[Python-Dev] Move encoding_decl to the top of Grammar/Grammar?

[Thomas Lee]

Hi all,

Currently, Parser/parsetok.c has a dependency on graminit.h. This can 
cause headaches when rebuilding after adding new syntax  to 
Grammar/Grammar because parsetok.c is part of pgen, which is responsible 
for *generating* graminit.h.

This circular dependency can result in parsetok.c using a different 
value for encoding_decl to what is used in ast.c, which causes 
PyAST_FromNode to fall over at runtime. It effectively looks something 
like this:

* Grammar/Grammar is modified
* build begins -- pgen compiles, parsetok.c uses encoding_decl=X
* graminit.h is rebuilt with encoding_decl=Y
* ast.c is compiled using encoding_decl=Y
* when python runs, parsetok() emits encoding_decl nodes that 
PyAST_FromNode can't recognize:

SystemError: invalid node XXX for PyAST_FromNode

A nice, easy short term solution that doesn't require unwinding this 
dependency would be to simply move encoding_decl to the top of 
Grammar/Grammar and add a big warning noting that it needs to come 
before everything else. This will help to ensure its value never changes 
when syntax is added/removed.

I'm happy to provide a patch for this (including some additional 
dependency info for files dependent upon graminit.h and Python-ast.h), 
but was wondering if there were any opinions about how this should be 
resolved.

Cheers,
Tom