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/server/volume_grpc_erasure_coding.go

@@ -20,6 +20,9 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/storage/types"
 	"github.com/seaweedfs/seaweedfs/weed/storage/volume_info"
 	"github.com/seaweedfs/seaweedfs/weed/util"
+
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
 )
 
 /*
@@ -506,6 +509,17 @@ func (vs *VolumeServer) VolumeEcShardsToVolume(ctx context.Context, req *volume_
 	}
 
 	dataBaseFileName, indexBaseFileName := v.DataBaseFileName(), v.IndexBaseFileName()
+
+	// If the EC index contains no live entries, decoding should be a no-op:
+	// just allow the caller to purge EC shards and do not generate an empty normal volume.
+	hasLive, err := erasure_coding.HasLiveNeedles(indexBaseFileName)
+	if err != nil {
+		return nil, fmt.Errorf("HasLiveNeedles %s: %w", indexBaseFileName, err)
+	}
+	if !hasLive {
+		return nil, status.Errorf(codes.FailedPrecondition, "ec volume %d %s", req.VolumeId, erasure_coding.EcNoLiveEntriesSubstring)
+	}
+
 	// calculate .dat file size
 	datFileSize, err := erasure_coding.FindDatFileSize(dataBaseFileName, indexBaseFileName)
 	if err != nil {