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: cockroachdb#103245
Release note (sql change): cluster_logical_timestamp now returns an
error when called at isolation levels lower than SERIALIZABLE.

Author: chrisseto

docs/generated/sql/functions.md

@@ -3116,6 +3116,7 @@ Case mode values range between 0 - 1, representing lower casing and upper casing
 a CockroachDB HLC in decimal form.</p>
 <p>Note that uses of this function disable server-side optimizations and
 may increase either contention or retry errors, or both.</p>
+<p>Returns an error if run in a transaction with an isolation level weaker than SERIALIZABLE.</p>
 </span></td><td>Volatile</td></tr>
 <tr><td><a name="crdb_internal.active_version"></a><code>crdb_internal.active_version() &rarr; jsonb</code></td><td><span class="funcdesc"><p>Returns the current active cluster version.</p>
 </span></td><td>Volatile</td></tr>

pkg/sql/conn_executor.go

@@ -3433,21 +3433,6 @@ func (ex *connExecutor) txnIsolationLevelToKV(
 	return ret
 }
 
-func kvTxnIsolationLevelToTree(level isolation.Level) tree.IsolationLevel {
-	var ret tree.IsolationLevel
-	switch level {
-	case isolation.Serializable:
-		ret = tree.SerializableIsolation
-	case isolation.ReadCommitted:
-		ret = tree.ReadCommittedIsolation
-	case isolation.Snapshot:
-		ret = tree.SnapshotIsolation
-	default:
-		log.Fatalf(context.Background(), "unknown isolation level: %s", level)
-	}
-	return ret
-}
-
 func txnPriorityToProto(mode tree.UserPriority) roachpb.UserPriority {
 	var pri roachpb.UserPriority
 	switch mode {
@@ -4003,7 +3988,7 @@ func (ex *connExecutor) serialize() serverpb.Session {
 			Priority:              ex.state.priority.String(),
 			QualityOfService:      sessiondatapb.ToQoSLevelString(txn.AdmissionHeader().Priority),
 			LastAutoRetryReason:   autoRetryReasonStr,
-			IsolationLevel:        kvTxnIsolationLevelToTree(ex.state.isolationLevel).String(),
+			IsolationLevel:        tree.IsolationLevelFromKVTxnIsolationLevel(ex.state.isolationLevel).String(),
 		}
 	}
 

pkg/sql/crdb_internal.go

@@ -7705,7 +7705,7 @@ func genClusterLocksGenerator(
 				tree.MakeDBool(tree.DBool(granted)),          /* granted */
 				tree.MakeDBool(len(curLock.Waiters) > 0),     /* contended */
 				durationDatum,                                /* duration */
-				tree.NewDString(kvTxnIsolationLevelToTree(curLock.LockHolder.IsoLevel).String()), /* isolation_level */
+				tree.NewDString(tree.IsolationLevelFromKVTxnIsolationLevel(curLock.LockHolder.IsoLevel).String()), /* isolation_level */
 			}, nil
 
 		}, nil, nil

pkg/sql/logictest/testdata/logic_test/txn

@@ -1523,3 +1523,20 @@ 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 ok
+BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED;
+
+skipif config local-mixed-22.2-23.1
+statement error pq: cluster_logical_timestamp\(\): unsupported in READ COMMITTED isolation
+SELECT cluster_logical_timestamp();
+
+statement ok
+ROLLBACK

