[TrueCloudLab#22] datagen: Allocate buffer once

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

internal/datagen/generator.go

@@ -43,7 +43,11 @@ func NewGenerator(vu modules.VU, size int) Generator {
 	if size <= 0 {
 		panic("size should be positive")
 	}
-	return Generator{vu: vu, size: size, buf: nil, offset: 0}
+	return Generator{
+		vu:   vu,
+		size: size,
+		buf:  make([]byte, size+TailSize),
+	}
 }
 
 func (g *Generator) GenPayload(calcHash bool) GenPayloadResponse {
@@ -60,9 +64,8 @@ func (g *Generator) GenPayload(calcHash bool) GenPayloadResponse {
 }
 
 func (g *Generator) nextSlice() []byte {
-	if g.buf == nil {
-		// Allocate buffer with extra tail for sliding and populate it with random bytes
-		g.buf = make([]byte, g.size+TailSize)
+	if g.offset >= TailSize {
+		g.offset = 0
 		rand.Read(g.buf) // Per docs, err is always nil here
 	}
 
@@ -71,10 +74,5 @@ func (g *Generator) nextSlice() []byte {
 	// Shift the offset for the next call. If we've used our entire tail, then erase
 	// the buffer so that on the next call it is regenerated anew
 	g.offset += 1
-	if g.offset >= TailSize {
-		g.buf = nil
-		g.offset = 0
-	}
-
 	return result
 }