sql/schemachanger: add decoder for schemaChangerUserError

Previously, if a schemaChangerUserError was sent over the network during
a backfill, it was not properly recognized on the receiving node.
Without a decoder, the error would be an unresolved opaqueWrapper. As a
result, the HasSchemaChangerUserError function, which confirms the error
by checking its type, would not work correctly. This patch adds a
decoder for this error type, allowing it to be decoded properly on
remote nodes.

Fixes: #141352

Release note: None

Author: fqazi

pkg/BUILD.bazel

@@ -598,6 +598,7 @@ ALL_TESTS = [
     "//pkg/sql/schemachanger/scbuild/internal/scbuildstmt:scbuildstmt_test",
     "//pkg/sql/schemachanger/scbuild:scbuild_test",
     "//pkg/sql/schemachanger/scdecomp:scdecomp_test",
+    "//pkg/sql/schemachanger/scerrors:scerrors_test",
     "//pkg/sql/schemachanger/scexec/backfiller:backfiller_test",
     "//pkg/sql/schemachanger/scexec:scexec_test",
     "//pkg/sql/schemachanger/scpb:scpb_test",
@@ -2239,6 +2240,7 @@ GO_TARGETS = [
     "//pkg/sql/schemachanger/scdeps/sctestutils:sctestutils",
     "//pkg/sql/schemachanger/scdeps:scdeps",
     "//pkg/sql/schemachanger/scerrors:scerrors",
+    "//pkg/sql/schemachanger/scerrors:scerrors_test",
     "//pkg/sql/schemachanger/scexec/backfiller:backfiller",
     "//pkg/sql/schemachanger/scexec/backfiller:backfiller_test",
     "//pkg/sql/schemachanger/scexec/scmutationexec:scmutationexec",

pkg/sql/schemachanger/scerrors/BUILD.bazel

@@ -1,4 +1,4 @@
-load("@io_bazel_rules_go//go:def.bzl", "go_library")
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
 
 go_library(
     name = "scerrors",
@@ -14,5 +14,17 @@ go_library(
         "//pkg/util/timeutil",
         "@com_github_cockroachdb_errors//:errors",
         "@com_github_cockroachdb_redact//:redact",
+        "@com_github_gogo_protobuf//proto",
+    ],
+)
+
+go_test(
+    name = "scerrors_test",
+    srcs = ["errors_test.go"],
+    deps = [
+        ":scerrors",
+        "@com_github_cockroachdb_errors//:errors",
+        "@com_github_cockroachdb_errors//errbase",
+        "@com_github_stretchr_testify//require",
     ],
 )

pkg/sql/schemachanger/scerrors/errors.go

@@ -19,6 +19,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/util/timeutil"
 	"github.com/cockroachdb/errors"
 	"github.com/cockroachdb/redact"
+	"github.com/gogo/protobuf/proto"
 )
 
 // EventLogger is a convenience object used for logging schema changer events.
@@ -206,12 +207,26 @@ func (e *schemaChangerUserError) SafeFormatError(p errors.Printer) (next error)
 	return e.err
 }
 
+// Error implements error.
 func (e *schemaChangerUserError) Error() string {
 	// We don't want to print the schemaChangerUserError wrapper in the error,
 	// this only serves as a marker to the declarative schema changer to surface.
 	return fmt.Sprintf("%v", e.err)
 }
 
+// Unwrap implements errors.Wrapper.
 func (e *schemaChangerUserError) Unwrap() error {
 	return e.err
 }
+
+// schemaChangerUserErrorDecodeWrapper is a wrapper decoder for
+// schemaChangerUserError.
+func schemaChangerUserErrorDecodeWrapper(
+	_ context.Context, cause error, _ string, _ []string, payload proto.Message,
+) error {
+	return &schemaChangerUserError{err: cause}
+}
+
+func init() {
+	errors.RegisterWrapperDecoder(errors.GetTypeKey((*schemaChangerUserError)(nil)), schemaChangerUserErrorDecodeWrapper)
+}

pkg/sql/schemachanger/scerrors/errors_test.go

@@ -0,0 +1,26 @@
+// Copyright 2025 The Cockroach Authors.
+//
+// Use of this software is governed by the CockroachDB Software License
+// included in the /LICENSE file.
+
+package scerrors_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/cockroachdb/cockroach/pkg/sql/schemachanger/scerrors"
+	"github.com/cockroachdb/errors"
+	"github.com/cockroachdb/errors/errbase"
+	"github.com/stretchr/testify/require"
+)
+
+// TestEncodeSchemaChangeUserError tests that the error is encoded and decoded
+// correctly if sent over the network.
+func TestEncodeSchemaChangeUserError(t *testing.T) {
+	ctx := context.Background()
+	base := scerrors.SchemaChangerUserError(errors.New("boom"))
+	err := errbase.EncodeError(ctx, base)
+	decodeError := errbase.DecodeError(ctx, err)
+	require.True(t, scerrors.HasSchemaChangerUserError(decodeError))
+}

pkg/testutils/lint/lint_test.go

@@ -1583,6 +1583,7 @@ func TestLint(t *testing.T) {
 			":!sql/plpgsql/plpgsql_error.go",
 			":!sql/protoreflect/redact.go",
 			":!sql/colexecerror/error.go",
+			":!sql/schemachanger/scerrors/errors.go",
 			":!util/timeutil/timeout_error.go",
 			":!util/protoutil/jsonpb_marshal.go",
 			":!util/protoutil/marshal.go",