sql: fix current_setting(..., true) for custom options

Release note (bug fix): The `current_setting` builtin function now
properly does not result in an error when checking a custom session
setting that does not exist and the `missing_ok` argument is true.

pkg/sql/logictest/testdata/logic_test/pg_builtins

@@ -268,6 +268,34 @@ SELECT current_setting('statement_timeout'), current_setting('search_path')
 query error unrecognized configuration parameter
 SELECT pg_catalog.current_setting('woo', false)
 
+# Check that current_setting handles custom settings correctly.
+query T
+SELECT current_setting('my.custom', true)
+----
+NULL
+
+statement ok
+PREPARE check_custom AS SELECT current_setting('my.custom', true)
+
+query T
+EXECUTE check_custom
+----
+NULL
+
+statement ok
+BEGIN;
+SET LOCAL my.custom = 'foo'
+
+# Check that the existence of my.custom is checked depending on the execution
+# context, and not at PREPARE time.
+query T
+EXECUTE check_custom
+----
+foo
+
+statement ok
+COMMIT
+
 # check error on unsupported session var.
 query error configuration setting.*not supported
 SELECT current_setting('vacuum_cost_delay', false)

pkg/sql/vars.go

@@ -79,6 +79,11 @@ type sessionVar struct {
 	// either by SHOW or in the pg_catalog table.
 	Get func(evalCtx *extendedEvalContext, kv *kv.Txn) (string, error)
 
+	// Exists returns true if this custom session option exists in the current
+	// context. It's needed to support the current_setting builtin for custom
+	// options. It is only defined for custom options.
+	Exists func(evalCtx *extendedEvalContext, kv *kv.Txn) bool
+
 	// GetFromSessionData returns a string representation of a given variable to
 	// be used by BufferParamStatus. This is only required if the variable
 	// is expected to send updates through ParamStatusUpdate in pgwire.
@@ -2607,6 +2612,10 @@ func getCustomOptionSessionVar(varName string) (sv sessionVar, isCustom bool) {
 				}
 				return v, nil
 			},
+			Exists: func(evalCtx *extendedEvalContext, _ *kv.Txn) bool {
+				_, ok := evalCtx.SessionData().CustomOptions[varName]
+				return ok
+			},
 			Set: func(ctx context.Context, m sessionDataMutator, val string) error {
 				// TODO(#72026): do some memory accounting.
 				m.SetCustomOption(varName, val)
@@ -2629,6 +2638,11 @@ func (p *planner) GetSessionVar(
 	if err != nil || !ok {
 		return ok, "", err
 	}
+	if existsFn := v.Exists; existsFn != nil {
+		if missingOk && !existsFn(&p.extendedEvalCtx, p.Txn()) {
+			return false, "", nil
+		}
+	}
 	val, err := v.Get(&p.extendedEvalCtx, p.Txn())
 	return true, val, err
 }