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.go

@@ -14,6 +14,23 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/util"
 )
 
+// EcNoLiveEntriesSubstring is used for server/client coordination when ec.decode determines that
+// decoding should be a no-op (all entries are deleted).
+const EcNoLiveEntriesSubstring = "has no live entries"
+
+// HasLiveNeedles returns whether the EC index (.ecx) contains at least one live (non-deleted) entry.
+// This is used by ec.decode to avoid generating an empty normal volume when all entries were deleted.
+func HasLiveNeedles(indexBaseFileName string) (hasLive bool, err error) {
+	err = iterateEcxFile(indexBaseFileName, func(_ types.NeedleId, _ types.Offset, size types.Size) error {
+		if !size.IsDeleted() {
+			hasLive = true
+			return io.EOF // stop early
+		}
+		return nil
+	})
+	return
+}
+
 // write .idx file from .ecx and .ecj files
 func WriteIdxFileFromEcIndex(baseFileName string) (err error) {
 
@@ -52,6 +69,11 @@ func FindDatFileSize(dataBaseFileName, indexBaseFileName string) (datSize int64,
 		return 0, fmt.Errorf("read ec volume %s version: %v", dataBaseFileName, err)
 	}
 
+	// Safety: ensure datSize is at least SuperBlockSize. While the caller typically
+	// checks HasLiveNeedles first, this protects against direct calls to FindDatFileSize
+	// when all needles are deleted (see issue #7748).
+	datSize = int64(super_block.SuperBlockSize)
+
 	err = iterateEcxFile(indexBaseFileName, func(key types.NeedleId, offset types.Offset, size types.Size) error {
 
 		if size.IsDeleted() {