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

@@ -540,6 +540,15 @@ 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() {
+		treeIso := tree.IsolationLevelFromKVTxnIsolationLevel(ec.TxnIsoLevel)
+		return nil, pgerror.Newf(pgcode.FeatureNotSupported, "unsupported in %s isolation", treeIso.String())
+	}
+
 	ts := ec.Txn.CommitTimestamp()
 	if ts.IsEmpty() {
 		return nil, errors.AssertionFailedf("zero cluster timestamp in txn")

pkg/sql/sem/tree/BUILD.bazel

@@ -125,6 +125,7 @@ go_library(
         "//pkg/col/typeconv",  # keep
         "//pkg/geo",
         "//pkg/geo/geopb",
+        "//pkg/kv/kvserver/concurrency/isolation",
         "//pkg/sql/lex",
         "//pkg/sql/lexbase",
         "//pkg/sql/pgrepl/lsn",
@@ -203,6 +204,7 @@ go_test(
         "pretty_test.go",
         "table_name_test.go",
         "time_test.go",
+        "txn_test.go",
         "type_check_internal_test.go",
         "type_check_test.go",
         "type_name_test.go",
@@ -218,6 +220,7 @@ go_test(
         "//pkg/col/coldata",
         "//pkg/col/coldataext",
         "//pkg/internal/rsg",
+        "//pkg/kv/kvserver/concurrency/isolation",
         "//pkg/security/securityassets",
         "//pkg/security/securitytest",
         "//pkg/settings/cluster",

pkg/sql/sem/tree/txn.go

@@ -14,6 +14,7 @@ import (
 	"fmt"
 	"strings"
 
+	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/concurrency/isolation"
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgcode"
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgerror"
 	"github.com/cockroachdb/redact"
@@ -63,6 +64,24 @@ func (i IsolationLevel) String() string {
 	return isolationLevelNames[i]
 }
 
+// IsolationLevelFromKVTxnIsolationLevel converts a kv level isolation.Level to
+// its SQL semantic equivalent.
+func IsolationLevelFromKVTxnIsolationLevel(level isolation.Level) IsolationLevel {
+	var ret IsolationLevel
+	switch level {
+	case isolation.Serializable:
+		ret = SerializableIsolation
+	case isolation.ReadCommitted:
+		ret = ReadCommittedIsolation
+	case isolation.Snapshot:
+		ret = SnapshotIsolation
+	default:
+		panic("What to do here? Log is a banned import")
+		// log.Fatalf(context.Background(), "unknown isolation level: %s", level)
+	}
+	return ret
+}
+
 // UserPriority holds the user priority for a transaction.
 type UserPriority int
 

pkg/sql/sem/tree/txn_test.go

@@ -0,0 +1,46 @@
+// Copyright 2015 The Cockroach Authors.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+package tree_test
+
+import (
+	"testing"
+
+	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/concurrency/isolation"
+	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
+	"github.com/cockroachdb/cockroach/pkg/util/leaktest"
+	"github.com/stretchr/testify/require"
+)
+
+func TestIsolationLevelFromKVTxnIsolationLevel(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+
+	testCases := []struct {
+		In  isolation.Level
+		Out tree.IsolationLevel
+	}{
+		{
+			In:  isolation.Serializable,
+			Out: tree.SerializableIsolation,
+		},
+		{
+			In:  isolation.ReadCommitted,
+			Out: tree.ReadCommittedIsolation,
+		},
+		{
+			In:  isolation.Snapshot,
+			Out: tree.SnapshotIsolation,
+		},
+	}
+
+	for _, tc := range testCases {
+		require.Equal(t, tc.Out, tree.IsolationLevelFromKVTxnIsolationLevel(tc.In))
+	}
+}

pkg/sql/vars.go

@@ -1491,7 +1491,7 @@ var varGen = map[string]sessionVar{
 	// See https://github.com/postgres/postgres/blob/REL_10_STABLE/src/backend/utils/misc/guc.c#L3401-L3409
 	`transaction_isolation`: {
 		Get: func(evalCtx *extendedEvalContext, _ *kv.Txn) (string, error) {
-			level := kvTxnIsolationLevelToTree(evalCtx.Txn.IsoLevel())
+			level := tree.IsolationLevelFromKVTxnIsolationLevel(evalCtx.Txn.IsoLevel())
 			return strings.ToLower(level.String()), nil
 		},
 		RuntimeSet: func(ctx context.Context, evalCtx *extendedEvalContext, local bool, s string) error {