tools: Import stand-alone sd_notify() implementation from systemd

... in order to avoid linking against the whole of libsystemd.

Only minimal changes to the upstream copy, to function as a drop-in
replacement for sd_notify() and as a header-only library.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Christian Lindig <christian.lindig@cloud.com>

tools/include/xen-sd-notify.h

@@ -0,0 +1,98 @@
+/* SPDX-License-Identifier: MIT-0 */
+
+/*
+ * Implement the systemd notify protocol without external dependencies.
+ * Supports both readiness notification on startup and on reloading,
+ * according to the protocol defined at:
+ * https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html
+ * This protocol is guaranteed to be stable as per:
+ * https://systemd.io/PORTABILITY_AND_STABILITY/
+ *
+ * Differences from the upstream copy:
+ * - Rename/rework as a drop-in replacement for systemd/sd-daemon.h
+ * - Only take the subset Xen cares about
+ * - Respect -Wdeclaration-after-statement
+ */
+
+#ifndef XEN_SD_NOTIFY
+#define XEN_SD_NOTIFY
+
+#include <errno.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <unistd.h>
+
+static inline void xen_sd_closep(int *fd) {
+  if (!fd || *fd < 0)
+    return;
+
+  close(*fd);
+  *fd = -1;
+}
+
+static inline int xen_sd_notify(const char *message) {
+  union sockaddr_union {
+    struct sockaddr sa;
+    struct sockaddr_un sun;
+  } socket_addr = {
+    .sun.sun_family = AF_UNIX,
+  };
+  size_t path_length, message_length;
+  ssize_t written;
+  const char *socket_path;
+  int __attribute__((cleanup(xen_sd_closep))) fd = -1;
+
+  /* Verify the argument first */
+  if (!message)
+    return -EINVAL;
+
+  message_length = strlen(message);
+  if (message_length == 0)
+    return -EINVAL;
+
+  /* If the variable is not set, the protocol is a noop */
+  socket_path = getenv("NOTIFY_SOCKET");
+  if (!socket_path)
+    return 0; /* Not set? Nothing to do */
+
+  /* Only AF_UNIX is supported, with path or abstract sockets */
+  if (socket_path[0] != '/' && socket_path[0] != '@')
+    return -EAFNOSUPPORT;
+
+  path_length = strlen(socket_path);
+  /* Ensure there is room for NUL byte */
+  if (path_length >= sizeof(socket_addr.sun.sun_path))
+    return -E2BIG;
+
+  memcpy(socket_addr.sun.sun_path, socket_path, path_length);
+
+  /* Support for abstract socket */
+  if (socket_addr.sun.sun_path[0] == '@')
+    socket_addr.sun.sun_path[0] = 0;
+
+  fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC, 0);
+  if (fd < 0)
+    return -errno;
+
+  if (connect(fd, &socket_addr.sa, offsetof(struct sockaddr_un, sun_path) + path_length) != 0)
+    return -errno;
+
+  written = write(fd, message, message_length);
+  if (written != (ssize_t) message_length)
+    return written < 0 ? -errno : -EPROTO;
+
+  return 1; /* Notified! */
+}
+
+static inline int sd_notify(int unset_environment, const char *message) {
+    int r = xen_sd_notify(message);
+
+    if (unset_environment)
+        unsetenv("NOTIFY_SOCKET");
+
+    return r;
+}
+
+#endif /* XEN_SD_NOTIFY */