fix: keep the CORS lookup fail-closed until bucket metadata is loaded

Restore the startup guard removed by the previous cleanup: while bucket
metadata is still loading, a non-resident name may be a bucket with a
restrictive CORS document, so the request gets no CORS answer instead of the
global policy. After startup a non-resident name still falls back to the
global policy without any metadata I/O; the separate load-failure set stays
removed.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PvgysXDmhPBBimCReYtA8q
Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/api-router.go

@@ -789,9 +789,10 @@ func corsHandler(handler http.Handler) http.Handler {
 			if bucket, _ := request2BucketObjectName(r); bucket != "" && globalBucketMetadataSys != nil {
 				// Resident-only lookup: this runs before authentication with a
 				// client-supplied path segment as the bucket name, so it must
-				// never load or cache metadata. A bucket with a stored CORS
-				// document that failed to parse gets no CORS headers; any other
-				// non-resident name falls back to the global policy below.
+				// never load or cache metadata. While startup loading is still
+				// running, and for a bucket whose stored CORS document failed to
+				// parse, the request gets no CORS headers; any other non-resident
+				// name falls back to the global policy below.
 				cfg, _, err := globalBucketMetadataSys.GetResidentCorsConfig(bucket)
 				if err == nil && cfg != nil {
 					if applyBucketCors(w, r, cfg) {

cmd/bucket-cors-middleware_test.go

@@ -598,15 +598,15 @@ func testBucketCorsUnknownBucketDoesNotGrowMetadata(obj ObjectLayer, _ string, _
 	}
 }
 
-func TestBucketCorsStartupMissUsesGlobalFallbackWithoutIO(t *testing.T) {
+func TestBucketCorsStartupMissFailsClosedWithoutIO(t *testing.T) {
 	ExecObjectLayerAPITest(ExecObjectLayerAPITestArgs{
 		t:          t,
-		objAPITest: testBucketCorsStartupMissUsesGlobalFallbackWithoutIO,
+		objAPITest: testBucketCorsStartupMissFailsClosedWithoutIO,
 		endpoints:  []string{"GetBucketCors"},
 	})
 }
 
-func testBucketCorsStartupMissUsesGlobalFallbackWithoutIO(obj ObjectLayer, _ string, _ string, _ http.Handler, _ auth.Credentials, t *testing.T) {
+func testBucketCorsStartupMissFailsClosedWithoutIO(obj ObjectLayer, _ string, _ string, _ http.Handler, _ auth.Credentials, t *testing.T) {
 	oldObjectAPI := newObjectLayerFn()
 	oldMetadataSys := globalBucketMetadataSys
 	counting := &corsLookupCountingObjectLayer{ObjectLayer: obj}
@@ -630,8 +630,8 @@ func testBucketCorsStartupMissUsesGlobalFallbackWithoutIO(obj ObjectLayer, _ str
 	if !innerCalled || rec.Code != http.StatusNoContent {
 		t.Fatalf("startup miss did not reach inner handler: called=%v status=%d", innerCalled, rec.Code)
 	}
-	if got := rec.Header().Get("Access-Control-Allow-Origin"); got != "https://app.example.com" {
-		t.Fatalf("startup miss did not fall back to the global policy: %q", got)
+	if got := rec.Header().Get("Access-Control-Allow-Origin"); got != "" {
+		t.Fatalf("startup miss used permissive global CORS: %q", got)
 	}
 	if got := counting.getObjectNInfoCalls.Load(); got != 0 {
 		t.Fatalf("startup miss performed %d metadata reads", got)

cmd/bucket-metadata-sys.go

@@ -400,18 +400,23 @@ func (sys *BucketMetadataSys) GetSSEConfig(bucket string) (*bucketsse.BucketSSEC
 // GetResidentCorsConfig returns the CORS configuration of a bucket whose
 // metadata is already resident in memory. It runs before authentication for
 // every Origin-bearing request with a client-supplied path segment, so it
-// never loads or caches metadata. A name that is not resident, including any
-// real bucket while startup loading is still in progress, reports
-// errConfigNotFound and the caller applies the global CORS policy exactly as
-// releases without per-bucket CORS did.
+// never loads or caches metadata. Until startup loading has completed, a
+// non-resident name may still be a bucket with a restrictive document, so it
+// reports errBucketMetadataNotInitialized and gets no CORS answer. After
+// that, a non-resident name reports errConfigNotFound and the caller applies
+// the global CORS policy exactly as releases without per-bucket CORS did.
 func (sys *BucketMetadataSys) GetResidentCorsConfig(bucket string) (*cors.Config, time.Time, error) {
 	if isReservedOrInvalidBucket(bucket, true) {
 		return nil, time.Time{}, errConfigNotFound
 	}
 	sys.RLock()
 	meta, ok := sys.metadataMap[bucket]
+	initialized := sys.initialized
 	sys.RUnlock()
 	if !ok {
+		if !initialized {
+			return nil, time.Time{}, errBucketMetadataNotInitialized
+		}
 		return nil, time.Time{}, errConfigNotFound
 	}
 	if meta.corsConfigErr != nil {