Add serial port example

Add an example connecting to a serial port and writing a message. This
is tested by creating a virtual serial port with the output written to
file.

Bazel test interaction with the filesystem prevents creation of a RW
device. As a result, `//test:virtual_serial_port_test` is defined as a
`sh_binary` that can be run with:

  bazel run //test:virtual_serial_port_test

Change-Id: I1be9cb2e8b13fa972084d66fa2c6ddffb23ae3cc

WORKSPACE.bazel

@@ -110,6 +110,27 @@ register_toolchains(
     "@gcc_12_toolchain//:toolchain",
 )
 
+ASIO_VERSION = "d6b95c0188e0359a8cdbdb6571f0cbacf11a538c"
+
+http_archive(
+    name = "asio",
+    build_file_content = """
+load("@rules_cc//cc:defs.bzl", "cc_library")
+
+cc_library(
+    name = "asio",
+    hdrs = glob(["asio/include/**"]),
+    includes = ["asio/include"],
+    defines = [
+    ],
+    visibility = ["//visibility:public"],
+)
+""",
+    sha256 = "c4eae0910b8fcf2df8d32dc0ab67b032e9876491a5968faad32d330fd41e119c",
+    strip_prefix = "asio-%s" % ASIO_VERSION,
+    url = "https://github.com/chriskohlhoff/asio/archive/%s.tar.gz" % ASIO_VERSION,
+)
+
 FMT_VERSION = "dd5a9691f992f7f257243cbecb516a4ab584dcde"
 
 http_archive(

test/BUILD.bazel

@@ -1,4 +1,34 @@
-load("@rules_cc//cc:defs.bzl", "cc_test")
+load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_test")
+
+# With GCC and tsan:
+# error: 'atomic_thread_fence' is not supported with '-fsanitize=thread' [-Werror=tsan]
+config_setting(
+    name = "gcc_tsan",
+    flag_values = {"@bazel_tools//tools/cpp:compiler": "gcc"},
+    values = {"features": "tsan"},
+)
+
+cc_binary(
+    name = "serial_test",
+    srcs = ["serial_test.cpp"],
+    target_compatible_with = select({
+        ":gcc_tsan": ["@platforms//:incompatible"],
+        "//conditions:default": [],
+    }),
+    deps = [
+        "@asio",
+        "@fmt",
+    ],
+)
+
+# creating a file/device with socat will fail as a `sh_test`
+# https://bazel.build/reference/test-encyclopedia#test-interaction-filesystem
+sh_binary(
+    name = "virtual_serial_port_test",
+    srcs = ["test_virtual_serial_port.sh"],
+    args = ["$(rootpath :serial_test)"],
+    data = [":serial_test"],
+)
 
 cc_test(
     name = "dummy_test",

test/serial_test.cpp

@@ -0,0 +1,35 @@
+#include <asio.hpp>
+#include <cstdlib>
+#include <fmt/core.h>
+#include <stdexcept>
+#include <string_view>
+
+auto main(int argc, const char** argv) -> int
+{
+  if (argc < 2) {
+    throw std::runtime_error{"device path not specified"};
+  }
+
+  auto ctx = asio::io_context{};
+
+  // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+  auto serial = asio::serial_port{ctx, argv[1]};
+  {
+    using opt = asio::serial_port_base;
+
+    // NOLINTBEGIN(readability-magic-numbers)
+    serial.set_option(opt::baud_rate{115200});
+    serial.set_option(opt::character_size{8});
+    serial.set_option(opt::parity{opt::parity::none});
+    serial.set_option(opt::stop_bits{opt::stop_bits::one});
+    // NOLINTEND(readability-magic-numbers)
+  }
+
+  constexpr auto sv = std::string_view{"hello, world!\n"};
+
+  for (auto buf = asio::buffer(sv.data(), sv.size()); buf.size() != 0;) {
+    buf += serial.write_some(asio::buffer(sv.data(), sv.size()));
+  }
+
+  return 0;
+}

test/test_virtual_serial_port.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+# https://bazel.build/reference/test-encyclopedia#test-interaction-filesystem
+TEST_TMPDIR="${TEST_TEMPDIR:-.}"
+
+dev="$TEST_TMPDIR/virt0"
+log="$TEST_TMPDIR/out"
+bin=$1
+
+rm -f "$log"
+
+socat PTY,link="$dev",raw,echo=0 OPEN:"$log",creat=1,excl=1 &
+socat_pid=$!
+
+while [ ! -e "$dev" ]; do
+    sleep 0.1
+done
+
+"$bin" "$dev"
+
+kill "$socat_pid"
+
+grep -q "hello, world!" "$log"