Prevent regexp back-refs from sometimes matching when they shouldn't.

The recursion in cdissect() was careless about clearing match data
for capturing parentheses after rejecting a partial match.  This
could allow a later back-reference to succeed when by rights it
should fail for lack of a defined referent.

To fix, think a little more rigorously about what the contract
between different levels of cdissect's recursion needs to be.
With the right spec, we can fix this using fewer rather than more
resets of the match data; the key decision being that a failed
sub-match is now explicitly responsible for clearing any matches
it may have set.

There are enough other cross-checks and optimizations in the code
that it's not especially easy to exhibit this problem; usually, the
match will fail as-expected.  Plus, regexps that are even potentially
vulnerable are most likely user errors, since there's just not much
point in writing a back-ref that doesn't always have a referent.
These facts perhaps explain why the issue hasn't been detected,
even though it's almost certainly a couple of decades old.

Discussion: https://postgr.es/m/151435.1629733387@sss.pgh.pa.us

2 files changed, 17 insertions, 0 deletions

diff --git a/src/test/regress/expected/regex.out b/src/test/regress/expected/regex.out
index 86477cc506c..cbe2cfc3ea1 100644
--- a/src/test/regress/expected/regex.out
+++ b/src/test/regress/expected/regex.out
@@ -567,6 +567,19 @@ select 'a' ~ '()+\1';
  t
 (1 row)
 
+-- Test ancient oversight in when to apply zaptreesubs
+select 'abcdef' ~ '^(.)\1|\1.' as f;
+ f 
+---
+ f
+(1 row)
+
+select 'abadef' ~ '^((.)\2|..)\2' as f;
+ f 
+---
+ f
+(1 row)
+
 -- Add coverage for some cases in checkmatchall
 select regexp_match('xy', '.|...');
  regexp_match 
diff --git a/src/test/regress/sql/regex.sql b/src/test/regress/sql/regex.sql
index b03a8d9ac22..c6974a43d11 100644
--- a/src/test/regress/sql/regex.sql
+++ b/src/test/regress/sql/regex.sql
@@ -135,6 +135,10 @@ select 'a' ~ '.. ()|\1';
 select 'a' ~ '()*\1';
 select 'a' ~ '()+\1';
 
+-- Test ancient oversight in when to apply zaptreesubs
+select 'abcdef' ~ '^(.)\1|\1.' as f;
+select 'abadef' ~ '^((.)\2|..)\2' as f;
+
 -- Add coverage for some cases in checkmatchall
 select regexp_match('xy', '.|...');
 select regexp_match('xyz', '.|...');