Prevent crash when ts_rewrite() replaces a non-top-level subtree with null.

When ts_rewrite()'s replacement argument is an empty tsquery, it's supposed
to simplify any operator nodes whose operand(s) become NULL; but it failed
to do that reliably, because dropvoidsubtree() only examined the top level
of the result tree.  Rather than make a second recursive pass, let's just
give the responsibility to dofindsubquery() to simplify while it's doing
the main replacement pass.  Per report from Andreas Seltenreich.

Artur Zakirov, with some cosmetic changes by me.  Back-patch to all
supported branches.

Discussion: https://postgr.es/m/8737i01dew.fsf@credativ.de

1 files changed, 3 insertions, 0 deletions

diff --git a/src/test/regress/sql/tsearch.sql b/src/test/regress/sql/tsearch.sql
index afd990e696..de43860c70 100644
--- a/src/test/regress/sql/tsearch.sql
+++ b/src/test/regress/sql/tsearch.sql
@@ -418,6 +418,9 @@ SELECT ts_rewrite('1 & (2 <2> 3)', 'SELECT keyword, sample FROM test_tsquery'::t
 SELECT ts_rewrite('5 <-> (1 & (2 <-> 3))', 'SELECT keyword, sample FROM test_tsquery'::text );
 SELECT ts_rewrite('5 <-> (6 | 8)', 'SELECT keyword, sample FROM test_tsquery'::text );
 
+-- Check empty substitution
+SELECT ts_rewrite(to_tsquery('5 & (6 | 5)'), to_tsquery('5'), to_tsquery(''));
+SELECT ts_rewrite(to_tsquery('!5'), to_tsquery('5'), to_tsquery(''));
 
 SELECT keyword FROM test_tsquery WHERE keyword @> 'new';
 SELECT keyword FROM test_tsquery WHERE keyword @> 'moscow';