Fix volume repeatedly toggling between crowded and uncrowded (#7793)

* Fix volume repeatedly toggling between crowded and uncrowded

Fixes #6712

The issue was that removeFromCrowded() was called in removeFromWritable(),
which is invoked whenever a volume temporarily becomes unwritable (due to
replica count fluctuations, heartbeat issues, or read-only state changes).

This caused unnecessary toggling:
1. Volume becomes temporarily unwritable → removeFromWritable() →
   removeFromCrowded() logs 'becomes uncrowded'
2. Volume becomes writable again
3. CollectDeadNodeAndFullVolumes() runs → setVolumeCrowded() logs
   'becomes crowded'

The fix:
- Remove removeFromCrowded() call from removeFromWritable()
- Only clear crowded status when volume is fully unregistered from
  the layout (when location.Length() == 0 in UnRegisterVolume)

This ensures transient state changes don't cause log spam and the
crowded status accurately reflects the volume's size relative to
the grow threshold.

* Refactor test to use subtests for better readability

Address review feedback: use t.Run subtests to make the test's intent
clearer by giving each verification step a descriptive name.

weed/topology/volume_layout.go

@@ -206,6 +206,7 @@ func (vl *VolumeLayout) UnRegisterVolume(v *storage.VolumeInfo, dn *DataNode) {
 
 		if location.Length() == 0 {
 			delete(vl.vid2location, v.Id)
+			vl.removeFromCrowded(v.Id)
 		}
 
 	}
@@ -400,7 +401,6 @@ func (vl *VolumeLayout) removeFromWritable(vid needle.VolumeId) bool {
 			break
 		}
 	}
-	vl.removeFromCrowded(vid)
 	if toDeleteIndex >= 0 {
 		glog.V(0).Infoln("Volume", vid, "becomes unwritable")
 		vl.writables = append(vl.writables[0:toDeleteIndex], vl.writables[toDeleteIndex+1:]...)

weed/topology/volume_layout_test.go

@@ -3,8 +3,10 @@ package topology
 import (
 	"testing"
 
+	"github.com/seaweedfs/seaweedfs/weed/storage"
 	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
 	"github.com/seaweedfs/seaweedfs/weed/storage/super_block"
+	"github.com/seaweedfs/seaweedfs/weed/storage/types"
 )
 
 func TestVolumesBinaryState(t *testing.T) {
@@ -114,3 +116,75 @@ func TestVolumesBinaryState(t *testing.T) {
 		})
 	}
 }
+
+func TestVolumeLayoutCrowdedState(t *testing.T) {
+	rp, _ := super_block.NewReplicaPlacementFromString("000")
+	ttl, _ := needle.ReadTTL("")
+	diskType := types.HardDriveType
+
+	vl := NewVolumeLayout(rp, ttl, diskType, 1024*1024*1024, false)
+
+	vid := needle.VolumeId(1)
+	dn := &DataNode{
+		NodeImpl: NodeImpl{
+			id: "test-node",
+		},
+		Ip:   "127.0.0.1",
+		Port: 8080,
+	}
+
+	// Create a volume info
+	volumeInfo := &storage.VolumeInfo{
+		Id:               vid,
+		ReplicaPlacement: rp,
+		Ttl:              ttl,
+		DiskType:         string(diskType),
+	}
+
+	// Register the volume
+	vl.RegisterVolume(volumeInfo, dn)
+
+	// Add the volume to writables
+	vl.accessLock.Lock()
+	vl.setVolumeWritable(vid)
+	vl.accessLock.Unlock()
+
+	// Mark the volume as crowded
+	vl.SetVolumeCrowded(vid)
+
+	t.Run("should be crowded after being marked", func(t *testing.T) {
+		vl.accessLock.RLock()
+		_, isCrowded := vl.crowded[vid]
+		vl.accessLock.RUnlock()
+		if !isCrowded {
+			t.Fatal("Volume should be marked as crowded after SetVolumeCrowded")
+		}
+	})
+
+	// Remove from writable (simulating temporary unwritable state)
+	vl.accessLock.Lock()
+	vl.removeFromWritable(vid)
+	vl.accessLock.Unlock()
+
+	t.Run("should remain crowded after becoming unwritable", func(t *testing.T) {
+		// This is the fix for issue #6712 - crowded state should persist
+		vl.accessLock.RLock()
+		_, stillCrowded := vl.crowded[vid]
+		vl.accessLock.RUnlock()
+		if !stillCrowded {
+			t.Fatal("Volume should remain crowded after becoming unwritable (fix for issue #6712)")
+		}
+	})
+
+	// Now unregister the volume completely
+	vl.UnRegisterVolume(volumeInfo, dn)
+
+	t.Run("should not be crowded after unregistering", func(t *testing.T) {
+		vl.accessLock.RLock()
+		_, stillCrowdedAfterUnregister := vl.crowded[vid]
+		vl.accessLock.RUnlock()
+		if stillCrowdedAfterUnregister {
+			t.Fatal("Volume should be removed from crowded map after full unregistration")
+		}
+	})
+}