sql: error in cluster_logical_timestamp when not SERIALIZABLE

Previously, running `cluster_logical_timestamp` within an isolation
level that tolerated write skew would result in an unhandled panic.

This commit adds a gate at the SQL level that will instead return a
FeatureNotSupported pgerror. This behavior may be permanent or may be
removed once we determine how CommitTimestamp should function at these
isolation levels.

Epic:  CRDB-26546
Part of: #103245
Release note (sql change): cluster_logical_timestamp now returns an
error when called at isolation levels lower than SERIALIZABLE.

pkg/sql/logictest/testdata/logic_test/txn

@@ -1523,3 +1523,14 @@ regular
 statement error pq: SET CLUSTER SETTING cannot be used inside a multi-statement transaction
 SET CLUSTER SETTING sql.defaults.use_declarative_schema_changer = 'on';
 SET CLUSTER SETTING sql.defaults.use_declarative_schema_changer = 'off';
+
+statement ok
+BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+SELECT cluster_logical_timestamp();
+
+statement ok
+ROLLBACK
+
+statement error pq: cluster_logical_timestamp\(\): unsupported in ReadCommitted isolation
+BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED;
+SELECT cluster_logical_timestamp();

pkg/sql/sem/builtins/builtins.go

@@ -2779,7 +2779,9 @@ nearest replica.`, builtinconstants.DefaultFollowerReadDuration),
 a CockroachDB HLC in decimal form.
 
 Note that uses of this function disable server-side optimizations and
-may increase either contention or retry errors, or both.`,
+may increase either contention or retry errors, or both.
+
+Returns an error if run in a transaction with an isolation level weaker than SERIALIZABLE.`
 			Volatility: volatility.Volatile,
 		},
 	),

pkg/sql/sem/eval/context.go

@@ -535,6 +535,14 @@ func (ec *Context) GetClusterTimestamp() (*tree.DDecimal, error) {
 	if ec.Txn == nil {
 		return nil, ErrNilTxnInClusterContext
 	}
+
+	// CommitTimestamp panics for isolation levels that can operate across
+	// multiple timestamps. Prevent this with a gate at the SQL level and return
+	// a pgerror until we decide how this will officially behave. See #103245.
+	if ec.TxnIsoLevel.ToleratesWriteSkew() {
+		return nil, pgerror.Newf(pgcode.FeatureNotSupported, "unsupported in %s isolation", ec.TxnIsoLevel.String())
+	}
+
 	ts := ec.Txn.CommitTimestamp()
 	if ts.IsEmpty() {
 		return nil, errors.AssertionFailedf("zero cluster timestamp in txn")