s3: keep an unset CORS MaxAgeSeconds unset (#12272)

A rule without MaxAgeSeconds was stored as 0, so GetBucketCors returned
MaxAgeSeconds 0 and preflights sent Access-Control-Max-Age: 0. Store -1
for unset, which corsRuleFromProto already reads back as nil.

weed/s3api/s3api_bucket_config.go

@@ -860,10 +860,10 @@ func corsConfigFromProto(protoConfig *s3_pb.CORSConfiguration) *cors.CORSConfigu
 	}
 }
 
-// getMaxAgeSecondsValue safely extracts max age seconds value
+// getMaxAgeSecondsValue safely extracts max age seconds value; -1 stores an unset MaxAgeSeconds
 func getMaxAgeSecondsValue(maxAge *int) int {
 	if maxAge == nil {
-		return 0
+		return -1
 	}
 	return *maxAge
 }

weed/s3api/s3api_bucket_cors_proto_test.go

@@ -0,0 +1,22 @@
+package s3api
+
+import (
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/s3api/cors"
+)
+
+func TestCorsRuleProtoRoundTripMaxAge(t *testing.T) {
+	unset := corsRuleFromProto(corsRuleToProto(cors.CORSRule{AllowedMethods: []string{"GET"}, AllowedOrigins: []string{"*"}}))
+	if unset.MaxAgeSeconds != nil {
+		t.Errorf("unset MaxAgeSeconds came back as %d", *unset.MaxAgeSeconds)
+	}
+
+	for _, age := range []int{0, 3000} {
+		age := age
+		got := corsRuleFromProto(corsRuleToProto(cors.CORSRule{AllowedMethods: []string{"GET"}, AllowedOrigins: []string{"*"}, MaxAgeSeconds: &age}))
+		if got.MaxAgeSeconds == nil || *got.MaxAgeSeconds != age {
+			t.Errorf("MaxAgeSeconds %d came back as %v", age, got.MaxAgeSeconds)
+		}
+	}
+}