deduplicate code and add comments

pgtype/json.go

@@ -71,37 +71,12 @@ func (c *JSONCodec) PlanEncode(m *Map, oid uint32, format int16, value any) Enco
 	}
 }
 
+// JSON needs its on scan plan for pointers to handle 'null'::json(b).
+// Consider making pointerPointerScanPlan more flexible in the future.
 type jsonPointerScanPlan struct {
 	next ScanPlan
 }
 
-func (p jsonPointerScanPlan) planNext(c *JSONCodec, m *Map, oid uint32, formatCode int16, target any, depth int) ScanPlan {
-	if depth > 8 {
-		return &scanPlanFail{m: m, oid: oid, formatCode: formatCode}
-	}
-
-	switch target.(type) {
-	case *string:
-		p.next = &scanPlanAnyToString{}
-	case *[]byte:
-		p.next = &scanPlanJSONToByteSlice{}
-	case BytesScanner:
-		p.next = &scanPlanBinaryBytesToBytesScanner{}
-	case sql.Scanner:
-		p.next = &scanPlanSQLScanner{formatCode: formatCode}
-	default:
-		rv := reflect.ValueOf(target)
-		if rv.Kind() == reflect.Pointer && rv.Elem().Kind() == reflect.Pointer {
-			var plan jsonPointerScanPlan
-			p.next = plan.planNext(c, m, oid, formatCode, rv.Elem().Interface(), depth+1)
-		} else {
-			p.next = &scanPlanJSONToJSONUnmarshal{unmarshal: c.Unmarshal}
-		}
-	}
-
-	return p
-}
-
 func (p jsonPointerScanPlan) Scan(src []byte, dst any) error {
 	el := reflect.ValueOf(dst).Elem()
 	if src == nil || string(src) == "null" {
@@ -163,7 +138,17 @@ func (e *encodePlanJSONCodecEitherFormatMarshal) Encode(value any, buf []byte) (
 	return buf, nil
 }
 
-func (c *JSONCodec) PlanScan(m *Map, oid uint32, format int16, target any) ScanPlan {
+func (c *JSONCodec) PlanScan(m *Map, oid uint32, formatCode int16, target any) ScanPlan {
+	return c.planScan(m, oid, formatCode, target, 0)
+}
+
+// JSON cannot fallback to pointerPointerScanPlan because of 'null'::json(b),
+// so we need to duplicate the logic here.
+func (c *JSONCodec) planScan(m *Map, oid uint32, formatCode int16, target any, depth int) ScanPlan {
+	if depth > 8 {
+		return &scanPlanFail{m: m, oid: oid, formatCode: formatCode}
+	}
+
 	switch target.(type) {
 	case *string:
 		return &scanPlanAnyToString{}
@@ -172,13 +157,14 @@ func (c *JSONCodec) PlanScan(m *Map, oid uint32, format int16, target any) ScanP
 	case BytesScanner:
 		return &scanPlanBinaryBytesToBytesScanner{}
 	case sql.Scanner:
-		return &scanPlanSQLScanner{formatCode: format}
+		return &scanPlanSQLScanner{formatCode: formatCode}
 	}
 
 	rv := reflect.ValueOf(target)
 	if rv.Kind() == reflect.Pointer && rv.Elem().Kind() == reflect.Pointer {
 		var plan jsonPointerScanPlan
-		return plan.planNext(c, m, oid, format, rv.Elem().Interface(), 0)
+		plan.next = c.planScan(m, oid, formatCode, rv.Elem().Interface(), depth+1)
+		return plan
 	} else {
 		return &scanPlanJSONToJSONUnmarshal{unmarshal: c.Unmarshal}
 	}