fix: check freeEcSlot before evacuating EC shards to prevent data loss

Related to #7619

The moveAwayOneEcVolume function was missing the freeEcSlot check that
exists in other EC shard placement functions. This could cause EC shards
to be moved to volume servers that have no capacity, resulting in:
1. 0-byte shard files when disk is full
2. Data loss when source shards are deleted after 'successful' copy

Changes:
- Add freeEcSlot check before attempting to move EC shards
- Sort destinations by both shard count and free slots
- Refresh topology during evacuation to get updated slot counts
- Log when nodes are skipped due to no free slots
- Update freeEcSlot count after successful moves

weed/shell/command_volume_server_evacuate.go

@@ -4,7 +4,6 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"os"
 
 	"slices"
 
@@ -168,9 +167,23 @@ func (c *commandVolumeServerEvacuate) evacuateEcVolumes(commandEnv *CommandEnv,
 	for _, thisNode := range thisNodes {
 		for _, diskInfo := range thisNode.info.DiskInfos {
 			for _, ecShardInfo := range diskInfo.EcShardInfos {
-				hasMoved, err := c.moveAwayOneEcVolume(commandEnv, ecShardInfo, thisNode, otherNodes, applyChange)
+				// Refresh topology to get updated free slot counts after each move
+				if applyChange {
+					if topologyInfo, _, err := collectTopologyInfo(commandEnv, 0); err != nil {
+						fmt.Fprintf(writer, "update topologyInfo for EC: %v\n", err)
+					} else {
+						ecNodesNew, _ := collectEcVolumeServersByDc(topologyInfo, "")
+						_, otherNodesNew := c.ecNodesOtherThan(ecNodesNew, volumeServer)
+						if len(otherNodesNew) > 0 {
+							otherNodes = otherNodesNew
+							c.topologyInfo = topologyInfo
+						}
+					}
+				}
+
+				hasMoved, err := c.moveAwayOneEcVolume(commandEnv, ecShardInfo, thisNode, otherNodes, applyChange, writer)
 				if err != nil {
-					fmt.Fprintf(writer, "move away volume %d from %s: %v", ecShardInfo.Id, volumeServer, err)
+					fmt.Fprintf(writer, "move away volume %d from %s: %v\n", ecShardInfo.Id, volumeServer, err)
 				}
 				if !hasMoved {
 					if skipNonMoveable {
@@ -185,14 +198,31 @@ func (c *commandVolumeServerEvacuate) evacuateEcVolumes(commandEnv *CommandEnv,
 	return nil
 }
 
-func (c *commandVolumeServerEvacuate) moveAwayOneEcVolume(commandEnv *CommandEnv, ecShardInfo *master_pb.VolumeEcShardInformationMessage, thisNode *EcNode, otherNodes []*EcNode, applyChange bool) (hasMoved bool, err error) {
+func (c *commandVolumeServerEvacuate) moveAwayOneEcVolume(commandEnv *CommandEnv, ecShardInfo *master_pb.VolumeEcShardInformationMessage, thisNode *EcNode, otherNodes []*EcNode, applyChange bool, writer io.Writer) (hasMoved bool, err error) {
 
 	for _, shardId := range erasure_coding.ShardBits(ecShardInfo.EcIndexBits).ShardIds() {
+		// Sort by: 1) fewest shards of this volume, 2) most free EC slots
+		// This ensures we prefer nodes with capacity and balanced shard distribution
 		slices.SortFunc(otherNodes, func(a, b *EcNode) int {
-			return a.localShardIdCount(ecShardInfo.Id) - b.localShardIdCount(ecShardInfo.Id)
+			aShards := a.localShardIdCount(ecShardInfo.Id)
+			bShards := b.localShardIdCount(ecShardInfo.Id)
+			if aShards != bShards {
+				return aShards - bShards // Prefer fewer shards
+			}
+			return b.freeEcSlot - a.freeEcSlot // Then prefer more free slots
 		})
+
+		shardMoved := false
+		skippedNodes := 0
 		for i := 0; i < len(otherNodes); i++ {
 			emptyNode := otherNodes[i]
+
+			// Skip nodes with no free EC slots
+			if emptyNode.freeEcSlot <= 0 {
+				skippedNodes++
+				continue
+			}
+
 			collectionPrefix := ""
 			if ecShardInfo.Collection != "" {
 				collectionPrefix = ecShardInfo.Collection + "_"
@@ -200,19 +230,26 @@ func (c *commandVolumeServerEvacuate) moveAwayOneEcVolume(commandEnv *CommandEnv
 			vid := needle.VolumeId(ecShardInfo.Id)
 			destDiskId := pickBestDiskOnNode(emptyNode, vid)
 			if destDiskId > 0 {
-				fmt.Fprintf(os.Stdout, "moving ec volume %s%d.%d %s => %s (disk %d)\n", collectionPrefix, ecShardInfo.Id, shardId, thisNode.info.Id, emptyNode.info.Id, destDiskId)
+				fmt.Fprintf(writer, "moving ec volume %s%d.%d %s => %s (disk %d)\n", collectionPrefix, ecShardInfo.Id, shardId, thisNode.info.Id, emptyNode.info.Id, destDiskId)
 			} else {
-				fmt.Fprintf(os.Stdout, "moving ec volume %s%d.%d %s => %s\n", collectionPrefix, ecShardInfo.Id, shardId, thisNode.info.Id, emptyNode.info.Id)
+				fmt.Fprintf(writer, "moving ec volume %s%d.%d %s => %s\n", collectionPrefix, ecShardInfo.Id, shardId, thisNode.info.Id, emptyNode.info.Id)
 			}
 			err = moveMountedShardToEcNode(commandEnv, thisNode, ecShardInfo.Collection, vid, shardId, emptyNode, destDiskId, applyChange)
 			if err != nil {
 				return
 			} else {
 				hasMoved = true
+				shardMoved = true
+				// Update the node's free slot count after successful move
+				emptyNode.freeEcSlot--
 				break
 			}
 		}
-		if !hasMoved {
+		if !shardMoved {
+			if skippedNodes > 0 {
+				fmt.Fprintf(writer, "no available destination for ec shard %d.%d: %d nodes have no free slots\n",
+					ecShardInfo.Id, shardId, skippedNodes)
+			}
 			return
 		}
 	}