sql: support customized session options

[nvanbenschoten]

Needed for #69010.

Postgres allows arbitrary session parameters to be set for a session, as long as they have a two-part name (i.e. contain a period). Tools can use this to store and retrieve arbitrary local session state.

We should ensure the arbitrary state has memory accounting.

> SELECT set_config('mytool.state', 'some val', false);
 set_config
------------
 some val
(1 row)

> SELECT current_setting('mytool.state');
 current_setting
-----------------
 some val
(1 row)

PG docs: https://www.postgresql.org/docs/current/runtime-config-custom.html

Prototype:

diff --git a/pkg/sql/exec_util.go b/pkg/sql/exec_util.go
index 42dded41fb..de40f98631 100644
--- a/pkg/sql/exec_util.go
+++ b/pkg/sql/exec_util.go
@@ -2788,6 +2788,13 @@ func (m *sessionDataMutator) SetLocation(loc *time.Location) {
 	m.bufferParamStatusUpdate("TimeZone", sessionDataTimeZoneFormat(loc))
 }
 
+func (m *sessionDataMutator) SetCustomOption(name, val string) {
+	if m.data.CustomOptions == nil {
+		m.data.CustomOptions = make(map[string]string)
+	}
+	m.data.CustomOptions[name] = val
+}
+
 func (m *sessionDataMutator) SetReadOnly(val bool) {
 	// The read-only state is special; it's set as a session variable (SET
 	// transaction_read_only=<>), but it represents per-txn state, not
diff --git a/pkg/sql/sessiondata/session_data.go b/pkg/sql/sessiondata/session_data.go
index a86e467b5f..67964914af 100644
--- a/pkg/sql/sessiondata/session_data.go
+++ b/pkg/sql/sessiondata/session_data.go
@@ -48,6 +48,9 @@ type SessionData struct {
 	// SequenceState gives access to the SQL sequences that have been
 	// manipulated by the session.
 	SequenceState *SequenceState
+	CustomOptions map[string]string
 }
 
 // Clone returns a clone of SessionData.
diff --git a/pkg/sql/vars.go b/pkg/sql/vars.go
index 16de1f2361..b38b0863d0 100644
--- a/pkg/sql/vars.go
+++ b/pkg/sql/vars.go
@@ -1956,6 +1956,14 @@ func (p *planner) GetSessionVar(
 	_ context.Context, varName string, missingOk bool,
 ) (bool, string, error) {
 	name := strings.ToLower(varName)
+	if isCustomOption(name) {
+		v, ok := p.SessionData().CustomOptions[name]
+		if !ok && !missingOk {
+			return false, "", pgerror.Newf(pgcode.UndefinedObject,
+				"unrecognized configuration parameter %q", name)
+		}
+		return true, v, nil
+	}
 	ok, v, err := getSessionVar(name, missingOk)
 	if err != nil || !ok {
 		return ok, "", err
@@ -1966,6 +1974,18 @@ func (p *planner) GetSessionVar(
 // SetSessionVar implements the EvalSessionAccessor interface.
 func (p *planner) SetSessionVar(ctx context.Context, varName, newVal string, isLocal bool) error {
 	name := strings.ToLower(varName)
+	if isCustomOption(name) {
+		// Ignore isLocal.
+		_ = isLocal
+		return p.applyOnSessionDataMutators(
+			ctx,
+			isLocal,
+			func(m sessionDataMutator) error {
+				m.SetCustomOption(name, newVal)
+				return nil
+			},
+		)
+	}
 	_, v, err := getSessionVar(name, false /* missingOk */)
 	if err != nil {
 		return err
@@ -1995,3 +2015,7 @@ func (p *planner) SetSessionVar(ctx context.Context, varName, newVal string, isL
 		},
 	)
 }
+
+func isCustomOption(varName string) bool {
+	return strings.Contains(varName, ".")
+}

Epic CRDB-10300