perf: GetField reduce allocations (#1195)

reuse slice

internal/corazawaf/transaction.go

@@ -587,9 +587,9 @@ func (tx *Transaction) GetField(rv ruleVariableParams) []types.MatchData {
 	}
 
 	// in the most common scenario filteredMatches length will be
-	// the same as matches length, so we avoid allocating per result
-	filteredMatches := make([]types.MatchData, 0, len(matches))
-
+	// the same as matches length, so we avoid allocating per result.
+	// We reuse the matches slice to store filtered results avoiding extra allocation.
+	filteredCount := 0
 	for _, c := range matches {
 		isException := false
 		lkey := strings.ToLower(c.Key())
@@ -600,10 +600,11 @@ func (tx *Transaction) GetField(rv ruleVariableParams) []types.MatchData {
 			}
 		}
 		if !isException {
-			filteredMatches = append(filteredMatches, c)
+			matches[filteredCount] = c
+			filteredCount++
 		}
 	}
-	matches = filteredMatches
+	matches = matches[:filteredCount]
 
 	if rv.Count {
 		count := len(matches)

internal/corazawaf/transaction_test.go

@@ -1291,6 +1291,20 @@ func TestTxGetField(t *testing.T) {
 	}
 }
 
+func BenchmarkTxGetField(b *testing.B) {
+	tx := makeTransaction(b)
+	rvp := ruleVariableParams{
+		Variable: variables.Args,
+	}
+	for i := 0; i < b.N; i++ {
+		tx.GetField(rvp)
+	}
+	if err := tx.Close(); err != nil {
+		b.Fatalf("Failed to close transaction: %s", err.Error())
+	}
+	b.ReportAllocs()
+}
+
 func TestTxProcessURI(t *testing.T) {
 	waf := NewWAF()
 	tx := waf.NewTransaction()