exe = sys.executable
         prefix = os.path.realpath(sys.prefix)
         while True:
             try:
                 print(relpath_nodots(exe, prefix))
                 break
elif args.variable_name == "LLDB_PYTHON_EXE_RELATIVE_PATH":
                     print("sys.prefix:", sys.prefix, file=sys.stderr)
                     sys.exit(1)
     elif args.variable_name == "LLDB_PYTHON_EXT_SUFFIX":
         print(sysconfig.get_config_var('EXT_SUFFIX'))
     else:
         parser.error(f"unknown variable {args.variable_name}")
 
 if __name__ == '__main__':
-    main()
+    main()