sql: link to issue cockroachdb#42061 from the XXA00 error hint

CockroachDB currently allows an explicit BEGIN..COMMIT txn containing
DDL to fail atomicity in certain cases. These cases are detected
during COMMIT and reported to the client as error XXA00.

This patch extends the error message to link to issue cockroachdb#42061 which
describes and handles this situation further.

Release note (sql change): the error message generated when a
txn containing DDL is both partially committed and partially rolled back
(error XXA00) now contains a link to github issue cockroachdb#42061 where this
situation is discussed further.

pkg/sql/conn_executor.go

@@ -2035,6 +2035,8 @@ func (ex *connExecutor) txnStateTransitionsApplyWrapper(
 					newErr = errors.WithHint(newErr,
 						"Some of the non-DDL statements may have committed successfully, but some of the DDL statement(s) failed.\n"+
 							"Manual inspection may be required to determine the actual state of the database.")
+					newErr = errors.WithIssueLink(newErr,
+						errors.IssueLink{IssueURL: "https://github.com/cockroachdb/cockroach/issues/42061"})
 					res.SetError(newErr)
 				}
 			}

pkg/sql/logictest/testdata/logic_test/strict_ddl_atomicity

@@ -0,0 +1,38 @@
+# Default behavior, where atomicity violations are allowed
+
+statement ok
+CREATE TABLE testing (k int, v string);
+  INSERT INTO testing (k,v) VALUES (1, 'a'), (2, 'b'), (3, 'a'), (4, 'b');
+  CREATE TABLE unrelated(x INT)
+
+
+statement ok
+BEGIN
+
+statement ok
+ALTER TABLE testing ADD CONSTRAINT "unique_values" UNIQUE(v)
+
+statement ok
+INSERT INTO testing (k,v) VALUES (5, 'c');
+INSERT INTO unrelated(x) VALUES (1);
+
+
+statement error pgcode XXA00 violates unique constraint.*\n.*\n.*\n.*\n.*issues/42061
+COMMIT
+
+# oops!
+query IT rowsort
+SELECT * FROM testing
+----
+1  a
+2  b
+3  a
+4  b
+5  c
+
+# oops again!
+query I
+SELECT * FROM unrelated
+----
+1
+