AP_MSP: add tests for msp_serial_send_frame

Check that a frame is sent whole when there is room for it, on both an
idle and a busy port, and dropped rather than truncated when there is
not. Uses a uart which truncates writes to the space available, as the
ChibiOS and Linux backends do.

libraries/AP_MSP/tests/test_msp_serial.cpp

@@ -0,0 +1,99 @@
+#include <AP_gtest.h>
+
+#include <AP_MSP/AP_MSP_config.h>
+
+#if HAL_MSP_ENABLED
+
+#include <AP_HAL/UARTDriver.h>
+#include <AP_Math/AP_Math.h>
+#include <AP_MSP/msp.h>
+
+const AP_HAL::HAL& hal = AP_HAL::get_HAL();
+
+// a uart which truncates writes to the space available, as the ChibiOS and
+// Linux backends do, and which records what made it onto the wire
+class DummyUart: public AP_HAL::UARTDriver {
+public:
+    bool is_initialized() override { return true; }
+    bool tx_pending() override { return _tx_pending; }
+    uint32_t txspace() override { return _txspace; }
+
+    void setup(uint32_t space, bool pending) {
+        _txspace = space;
+        _tx_pending = pending;
+        written = 0;
+    }
+
+    uint32_t written;
+
+protected:
+    uint32_t _available() override { return 0; }
+    void _begin(uint32_t baud, uint16_t rxSpace, uint16_t txSpace) override {}
+    void _end() override {}
+    void _flush() override {}
+    ssize_t _read(uint8_t *buf, uint16_t count) override { return 0; }
+    bool _discard_input() override { return false; }
+
+    size_t _write(const uint8_t *buffer, size_t size) override {
+        const size_t accepted = MIN(size_t(_txspace), size);
+        _txspace -= accepted;
+        written += accepted;
+        return accepted;
+    }
+
+private:
+    uint32_t _txspace;
+    bool _tx_pending;
+};
+
+static DummyUart test_uart;
+
+// a frame is 6 bytes of header, 4 of payload and 1 of checksum
+static uint32_t send_frame(uint32_t txspace, bool tx_pending)
+{
+    const uint8_t hdr[6] {};
+    const uint8_t data[4] {};
+    const uint8_t crc[1] {};
+
+    test_uart.setup(txspace, tx_pending);
+
+    MSP::msp_port_t port {};
+    port.uart = &test_uart;
+
+    return MSP::msp_serial_send_frame(&port, hdr, sizeof(hdr), data, sizeof(data), crc, sizeof(crc));
+}
+
+TEST(MSPSerial, SendFrameFits)
+{
+    // an idle port with room takes the whole frame
+    EXPECT_EQ(11U, send_frame(11, false));
+    EXPECT_EQ(11U, test_uart.written);
+
+    // so does a busy port which still has room for it
+    EXPECT_EQ(11U, send_frame(64, true));
+    EXPECT_EQ(11U, test_uart.written);
+}
+
+TEST(MSPSerial, SendFrameDoesNotFit)
+{
+    // a busy port without room must drop the frame rather than write part of
+    // it: writes do not block, so a partial write corrupts the stream
+    EXPECT_EQ(0U, send_frame(10, true));
+    EXPECT_EQ(0U, test_uart.written);
+
+    // and a frame larger than the buffer can never be written whole
+    EXPECT_EQ(0U, send_frame(10, false));
+    EXPECT_EQ(0U, test_uart.written);
+
+    // a port with no space at all is the same case
+    EXPECT_EQ(0U, send_frame(0, true));
+    EXPECT_EQ(0U, test_uart.written);
+}
+
+AP_GTEST_MAIN()
+
+#else
+
+AP_GTEST_MAIN()
+
+#endif  // HAL_MSP_ENABLED

libraries/AP_MSP/tests/wscript

@@ -0,0 +1,4 @@
+#!/usr/bin/env python3
+
+def build(bld):
+    bld.ap_find_tests(use='ap')