Inline_op should use shared dynamic stack
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Key: PIG-2138
URL: https://issues.apache.org/jira/browse/PIG-2138
Project: Pig
Issue Type: Improvement
Reporter: Gianmarco De Francisci Morales
The way inline_op saves and restores the statement state is brittle and can
lead to bugs.
To work around the issue one needs to be careful when writing rules not to
include operations that can include inline_op in the evaluation of syntactic
predicates for disambiguation of LL(*) rules.
The interaction between inline_op, the statement scope and the syntactic
predicates that get executed to choose between alternative op versions can null
the statement::alias without restoring it.
inline_op stores the statement::alias locally and sets it to null in @init, and
later in @after restores it to the previous value.
When inline_op gets executed inside a syntactic predicate, @init always gets
executed, @after does not instead.
So the statement::alias doesn't get back its value and it can lead to an NPE.
This behavior is dangerous, because it messes with global structures.
@init and @after are used to mimic a stack behavior.
What inline_op needs is a local statement scope, so that inline_op could just
push another initialized scope on the stack, and get it popped when the
syntactic predicate exits the rule, restoring the old state automatically.
This will also help in the long run to support arbitrarily nested statements.
Unfortunately this also requires to rewrite the whole grammar.
Every rule that accesses statement::alias (or any attribute in statement::
scope) should be rewritten to get a reference to the local scope of the
statement.
The implementation can make use of dynamic shared scope between inline_op and
statement (very much like GScope).
ANTLR will automatically push and pop a stack frame when the rule is invoked
and when it gets out of the rule.
The @init will get executed to initialize members in the scope. Even though the
@after doesn't get executed because we are in a syntactic predicate, the frame
is popped from the stack nonetheless.
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