Add memory leak tests

Makefile

@@ -68,6 +68,7 @@ $(GOBIN)/gocredits:
 .PHONY: test
 test: build
 	go test -v -race ./...
+	MEM_THRESHOLD=20 go test -bench=Leak -benchtime 999999x
 
 .PHONY: lint
 lint: $(GOBIN)/staticcheck

memory_test.go

@@ -0,0 +1,53 @@
+package gojq_test
+
+import (
+	"os"
+	"runtime"
+	"strconv"
+	"strings"
+	"testing"
+
+	"github.com/itchyny/gojq"
+)
+
+func BenchmarkMemoryLeak(b *testing.B) {
+	benchCases := []string{
+		`range(.) | select(false)`,
+		`range(.) | if (false) then . else empty end`,
+	}
+	const MB = 1024 * 1024
+	memThreshold := float32(10)
+	if memEnv := os.Getenv("MEM_THRESHOLD"); memEnv != "" {
+		num, err := strconv.Atoi(memEnv)
+		if err != nil {
+			b.Fatal("MEM_THRESHOLD:", err)
+		}
+		memThreshold = float32(num)
+	}
+	memThreshold *= MB
+	for _, bc := range benchCases {
+		query, err := gojq.Parse(bc)
+		if err != nil {
+			b.Fatal(err)
+		}
+		b.Run(strings.ReplaceAll(bc, " ", ""), func(b *testing.B) {
+			var memStats runtime.MemStats
+			runtime.GC()
+			runtime.ReadMemStats(&memStats)
+			memUsage1 := float32(memStats.HeapAlloc)
+			iter := query.Run(b.N)
+			for {
+				_, ok := iter.Next()
+				if !ok {
+					break
+				}
+			}
+			runtime.ReadMemStats(&memStats)
+			memUsage2 := float32(memStats.HeapAlloc)
+			b.Logf("%.1f MB => %.1f MB    (%d iterations)", memUsage1/MB, memUsage2/MB, b.N)
+			if memUsage2-memUsage1 > memThreshold {
+				b.Errorf("MEM_THRESHOLD (%.0f MB) passed, failing...", memThreshold/MB)
+			}
+		})
+	}
+}