Read served files with pread

Over TLS a file is copied out of its mapping in this process, so one
truncated while it was being served raised SIGBUS and took the whole
server down. Read it with pread, which comes up short instead.

test/test.cc

@@ -12797,6 +12797,45 @@ TEST(ResponseContentSourceTest, LastContentSetterWins) {
   }
 }
 
+#if defined(CPPHTTPLIB_SSL_ENABLED) && !defined(_WIN32)
+TEST(FileContentTest, TruncatedWhileServedEndsTheResponse) {
+  const char *path = "./truncated_while_served.txt";
+  const size_t size = 32 * 1024 * 1024;
+  {
+    std::ofstream f(path, std::ios::binary);
+    f << std::string(size, 'x');
+  }
+  auto se_file = detail::scope_exit([&] { std::remove(path); });
+
+  // Over TLS, where the bytes are encrypted in this process rather than
+  // copied by send().
+  SSLServer svr(SERVER_CERT_FILE, SERVER_PRIVATE_KEY_FILE);
+  svr.Get("/file", [&](const Request &, Response &res) {
+    res.set_file_content(path, "text/plain");
+  });
+  auto thread = std::thread([&]() { svr.listen(HOST, PORT); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    thread.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+  svr.wait_until_ready();
+
+  // The file shrinks once the first bytes are in, as a rewrite in place
+  // would, and reading the mapping past its new end used to raise SIGBUS.
+  SSLClient cli(HOST, PORT);
+  cli.enable_server_certificate_verification(false);
+  size_t received = 0;
+  auto res = cli.Get("/file", [&](const char *, size_t len) {
+    if (received == 0) { EXPECT_EQ(0, ::truncate(path, 0)); }
+    received += len;
+    return true;
+  });
+  EXPECT_FALSE(res);
+  EXPECT_LT(received, size);
+}
+#endif
+
 // Forward declaration (see the base64_encode note).
 namespace httplib {
 namespace detail {