Record dependencies of a cast on other casts that it requires.

When creating a cast that uses a conversion function, we've
historically allowed the input and result types to be
binary-compatible with the function's input and result types,
rather than necessarily being identical.  This means that the new
cast is logically dependent on the binary-compatible cast or casts
that it references: if those are defined by pg_cast entries, and you
try to restore the new cast without having defined them, it'll fail.
Hence, we should make pg_depend entries to record these dependencies
so that pg_dump knows that there is an ordering requirement.

This is not the only place where we allow such shortcuts; aggregate
functions for example are similarly lax, and in principle should gain
similar dependencies.  However, for now it seems sufficient to fix
the cast-versus-cast case, as pg_dump's other ordering heuristics
should keep it out of trouble for other object types.

Per report from David Turoň; thanks also to Robert Haas for
preliminary investigation.  I considered back-patching, but
seeing that this issue has existed for many years without
previous reports, it's not clear it's worth the trouble.
Moreover, back-patching wouldn't be enough to ensure that the
new pg_depend entries exist in existing databases anyway.

Discussion: https://postgr.es/m/OF0A160F3E.578B15D1-ONC12588DA.003E4857-C12588DA.0045A428@notes.linuxbox.cz

2 files changed, 50 insertions, 0 deletions

diff --git a/src/test/regress/expected/create_cast.out b/src/test/regress/expected/create_cast.out
index 88f94a63b48..9a56fe3f0fd 100644
--- a/src/test/regress/expected/create_cast.out
+++ b/src/test/regress/expected/create_cast.out
@@ -72,3 +72,32 @@ SELECT 1234::int4::casttesttype; -- Should work now
  foo1234
 (1 row)
 
+DROP FUNCTION int4_casttesttype(int4) CASCADE;
+NOTICE:  drop cascades to cast from integer to casttesttype
+-- Try it with a function that requires an implicit cast
+CREATE FUNCTION bar_int4_text(int4) RETURNS text LANGUAGE SQL AS
+$$ SELECT ('bar'::text || $1::text); $$;
+CREATE CAST (int4 AS casttesttype) WITH FUNCTION bar_int4_text(int4) AS IMPLICIT;
+SELECT 1234::int4::casttesttype; -- Should work now
+ casttesttype 
+--------------
+ bar1234
+(1 row)
+
+-- check dependencies generated for that
+SELECT pg_describe_object(classid, objid, objsubid) as obj,
+       pg_describe_object(refclassid, refobjid, refobjsubid) as objref,
+       deptype
+FROM pg_depend
+WHERE classid = 'pg_cast'::regclass AND
+      objid = (SELECT oid FROM pg_cast
+               WHERE castsource = 'int4'::regtype
+                 AND casttarget = 'casttesttype'::regtype)
+ORDER BY refclassid;
+                obj                |             objref              | deptype 
+-----------------------------------+---------------------------------+---------
+ cast from integer to casttesttype | type casttesttype               | n
+ cast from integer to casttesttype | function bar_int4_text(integer) | n
+ cast from integer to casttesttype | cast from text to casttesttype  | n
+(3 rows)
+
diff --git a/src/test/regress/sql/create_cast.sql b/src/test/regress/sql/create_cast.sql
index b11cf88b064..32187853cc7 100644
--- a/src/test/regress/sql/create_cast.sql
+++ b/src/test/regress/sql/create_cast.sql
@@ -52,3 +52,24 @@ $$ SELECT ('foo'::text || $1::text)::casttesttype; $$;
 
 CREATE CAST (int4 AS casttesttype) WITH FUNCTION int4_casttesttype(int4) AS IMPLICIT;
 SELECT 1234::int4::casttesttype; -- Should work now
+
+DROP FUNCTION int4_casttesttype(int4) CASCADE;
+
+-- Try it with a function that requires an implicit cast
+
+CREATE FUNCTION bar_int4_text(int4) RETURNS text LANGUAGE SQL AS
+$$ SELECT ('bar'::text || $1::text); $$;
+
+CREATE CAST (int4 AS casttesttype) WITH FUNCTION bar_int4_text(int4) AS IMPLICIT;
+SELECT 1234::int4::casttesttype; -- Should work now
+
+-- check dependencies generated for that
+SELECT pg_describe_object(classid, objid, objsubid) as obj,
+       pg_describe_object(refclassid, refobjid, refobjsubid) as objref,
+       deptype
+FROM pg_depend
+WHERE classid = 'pg_cast'::regclass AND
+      objid = (SELECT oid FROM pg_cast
+               WHERE castsource = 'int4'::regtype
+                 AND casttarget = 'casttesttype'::regtype)
+ORDER BY refclassid;