Make detach-partition-concurrently-4 less timing sensitive

Same as 5e0b1aeb, for the companion test file.

This one seems lower probability (only two failures in a month of runs);
I was hardly able to reproduce a failure without a patch, so the fact
that I was also unable to reproduce one with it doesn't say anything.
We'll have to wait for further buildfarm results to see if we need any
further adjustments.

Discussion: https://postgr.es/m/20210524090712.GA3771394@rfd.leadboat.com

src/test/isolation/specs/detach-partition-concurrently-4.spec

@@ -3,6 +3,11 @@
 # (The cases where the detaching transaction is cancelled is interesting
 # because the locking situation is completely different.  I didn't verify
 # that keeping both variants adds any extra coverage.)
+#
+# Note: Always keep "s2noop" right after "s1cancel" in permutations.  This
+# reduces the probability of the timing problem that the cancel error report
+# is shown together with the next query instead of with the cancel query.
+
 setup {
 	drop table if exists d4_primary, d4_primary1, d4_fk, d4_pid;
 	create table d4_primary (a int primary key) partition by list (a);
@@ -19,7 +24,8 @@ session "s1"
 step "s1b"			{ begin; }
 step "s1brr"		{ begin isolation level repeatable read; }
 step "s1s"			{ select * from d4_primary; }
-step "s1cancel" 	{ select pg_cancel_backend(pid) from d4_pid; }
+# Sleep 0.1s after sending cancel, to give s2 time to react
+step "s1cancel" 	{ select pg_cancel_backend(pid), pg_sleep(0.1) from d4_pid; }
 step "s1insert"		{ insert into d4_fk values (1); }
 step "s1c"			{ commit; }
 step "s1declare"	{ declare f cursor for select * from d4_primary; }
@@ -33,6 +39,7 @@ step "s1rollback"	{ rollback to f; }
 session "s2"
 step "s2snitch" { insert into d4_pid select pg_backend_pid(); }
 step "s2detach" { alter table d4_primary detach partition d4_primary1 concurrently; }
+step "s2noop"	{ UNLISTEN noop; }
 
 session "s3"
 step "s3brr"		{ begin isolation level repeatable read; }
@@ -41,34 +48,34 @@ step "s3commit"		{ commit; }
 step "s3vacfreeze"	{ vacuum freeze pg_catalog.pg_inherits; }
 
 # Trying to insert into a partially detached partition is rejected
-permutation "s2snitch" "s1b"   "s1s" "s2detach" "s1cancel" "s1insert" "s1c"
+permutation "s2snitch" "s1b"   "s1s" "s2detach" "s1cancel" "s2noop" "s1insert" "s1c"
 permutation "s2snitch" "s1b"   "s1s" "s2detach"            "s1insert" "s1c"
 # ... even under REPEATABLE READ mode.
-permutation "s2snitch" "s1brr" "s1s" "s2detach" "s1cancel" "s1insert" "s1c"
+permutation "s2snitch" "s1brr" "s1s" "s2detach" "s1cancel" "s2noop" "s1insert" "s1c"
 permutation "s2snitch" "s1brr" "s1s" "s2detach"            "s1insert" "s1c"
 
 # If you read the referenced table using a cursor, you can see a row that the
 # RI query does not see.
-permutation "s2snitch" "s1b" "s1declare" "s2detach" "s1cancel" "s1fetchall" "s1insert" "s1c"
+permutation "s2snitch" "s1b" "s1declare" "s2detach" "s1cancel" "s2noop" "s1fetchall" "s1insert" "s1c"
 permutation "s2snitch" "s1b" "s1declare" "s2detach"            "s1fetchall" "s1insert" "s1c"
-permutation "s2snitch" "s1b" "s1declare" "s2detach" "s1cancel" "s1svpt" "s1insert" "s1rollback" "s1fetchall" "s1c"
+permutation "s2snitch" "s1b" "s1declare" "s2detach" "s1cancel" "s2noop" "s1svpt" "s1insert" "s1rollback" "s1fetchall" "s1c"
 permutation "s2snitch" "s1b" "s1declare" "s2detach"            "s1svpt" "s1insert" "s1rollback" "s1fetchall" "s1c"
-permutation "s2snitch" "s1b" "s2detach" "s1declare" "s1cancel" "s1fetchall" "s1insert" "s1c"
+permutation "s2snitch" "s1b" "s2detach" "s1declare" "s1cancel" "s2noop" "s1fetchall" "s1insert" "s1c"
 permutation "s2snitch" "s1b" "s2detach" "s1declare"            "s1fetchall" "s1insert" "s1c"
-permutation "s2snitch" "s1b" "s2detach" "s1declare" "s1cancel" "s1svpt" "s1insert" "s1rollback" "s1fetchall" "s1c"
+permutation "s2snitch" "s1b" "s2detach" "s1declare" "s1cancel" "s2noop" "s1svpt" "s1insert" "s1rollback" "s1fetchall" "s1c"
 permutation "s2snitch" "s1b" "s2detach" "s1declare"            "s1svpt" "s1insert" "s1rollback" "s1fetchall" "s1c"
 
 # Creating the referencing row using a cursor
-permutation "s2snitch" "s1brr" "s1declare2" "s1fetchone" "s2detach" "s1cancel" "s1updcur" "s1c"
+permutation "s2snitch" "s1brr" "s1declare2" "s1fetchone" "s2detach" "s1cancel" "s2noop" "s1updcur" "s1c"
 permutation "s2snitch" "s1brr" "s1declare2" "s1fetchone" "s2detach"            "s1updcur" "s1c"
 permutation "s2snitch" "s1brr" "s1declare2" "s1fetchone" "s1updcur" "s2detach" "s1c"
 
 # Try reading the table from an independent session.
 permutation "s2snitch" "s1b" "s1s" "s2detach" "s3insert" "s1c"
-permutation "s2snitch" "s1b" "s1s" "s2detach" "s3brr" "s3insert" "s3commit" "s1cancel" "s1c"
+permutation "s2snitch" "s1b" "s1s" "s2detach" "s3brr" "s3insert" "s3commit" "s1cancel" "s2noop" "s1c"
 permutation "s2snitch" "s1b" "s1s" "s2detach" "s3brr" "s3insert" "s3commit" "s1c"
 
 # Try one where we VACUUM FREEZE pg_inherits (to verify that xmin change is
 # handled correctly).
-permutation "s2snitch" "s1brr" "s1s" "s2detach" "s1cancel" "s3vacfreeze" "s1s" "s1insert" "s1c"
-permutation "s2snitch" "s1b" "s1s" "s2detach" "s1cancel" "s3vacfreeze" "s1s" "s1insert" "s1c"
+permutation "s2snitch" "s1brr" "s1s" "s2detach" "s1cancel" "s2noop" "s3vacfreeze" "s1s" "s1insert" "s1c"
+permutation "s2snitch" "s1b" "s1s" "s2detach" "s1cancel" "s2noop" "s3vacfreeze" "s1s" "s1insert" "s1c"