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/storage/erasure_coding/ec_decoder_test.go

@@ -0,0 +1,81 @@
+package erasure_coding_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	erasure_coding "github.com/seaweedfs/seaweedfs/weed/storage/erasure_coding"
+	"github.com/seaweedfs/seaweedfs/weed/storage/types"
+)
+
+func TestHasLiveNeedles_AllDeletedIsFalse(t *testing.T) {
+	dir := t.TempDir()
+
+	collection := "foo"
+	base := filepath.Join(dir, collection+"_1")
+
+	// Build an ecx file with only deleted entries.
+	// ecx file entries are the same format as .idx entries.
+	ecx := makeNeedleMapEntry(types.NeedleId(1), types.Offset{}, types.TombstoneFileSize)
+	if err := os.WriteFile(base+".ecx", ecx, 0644); err != nil {
+		t.Fatalf("write ecx: %v", err)
+	}
+
+	hasLive, err := erasure_coding.HasLiveNeedles(base)
+	if err != nil {
+		t.Fatalf("HasLiveNeedles: %v", err)
+	}
+	if hasLive {
+		t.Fatalf("expected no live entries")
+	}
+}
+
+func TestHasLiveNeedles_WithLiveEntryIsTrue(t *testing.T) {
+	dir := t.TempDir()
+
+	collection := "foo"
+	base := filepath.Join(dir, collection+"_1")
+
+	// Build an ecx file containing at least one live entry.
+	// ecx file entries are the same format as .idx entries.
+	live := makeNeedleMapEntry(types.NeedleId(1), types.Offset{}, types.Size(1))
+	if err := os.WriteFile(base+".ecx", live, 0644); err != nil {
+		t.Fatalf("write ecx: %v", err)
+	}
+
+	hasLive, err := erasure_coding.HasLiveNeedles(base)
+	if err != nil {
+		t.Fatalf("HasLiveNeedles: %v", err)
+	}
+	if !hasLive {
+		t.Fatalf("expected live entries")
+	}
+}
+
+func TestHasLiveNeedles_EmptyFileIsFalse(t *testing.T) {
+	dir := t.TempDir()
+
+	base := filepath.Join(dir, "foo_1")
+
+	// Create an empty ecx file.
+	if err := os.WriteFile(base+".ecx", []byte{}, 0644); err != nil {
+		t.Fatalf("write ecx: %v", err)
+	}
+
+	hasLive, err := erasure_coding.HasLiveNeedles(base)
+	if err != nil {
+		t.Fatalf("HasLiveNeedles: %v", err)
+	}
+	if hasLive {
+		t.Fatalf("expected no live entries for empty file")
+	}
+}
+
+func makeNeedleMapEntry(key types.NeedleId, offset types.Offset, size types.Size) []byte {
+	b := make([]byte, types.NeedleIdSize+types.OffsetSize+types.SizeSize)
+	types.NeedleIdToBytes(b[0:types.NeedleIdSize], key)
+	types.OffsetToBytes(b[types.NeedleIdSize:types.NeedleIdSize+types.OffsetSize], offset)
+	types.SizeToBytes(b[types.NeedleIdSize+types.OffsetSize:types.NeedleIdSize+types.OffsetSize+types.SizeSize], size)
+	return b
+}