pgsql: seg: pure parser and reentrant scanner

seg: pure parser and reentrant scanner

Use the flex %option reentrant and the bison option %pure-parser to
make the generated scanner and parser pure, reentrant, and
thread-safe.

Make the generated scanner use palloc() etc. instead of malloc() etc.
Previously, we only used palloc() for the buffer, but flex would still
use malloc() for its internal structures. As a result, there could be
some small memory leaks in case of uncaught errors. (We do catch
normal syntax errors as soft errors.) Now, all the memory is under
palloc() control, so there are no more such issues.

Simplify flex scan buffer management: Instead of constructing the
buffer from pieces and then using yy_scan_buffer(), we can just use
yy_scan_string(), which does the same thing internally.

The previous code was necessary because we allocated the buffer with
palloc() and the rest of the state was handled by malloc(). But this
is no longer the case; everything is under palloc() now.

(We could even get rid of the yylex_destroy() call and just let the
memory context cleanup handle everything. But for now, we preserve
the existing behavior.)

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Andreas Karlsson <andreas@proxel.se>
Discussion: /messages/by-id/eb6faeac-2a8a-4b69-9189-c33c520e5b7b@eisentraut.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/1f0de66ea2a5549a3768c67434e28a136c280571

Modified Files
--------------
contrib/seg/seg.c | 9 ++---
contrib/seg/segdata.h | 13 +++++---
contrib/seg/segparse.y | 9 ++---
contrib/seg/segscan.l | 90 +++++++++++++++++++++++++++++---------------------
4 files changed, 72 insertions(+), 49 deletions(-)