MEDIUM: mworker: get rid of libsystemd

Given the xz drama which allowed liblzma to be linked to openssh, lets remove
libsystemd to get rid of useless dependencies.

The sd_notify API seems to be stable and is now documented. This patch replaces
the sd_notify() and sd_notifyf() function by a reimplementation inspired by the
systemd documentation.

This should not change anything functionnally. The function will be built when
haproxy is built using USE_SYSTEMD=1.

References:
  https://github.com/systemd/systemd/issues/32028
  https://www.freedesktop.org/software/systemd/man/devel/sd_notify.html#Notes

Before:

wla@kikyo:~% ldd /usr/sbin/haproxy
	linux-vdso.so.1 (0x00007ffcfaf65000)
	libcrypt.so.1 => /lib/x86_64-linux-gnu/libcrypt.so.1 (0x000074637fef4000)
	libssl.so.3 => /lib/x86_64-linux-gnu/libssl.so.3 (0x000074637fe4f000)
	libcrypto.so.3 => /lib/x86_64-linux-gnu/libcrypto.so.3 (0x000074637f400000)
	liblua5.4.so.0 => /lib/x86_64-linux-gnu/liblua5.4.so.0 (0x000074637fe0d000)
	libsystemd.so.0 => /lib/x86_64-linux-gnu/libsystemd.so.0 (0x000074637f92a000)
	libpcre2-8.so.0 => /lib/x86_64-linux-gnu/libpcre2-8.so.0 (0x000074637f365000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x000074637f000000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x000074637f27a000)
	libcap.so.2 => /lib/x86_64-linux-gnu/libcap.so.2 (0x000074637fdff000)
	libgcrypt.so.20 => /lib/x86_64-linux-gnu/libgcrypt.so.20 (0x000074637eeb8000)
	liblzma.so.5 => /lib/x86_64-linux-gnu/liblzma.so.5 (0x000074637fdcd000)
	libzstd.so.1 => /lib/x86_64-linux-gnu/libzstd.so.1 (0x000074637ee01000)
	liblz4.so.1 => /lib/x86_64-linux-gnu/liblz4.so.1 (0x000074637fda8000)
	/lib64/ld-linux-x86-64.so.2 (0x000074637ff5d000)
	libgpg-error.so.0 => /lib/x86_64-linux-gnu/libgpg-error.so.0 (0x000074637f904000)

After:

wla@kikyo:~% ldd /usr/sbin/haproxy
	linux-vdso.so.1 (0x00007ffd51901000)
	libcrypt.so.1 => /lib/x86_64-linux-gnu/libcrypt.so.1 (0x00007f758d6c0000)
	libssl.so.3 => /lib/x86_64-linux-gnu/libssl.so.3 (0x00007f758d61b000)
	libcrypto.so.3 => /lib/x86_64-linux-gnu/libcrypto.so.3 (0x00007f758ca00000)
	liblua5.4.so.0 => /lib/x86_64-linux-gnu/liblua5.4.so.0 (0x00007f758d5d9000)
	libpcre2-8.so.0 => /lib/x86_64-linux-gnu/libpcre2-8.so.0 (0x00007f758d365000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f758d5ba000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f758c600000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f758c915000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f758d729000)

A backport to all stable versions could be considered at some point.

Makefile

@@ -734,7 +734,7 @@ ifneq ($(USE_WURFL),)
 endif
 
 ifneq ($(USE_SYSTEMD),)
-  SYSTEMD_LDFLAGS = -lsystemd
+  OPTIONS_OBJS    += src/systemd.o
 endif
 
 ifneq ($(USE_PCRE)$(USE_STATIC_PCRE)$(USE_PCRE_JIT),)

include/haproxy/systemd.h

@@ -0,0 +1,7 @@
+#ifndef _HAPROXY_SYSTEMD_H
+#define _HAPROXY_SYSTEMD_H
+
+int sd_notify(int unset_environment, const char *message);
+int sd_notifyf(int unset_environment, const char *format, ...);
+
+#endif

src/mworker.c

@@ -20,10 +20,6 @@
 #include <sys/wait.h>
 #include <unistd.h>
 
-#if defined(USE_SYSTEMD)
-#include <systemd/sd-daemon.h>
-#endif
-
 #include <haproxy/api.h>
 #include <haproxy/cfgparse.h>
 #include <haproxy/cli.h>
@@ -45,6 +41,9 @@
 #include <haproxy/tools.h>
 #include <haproxy/version.h>
 
+#if defined(USE_SYSTEMD)
+#include <haproxy/systemd.h>
+#endif
 
 static int exitcode = -1;
 static int max_reloads = -1; /* number max of reloads a worker can have until they are killed */

src/systemd.c

@@ -0,0 +1,134 @@
+/* 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/
+ *
+ */
+
+#include <errno.h>
+#include <inttypes.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <time.h>
+#include <unistd.h>
+#include <stdarg.h>
+
+#include <haproxy/tools.h>
+
+/*
+ *  standalone reimplementation of sd_notify from the libsystemd
+ *  Return:
+ *     -errno in case of error
+ *     0 when ignored
+ *     >0 when succeeded
+ *
+ * Will send <message> over the NOTIFY_SOCKET.
+ * When unset_environement is set, unsetenv NOTIFY_SOCKET.
+ */
+int sd_notify(int unset_environment, const char *message)
+{
+	union sockaddr_union {
+		struct sockaddr sa;
+		struct sockaddr_un sun;
+	} socket_addr = {
+		.sun.sun_family = AF_UNIX,
+	};
+	int ret = 1;
+	int fd = -1;
+	size_t path_length, message_length;
+	const char *socket_path;
+	ssize_t written;
+
+	socket_path = getenv("NOTIFY_SOCKET");
+	if (!socket_path) {
+		ret = 0; /* Not running under systemd? Nothing to do */
+		goto end;
+	}
+
+	if (unset_environment)
+		unsetenv("NOTIFY_SOCKET");
+
+	if (!message) {
+		ret = -EINVAL;
+		goto end;
+	}
+
+	message_length = strlen(message);
+	if (message_length == 0) {
+		ret = -EINVAL;
+		goto end;
+	}
+
+	/* Only AF_UNIX is supported, with path or abstract sockets */
+	if (socket_path[0] != '/' && socket_path[0] != '@') {
+		ret = -EAFNOSUPPORT;
+		goto end;
+	}
+
+	path_length = strlen(socket_path);
+	/* Ensure there is room for NUL byte */
+	if (path_length >= sizeof(socket_addr.sun.sun_path)) {
+		ret = -E2BIG;
+		goto end;
+	}
+
+	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) {
+		ret = -errno;
+		goto end;
+	}
+
+	if (connect(fd, &socket_addr.sa, offsetof(struct sockaddr_un, sun_path) + path_length) != 0) {
+		ret = -errno;
+		goto end;
+	}
+
+	written = write(fd, message, message_length);
+	if (written != (ssize_t) message_length) {
+		ret = written < 0 ? -errno : -EPROTO;
+		goto end;
+	}
+
+end:
+	if (fd > -1)
+		close(fd);
+	return ret; /* Notified! */
+}
+
+/* va_args variant of sd_notify */
+int sd_notifyf(int unset_environment, const char *format, ...)
+{
+	int r;
+	va_list args;
+	char *strp = NULL;
+
+	va_start(args, format);
+	strp = memvprintf(&strp, format, args);
+	va_end(args);
+
+	if (strp == NULL) {
+		r = -ENOMEM;
+		goto end;
+	}
+
+	r = sd_notify(unset_environment, strp);
+	free(strp);
+end:
+	return r;
+}
+