fix(ec.decode): purge EC shards when volume is empty (#7749)

* fix(ec.decode): purge EC shards when volume is empty

When an EC volume has no live entries (all deleted), ec.decode should not generate an empty normal volume. Instead, treat decode as a no-op and allow shard purge to proceed cleanly.\n\nFixes: #7748

* chore: address PR review comments

* test: cover live EC index + avoid magic string

* chore: harden empty-EC handling

- Make shard cleanup best-effort (collect errors)\n- Remove unreachable EOF handling in HasLiveNeedles\n- Add empty ecx test case\n- Share no-live-entries substring between server/client\n

* perf: parallelize EC shard unmount/delete across locations

* refactor: combine unmount+delete into single goroutine per location

* refactor: use errors.Join for multi-error aggregation

* refactor: use existing ErrorWaitGroup for parallel execution

* fix: capture loop variables + clarify SuperBlockSize safety

weed/shell/command_ec_decode.go

@@ -5,11 +5,14 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"strings"
 
 	"github.com/seaweedfs/seaweedfs/weed/pb"
 	"github.com/seaweedfs/seaweedfs/weed/storage/types"
 
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
 
 	"github.com/seaweedfs/seaweedfs/weed/operation"
 	"github.com/seaweedfs/seaweedfs/weed/pb/master_pb"
@@ -118,6 +121,11 @@ func doEcDecode(commandEnv *CommandEnv, topoInfo *master_pb.TopologyInfo, collec
 	// generate a normal volume
 	err = generateNormalVolume(commandEnv.option.GrpcDialOption, vid, collection, targetNodeLocation)
 	if err != nil {
+		// Special case: if the EC index has no live entries, decoding is a no-op.
+		// Just purge EC shards and return success without generating/mounting an empty volume.
+		if isEcDecodeEmptyVolumeErr(err) {
+			return unmountAndDeleteEcShards(commandEnv.option.GrpcDialOption, collection, nodeToEcIndexBits, vid)
+		}
 		return fmt.Errorf("generate normal volume %d on %s: %v", vid, targetNodeLocation, err)
 	}
 
@@ -130,6 +138,44 @@ func doEcDecode(commandEnv *CommandEnv, topoInfo *master_pb.TopologyInfo, collec
 	return nil
 }
 
+func isEcDecodeEmptyVolumeErr(err error) bool {
+	st, ok := status.FromError(err)
+	if !ok {
+		return false
+	}
+	if st.Code() != codes.FailedPrecondition {
+		return false
+	}
+	// Keep this robust against wording tweaks while still being specific.
+	return strings.Contains(st.Message(), erasure_coding.EcNoLiveEntriesSubstring)
+}
+
+func unmountAndDeleteEcShards(grpcDialOption grpc.DialOption, collection string, nodeToEcIndexBits map[pb.ServerAddress]erasure_coding.ShardBits, vid needle.VolumeId) error {
+	return unmountAndDeleteEcShardsWithPrefix("unmountAndDeleteEcShards", grpcDialOption, collection, nodeToEcIndexBits, vid)
+}
+
+func unmountAndDeleteEcShardsWithPrefix(prefix string, grpcDialOption grpc.DialOption, collection string, nodeToEcIndexBits map[pb.ServerAddress]erasure_coding.ShardBits, vid needle.VolumeId) error {
+	ewg := NewErrorWaitGroup(len(nodeToEcIndexBits))
+
+	// unmount and delete ec shards in parallel (one goroutine per location)
+	for location, ecIndexBits := range nodeToEcIndexBits {
+		location, ecIndexBits := location, ecIndexBits // capture loop variables for goroutine
+		ewg.Add(func() error {
+			fmt.Printf("unmount ec volume %d on %s has shards: %+v\n", vid, location, ecIndexBits.ShardIds())
+			if err := unmountEcShards(grpcDialOption, vid, location, ecIndexBits.ToUint32Slice()); err != nil {
+				return fmt.Errorf("%s unmount ec volume %d on %s: %w", prefix, vid, location, err)
+			}
+
+			fmt.Printf("delete ec volume %d on %s has shards: %+v\n", vid, location, ecIndexBits.ShardIds())
+			if err := sourceServerDeleteEcShards(grpcDialOption, collection, vid, location, ecIndexBits.ToUint32Slice()); err != nil {
+				return fmt.Errorf("%s delete ec volume %d on %s: %w", prefix, vid, location, err)
+			}
+			return nil
+		})
+	}
+	return ewg.Wait()
+}
+
 func mountVolumeAndDeleteEcShards(grpcDialOption grpc.DialOption, collection string, targetNodeLocation pb.ServerAddress, nodeToEcIndexBits map[pb.ServerAddress]erasure_coding.ShardBits, vid needle.VolumeId) error {
 
 	// mount volume
@@ -142,24 +188,7 @@ func mountVolumeAndDeleteEcShards(grpcDialOption grpc.DialOption, collection str
 		return fmt.Errorf("mountVolumeAndDeleteEcShards mount volume %d on %s: %v", vid, targetNodeLocation, err)
 	}
 
-	// unmount ec shards
-	for location, ecIndexBits := range nodeToEcIndexBits {
-		fmt.Printf("unmount ec volume %d on %s has shards: %+v\n", vid, location, ecIndexBits.ShardIds())
-		err := unmountEcShards(grpcDialOption, vid, location, ecIndexBits.ToUint32Slice())
-		if err != nil {
-			return fmt.Errorf("mountVolumeAndDeleteEcShards unmount ec volume %d on %s: %v", vid, location, err)
-		}
-	}
-	// delete ec shards
-	for location, ecIndexBits := range nodeToEcIndexBits {
-		fmt.Printf("delete ec volume %d on %s has shards: %+v\n", vid, location, ecIndexBits.ShardIds())
-		err := sourceServerDeleteEcShards(grpcDialOption, collection, vid, location, ecIndexBits.ToUint32Slice())
-		if err != nil {
-			return fmt.Errorf("mountVolumeAndDeleteEcShards delete ec volume %d on %s: %v", vid, location, err)
-		}
-	}
-
-	return nil
+	return unmountAndDeleteEcShardsWithPrefix("mountVolumeAndDeleteEcShards", grpcDialOption, collection, nodeToEcIndexBits, vid)
 }
 
 func generateNormalVolume(grpcDialOption grpc.DialOption, vid needle.VolumeId, collection string, sourceVolumeServer pb.ServerAddress) error {