pgsql: Fix conversion of JSON strings to JSON output columns in json_to

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pgsql: Fix conversion of JSON strings to JSON output columns in json_to

Tom Lane-2
Fix conversion of JSON strings to JSON output columns in json_to_record().

json_to_record(), when an output column is declared as type json or jsonb,
should emit the corresponding field of the input JSON object.  But it got
this slightly wrong when the field is just a string literal: it failed to
escape the contents of the string.  That typically resulted in syntax
errors if the string contained any double quotes or backslashes.

jsonb_to_record() handles such cases correctly, but I added corresponding
test cases for it too, to prevent future backsliding.

Improve the documentation, as it provided only a very hand-wavy
description of the conversion rules used by these functions.

Per bug report from Robert Vollmert.  Back-patch to v10 where the
error was introduced (by commit cf35346e8).

Note that PG 9.4 - 9.6 also get this case wrong, but differently so:
they feed the de-escaped contents of the string literal to json[b]_in.
That behavior is less obviously wrong, so possibly it's being depended on
in the field, so I won't risk trying to make the older branches behave
like the newer ones.

Discussion: https://postgr.es/m/D6921B37-BD8E-4664-8D5F-DB3525765DCD@...

Branch
------
REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1c9034579c026562ae6db5a3f2d3a7678653b9ff

Modified Files
--------------
doc/src/sgml/func.sgml              | 87 +++++++++++++++++++++++++++----------
src/backend/utils/adt/jsonfuncs.c   | 37 +++++++---------
src/test/regress/expected/json.out  | 36 +++++++++++++++
src/test/regress/expected/jsonb.out | 36 +++++++++++++++
src/test/regress/sql/json.sql       |  7 +++
src/test/regress/sql/jsonb.sql      |  7 +++
6 files changed, 167 insertions(+), 43 deletions(-)