Auto-detect and link libsystemd at compile-time

This adds Makefile/build-system support for USE_SYSTEMD=(yes|no|*). This
variable's value determines whether or not libsystemd will be linked at
build-time.

If USE_SYSTEMD is set to "yes", make will use PKG_CONFIG to check for
libsystemd's presence, and fail the build early if it isn't
installed/detected properly.

If USE_SYSTEM is set to "no", libsystemd will *not* be linked, even if
support for it is available on the system redis is being built on.

For any other value that USE_SYSTEM might assume (e.g. "auto"),
PKG_CONFIG will try to determine libsystemd's presence, and set up the
build process to link against it, if it was indicated as being
installed/available.

This approach has a number of repercussions of its own, most importantly
the following: If you build redis on a system that actually has systemd
support, but no libsystemd-dev package(s) installed, you'll end up
*without* support for systemd notification/status reporting support in
redis-server. This changes established runtime behaviour.

I'm not sure if the build system and/or the server binary should
indicate this. I'm also wondering if not actually having
systemd-notify-support, but requesting it via the server's config,
should result in a fatal error now.

src/Makefile

@@ -32,6 +32,7 @@ OPT=$(OPTIMIZATION)
 PREFIX?=/usr/local
 INSTALL_BIN=$(PREFIX)/bin
 INSTALL=install
+PKG_CONFIG?=pkg-config
 
 # Default allocator defaults to Jemalloc if it's not an ARM
 MALLOC=libc
@@ -131,6 +132,30 @@ endif
 # Include paths to dependencies
 FINAL_CFLAGS+= -I../deps/hiredis -I../deps/linenoise -I../deps/lua/src
 
+# Determine systemd support and/or build preference (defaulting to auto-detection)
+BUILD_WITH_SYSTEMD=no
+# If 'USE_SYSTEMD' in the environment is neither "no" nor "yes", try to
+# auto-detect libsystemd's presence and link accordingly.
+ifneq ($(USE_SYSTEMD),no)
+	LIBSYSTEMD_PKGCONFIG := $(shell $(PKG_CONFIG) --exists libsystemd && echo $$?)
+# If libsystemd cannot be detected, continue building without support for it
+# (unless a later check tells us otherwise)
+ifeq ($(LIBSYSTEMD_PKGCONFIG),0)
+	BUILD_WITH_SYSTEMD=yes
+endif
+endif
+ifeq ($(USE_SYSTEMD),yes)
+ifneq ($(LIBSYSTEMD_PKGCONFIG),0)
+$(error USE_SYSTEMD is set to "$(USE_SYSTEMD)", but $(PKG_CONFIG) cannot find libsystemd)
+endif
+# Force building with libsystemd
+	BUILD_WITH_SYSTEMD=yes
+endif
+ifeq ($(BUILD_WITH_SYSTEMD),yes)
+	FINAL_LIBS+=$(shell $(PKG_CONFIG) --libs libsystemd)
+	FINAL_CFLAGS+= -DHAVE_LIBSYSTEMD
+endif
+
 ifeq ($(MALLOC),tcmalloc)
 	FINAL_CFLAGS+= -DUSE_TCMALLOC
 	FINAL_LIBS+= -ltcmalloc

src/server.c

@@ -55,7 +55,6 @@
 #include <sys/utsname.h>
 #include <locale.h>
 #include <sys/socket.h>
-#include <dlfcn.h>
 
 /* Our shared "common" objects */
 
@@ -4876,37 +4875,16 @@ int redisSupervisedUpstart(void) {
 
 int redisCommunicateSystemd(const char *sd_notify_msg) {
     const char *notify_socket = getenv("NOTIFY_SOCKET");
-    int (*dl_sd_notify)(int unset_environment, const char *state);
-    char *error;
-    void *handle;
-
     if (!notify_socket) {
         serverLog(LL_WARNING,
                 "systemd supervision requested, but NOTIFY_SOCKET not found");
-        return 0;
     }
 
-    handle = dlopen("libsystemd.so.0", RTLD_LAZY);
-    if (!handle) {
-        serverLog(LL_WARNING,
-                "systemd supervision requested, but could not dlopen() libsystemd.so");
-        (void) dlerror();
-        return 0;
-    }
-    (void) dlerror();
-
-    *(void **)(&dl_sd_notify) = dlsym(handle, "sd_notify");
-    error = dlerror();
-
-    if (error != NULL) {
-        serverLog(LL_WARNING,
-                "systemd supervision requested, but could not load sd_notify(3) from libsystemd.so");
-        dlclose(handle);
-        return 0;
-    }
-
-    (void) (*dl_sd_notify)(0, sd_notify_msg);
-    dlclose(handle);
+    #ifdef HAVE_LIBSYSTEMD
+    (void) sd_notify(0, sd_notify_msg);
+    #else
+    UNUSED(sd_notify_msg);
+    #endif
     return 0;
 }
 

src/server.h

@@ -49,6 +49,10 @@
 #include <lua.h>
 #include <signal.h>
 
+#ifdef HAVE_LIBSYSTEMD
+#include <systemd/sd-daemon.h>
+#endif
+
 typedef long long mstime_t; /* millisecond time type. */
 typedef long long ustime_t; /* microsecond time type. */