pgsql: Tighten data validity checks in tsvectorrecv().

Tighten data validity checks in tsvectorrecv().

Insist that the input lexemes be already sorted and non-duplicate.
There's no good reason for them not to be, since the required ordering
is simple and platform-independent. The previous coding was willing
to re-sort them, but that left a gap: it didn't de-duplicate them.
De-duplicating would potentially require merging position lists,
which would be painful; we can't easily re-use tsvectorin()'s logic
for that, since it uses a different temporary representation.
Let's just dodge the problem by insisting the lexeme list be fully
correct as-presented, as we were already doing for the position lists.

Also, there was an oversight in the position-list checking: it did
not verify that the first position isn't zero. tsvectorin() rejects
that, so this should too.

While these problems allow tsvectorrecv() to construct tsvectors
that would not be generated internally, the vectors aren't so broken
that we can't work with them. Therefore, the best course of action
seems to be to apply these tighter rules in master but not back-patch.
Back-patching might break application workflows that aren't causing
any serious problems. Any such users will need to clean their data
before loading it into PG >= v20, but they'd face that issue anyway,
and a major update is a better time for it than a minor update.

Bug: #19748
Reported-by: Ke <kehan5800(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/19748-6075f0508fa1ffc0@postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/10cc5aa96a6df7cb1d7c47e978cc94f01f785cb7

Modified Files
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src/backend/utils/adt/tsvector.c | 17 +++++++++--------
1 file changed, 9 insertions(+), 8 deletions(-)