iceberg maintenance: keep ordered runs when the full repack yields nothing

The bestEffort leftover fallback removed the ordered runs before checking
whether repacking the whole bin produced any bins, discarding valid
compaction work.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/worker/tasks/iceberg/compact.go

@@ -598,10 +598,13 @@ func buildCompactionBins(entries []iceberg.ManifestEntry, targetSize int64, minF
 				packed := splitOversizedBin(runtBin, targetSize, minFiles)
 				if len(packed) == 0 {
 					// Stragglers cannot merge even with each other; repacking
-					// the whole bin lets them pair across the ordered runs
-					// instead of stranding them for every later pass.
-					result = result[:len(result)-len(runs)]
-					packed = splitOversizedBin(*bin, targetSize, minFiles)
+					// the whole bin lets them pair across the ordered runs.
+					// Only replace the runs when the repack yields bins —
+					// otherwise the runs stay valid compaction work.
+					if full := splitOversizedBin(*bin, targetSize, minFiles); len(full) > 0 {
+						result = result[:len(result)-len(runs)]
+						packed = full
+					}
 				}
 				result = append(result, packed...)
 			}

weed/worker/tasks/iceberg/compact_order_test.go

@@ -363,3 +363,26 @@ func TestResolveCompactionOrderSkipsUncoveredSortField(t *testing.T) {
 		t.Fatalf("uncovered sort field: got %+v, want nil", order)
 	}
 }
+// A full-bin repack that produces nothing must not discard the ordered runs
+// it was meant to replace — they are still valid compaction work.
+func TestCompactDataFilesKeepsRunsWhenRepackFails(t *testing.T) {
+	fs, client := startFakeFiler(t)
+	setup := tableSetup{BucketName: "tb", Namespace: "ns", TableName: "tbl"}
+	populateBoundedTable(t, fs, setup, []boundedFile{
+		{Name: "r1.parquet", IDs: []int64{1}, Lower: 1, Upper: 2, SizeBytes: 4 << 20},
+		{Name: "r2.parquet", IDs: []int64{3}, Lower: 3, Upper: 4, SizeBytes: 3 << 20},
+		{Name: "r3.parquet", IDs: []int64{5}, Lower: 5, Upper: 6, SizeBytes: 3 << 20},
+		{Name: "r4.parquet", IDs: []int64{7}, Lower: 7, Upper: 8, SizeBytes: 6 << 20},
+		{Name: "r5.parquet", IDs: []int64{9}, Lower: 9, Upper: 10, SizeBytes: 6 << 20},
+	})
+
+	handler := NewHandler(nil)
+	config := Config{TargetFileSizeBytes: 10 << 20, MinInputFiles: 3, MaxCommitRetries: 3}
+	result, _, err := handler.compactDataFiles(context.Background(), client, setup.BucketName, setup.tablePath(), config, nil)
+	if err != nil {
+		t.Fatalf("compactDataFiles: %v", err)
+	}
+	if !strings.Contains(result, "compacted 3 files into 1") {
+		t.Fatalf("expected the ordered run to survive the failed repack, got %q", result)
+	}
+}