code review 5784046: go/parser: avoid endless loop in case of internal error

go/parser: avoid endless loop in case of internal error

Factored the error synchronization code into two functions
syncStmt and syncDecl. Because they may return w/o advancing
the scanner, there is potential for endless loops across
multiple parse functions; typically caused by an incorrect
token list in these functions (e.g., adding token.ELSE to
syncStmt will cause the parser to go into an endless loop
for test/syntax/semi7.go without this mechanism). This would
indicate a compiler bug, exposed only in an error situation
for very specific source files. Added a mechanism to force
scanner advance if an endless loop is detected. As a result,
error recovery will be less good in those cases, but the parser
reported a source error already and at least doesn't get stuck.

[gri, 2012-03-08 00:52:16]

Hello rsc@golang.org (cc: golang-dev@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go

[rsc1, 2012-03-08 02:16:44]

LGTM

http://codereview.appspot.com/5784046/diff/4003/src/pkg/go/parser/parser.go
File src/pkg/go/parser/parser.go (right):

http://codereview.appspot.com/5784046/diff/4003/src/pkg/go/parser/parser.go#n...
src/pkg/go/parser/parser.go:48: synCnt int       // number of calls to syncXXX
without progress
Not syncPos, syncCnt?

[gri, 2012-03-08 05:28:53]

*** Submitted as http://code.google.com/p/go/source/detail?r=ee68d043e7f5 ***

go/parser: avoid endless loop in case of internal error

Factored the error synchronization code into two functions
syncStmt and syncDecl. Because they may return w/o advancing
the scanner, there is potential for endless loops across
multiple parse functions; typically caused by an incorrect
token list in these functions (e.g., adding token.ELSE to
syncStmt will cause the parser to go into an endless loop
for test/syntax/semi7.go without this mechanism). This would
indicate a compiler bug, exposed only in an error situation
for very specific source files. Added a mechanism to force
scanner advance if an endless loop is detected. As a result,
error recovery will be less good in those cases, but the parser
reported a source error already and at least doesn't get stuck.

R=rsc, rsc
CC=golang-dev
http://codereview.appspot.com/5784046