go: cut Rows' remaining per-call allocations (#456)

go/CHANGELOG.md

@@ -11,6 +11,7 @@ The four bindings in this repository are released together and give one answer,
 ### Changed
 
 - `Schema.Rows` decodes its result with far fewer allocations, so it scales across goroutines (#456). It reads the batch document in one streaming pass into typed values, with no generic tree and no per-row path string, and falls back to the strict generic reader for any document it will not judge itself (an unknown key, a duplicate key, a value of the wrong type), so every refusal and its message are unchanged. On a real 100-row document the decode went from 4,476 to 173 allocations at `doc_flags` 0 and from 56,822 to 2,641 at `doc_flags` 7. The decoded values are identical, and no exported name changes.
+- `Schema.Rows` makes far fewer allocations around the decode too (#456). Measured on a full call over a 100-row body with a filter, a four-key settings map and an export (the shape of the report), it went from 412 allocations and 75 KB per call to 76 and 45 KB, about 40% fewer bytes for the collector to clear. The batch document is copied out of the library into a pooled buffer instead of a fresh slice, the row decoders and the settings encoder are kept between calls, the per-row verdict, partition id and span pointers come from shared chunks, a list of spans is decoded one element at a time with no allocation each, and the call options are small named types instead of closures. Every returned value is still Go-owned and independent of those pools, and results are byte-for-byte what they were; no exported name changes.
 
 ### Fixed
 

go/chtypes/capture_batch_test.go

@@ -64,4 +64,23 @@ func TestCaptureBatchDocuments(t *testing.T) {
 		}
 		t.Logf("wrote %s (%d bytes)", name, len(doc))
 	}
+	// The third document is the shape a filtering caller gets: a filter
+	// attached, so the document also carries rows_passed, rows_cut and one
+	// row_spans entry per row, and an export asked for.
+	flt, cerr := sch.lib.tbl.FilterCreate(sch.h, []byte("small < {m:Int8}"), []byte(`{"m":"50"}`), nil)
+	if cerr != nil {
+		t.Fatal(callError(cerr))
+	}
+	defer flt.Close()
+	docBuf, exportBuf, cerr := sch.lib.tbl.PreviewBatch(sch.h, int32(JSONEachRow), body, nil, nil, flt, int32(JSONCompactEachRow), 0)
+	if cerr != nil {
+		t.Fatal(callError(cerr))
+	}
+	doc := sch.lib.tbl.Take(docBuf)
+	sch.lib.tbl.Take(exportBuf)
+	name := filepath.Join(dir, "batch-100-filter.json")
+	if err := os.WriteFile(name, doc, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("wrote %s (%d bytes)", name, len(doc))
 }