StreamRequestBody shouldn't read more data than actual need.

http.go

@@ -2283,12 +2283,13 @@ func readBodyWithStreaming(r *bufio.Reader, contentLength, maxBodySize int, dst
 		readN = 8 * 1024
 	}
 
-	if contentLength >= 0 && maxBodySize >= contentLength {
-		b, err = appendBodyFixedSize(r, dst, readN)
-	} else {
-		b, err = readBodyIdentity(r, readN, dst)
-	}
-
+	// A fixed-length pre-read function should be used here; otherwise,
+	// it may read content beyond the request body into areas outside
+	// the br buffer. This could affect the handling of the next request
+	// in the br buffer, if there is one. The original two branches can
+	// be handled with this single branch. by the way,
+	// fix issue: https://github.com/valyala/fasthttp/issues/1816
+	b, err = appendBodyFixedSize(r, dst, readN)
 	if err != nil {
 		return b, err
 	}

server_test.go

@@ -4358,3 +4358,50 @@ func (cl *testLogger) Printf(format string, args ...any) {
 	cl.out += fmt.Sprintf(format, args...)[6:] + "\n"
 	cl.lock.Unlock()
 }
+
+func TestRequestBodyStreamReadIssue1816(t *testing.T) {
+	pcs := fasthttputil.NewPipeConns()
+	cliCon, serverCon := pcs.Conn1(), pcs.Conn2()
+	go func() {
+		req := AcquireRequest()
+		defer ReleaseRequest(req)
+		req.Header.SetContentLength(10)
+		req.Header.SetMethod("POST")
+		req.SetRequestURI("http://localhsot:8080")
+		req.SetBodyRaw(bytes.Repeat([]byte{'1'}, 10))
+		var pipelineReqBody []byte
+		reqBody := req.String()
+		pipelineReqBody = append(pipelineReqBody, reqBody...)
+		pipelineReqBody = append(pipelineReqBody, reqBody...)
+		_, err := cliCon.Write(pipelineReqBody)
+		if err != nil {
+			t.Error(err)
+		}
+		resp := AcquireResponse()
+		err = resp.Read(bufio.NewReader(cliCon))
+		if err != nil {
+			t.Error(err)
+		}
+		err = cliCon.Close()
+		if err != nil {
+			t.Error(err)
+		}
+	}()
+	server := Server{StreamRequestBody: true, MaxRequestBodySize: 5, Handler: func(ctx *RequestCtx) {
+		r := ctx.RequestBodyStream()
+		p := make([]byte, 1300)
+		for {
+			_, err := r.Read(p)
+			if err != nil {
+				if err != io.EOF {
+					t.Fatal(err)
+				}
+				break
+			}
+		}
+	}}
+	err := server.serveConn(serverCon)
+	if err != nil {
+		t.Fatal(err)
+	}
+}