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bug#22983: syntax-ppss returns wrong result.


From: Andreas Röhler
Subject: bug#22983: syntax-ppss returns wrong result.
Date: Tue, 5 Sep 2017 08:57:54 +0200
User-agent: Mozilla/5.0 (X11; Linux i686; rv:52.0) Gecko/20100101 Thunderbird/52.2.1



On 05.09.2017 01:34, Dmitry Gutov wrote:
On 9/2/17 8:40 PM, Alan Mackenzie wrote:
I'm not happy about this.  22983 is a serious design flaw, which has had
deleterious effects deep within Emacs.

I'm sure we want to fix design flaws. As long as there is a solid plan that does not swap one flaw for another.

One recorded example, resulting
in an infinite loop, is:

#########################################################################
From: Philipp Stephani <address@hidden>
To: address@hidden
Subject: [PATCH] Protect against an infloop in python-mode
Date: Tue, 28 Feb 2017 22:31:49 +0100

There appears to be an edge case caused by using `syntax-ppss' in a
narrowed buffer during JIT lock inside of Python triple-quote strings.
Unfortunately it is impossible to reproduce without manually
destroying the syntactic information in the Python buffer, but it has
been observed in practice.  In that case it can happen that the syntax
caches get sufficiently out of whack so that there appear to be
overlapping strings in the buffer.  As Python has no nested strings,
this situation is impossible and leads to an infloop in
`python-nav-end-of-statement'.  Protect against this by checking
whether the search for the end of the current string makes progress.
#########################################################################

In this case, Philipp had to apply a workaround.

The problem manifested during jit-lock. Do we understand why the (widen) call inside font-lock-default-fontify-region didn't help?





IIRC its about dissolving circular dependencies notably between syntax-propertize-function and syntax-ppss.

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