localed: downgrade libxkbcommon to an optional runtime dependency

Previously, libxkbcommon was a compile-time option. When enabled the localed
binary would strictly depend on it, thus pulling in libxkbcommon and its
dependencies, which are non-trivial in size.

With this change we dlopen() libxkbcommon when it is available instead. If the
library is available behaviour is as before. However, if it isn't the system is
considered "headless", i.e. without local hardware and all attempts to set the
local keyboard configuration will be refused.

This is useful for general-purpose distributions which want to support
"headless" (such as container systems) and "full" systems with the same build.

2 files changed, 67 insertions, 12 deletions

diff --git a/Makefile.am b/Makefile.am
index 5c25178aec..0c2de6f2d3 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -4729,8 +4729,7 @@ systemd_localed_SOURCES = \
 	src/locale/localed.c
 
 systemd_localed_LDADD = \
-	libshared.la \
-	$(XKBCOMMON_LIBS)
+	libshared.la
 
 systemd_localed_CFLAGS = \
 	$(AM_CFLAGS) \
diff --git a/src/locale/localed.c b/src/locale/localed.c
index 46405ca68a..3b22a582ac 100644
--- a/src/locale/localed.c
+++ b/src/locale/localed.c
@@ -24,6 +24,7 @@
 
 #ifdef HAVE_XKBCOMMON
 #include <xkbcommon/xkbcommon.h>
+#include <dlfcn.h>
 #endif
 
 #include "sd-bus.h"
@@ -1101,6 +1102,7 @@ static int method_set_vc_keyboard(sd_bus_message *m, void *userdata, sd_bus_erro
 }
 
 #ifdef HAVE_XKBCOMMON
+
 _printf_(3, 0)
 static void log_xkb(struct xkb_context *ctx, enum xkb_log_level lvl, const char *format, va_list args) {
         const char *fmt;
@@ -1109,7 +1111,24 @@ static void log_xkb(struct xkb_context *ctx, enum xkb_log_level lvl, const char
         log_internalv(LOG_DEBUG, 0, __FILE__, __LINE__, __func__, fmt, args);
 }
 
+#define LOAD_SYMBOL(symbol, dl, name)                                   \
+        ({                                                              \
+                (symbol) = (typeof(symbol)) dlvsym((dl), (name), "V_0.5.0"); \
+                (symbol) ? 0 : -EOPNOTSUPP;                             \
+        })
+
 static int verify_xkb_rmlvo(const char *model, const char *layout, const char *variant, const char *options) {
+
+        /* We dlopen() the library in order to make the dependency soft. The library (and what it pulls in) is huge
+         * after all, hence let's support XKB maps when the library is around, and refuse otherwise. The function
+         * pointers to the shared library are below: */
+
+        struct xkb_context* (*symbol_xkb_context_new)(enum xkb_context_flags flags) = NULL;
+        void (*symbol_xkb_context_unref)(struct xkb_context *context) = NULL;
+        void (*symbol_xkb_context_set_log_fn)(struct xkb_context *context, void (*log_fn)(struct xkb_context *context, enum xkb_log_level level, const char *format, va_list args)) = NULL;
+        struct xkb_keymap* (*symbol_xkb_keymap_new_from_names)(struct xkb_context *context, const struct xkb_rule_names *names, enum xkb_keymap_compile_flags flags) = NULL;
+        void (*symbol_xkb_keymap_unref)(struct xkb_keymap *keymap) = NULL;
+
         const struct xkb_rule_names rmlvo = {
                 .model          = model,
                 .layout         = layout,
@@ -1118,35 +1137,68 @@ static int verify_xkb_rmlvo(const char *model, const char *layout, const char *v
         };
         struct xkb_context *ctx = NULL;
         struct xkb_keymap *km = NULL;
+        void *dl;
         int r;
 
-        /* compile keymap from RMLVO information to check out its validity */
+        /* Compile keymap from RMLVO information to check out its validity */
+
+        dl = dlopen("libxkbcommon.so.0", RTLD_LAZY);
+        if (!dl)
+                return -EOPNOTSUPP;
+
+        r = LOAD_SYMBOL(symbol_xkb_context_new, dl, "xkb_context_new");
+        if (r < 0)
+                goto finish;
+
+        r = LOAD_SYMBOL(symbol_xkb_context_unref, dl, "xkb_context_unref");
+        if (r < 0)
+                goto finish;
+
+        r = LOAD_SYMBOL(symbol_xkb_context_set_log_fn, dl, "xkb_context_set_log_fn");
+        if (r < 0)
+                goto finish;
 
-        ctx = xkb_context_new(XKB_CONTEXT_NO_ENVIRONMENT_NAMES);
+        r = LOAD_SYMBOL(symbol_xkb_keymap_new_from_names, dl, "xkb_keymap_new_from_names");
+        if (r < 0)
+                goto finish;
+
+        r = LOAD_SYMBOL(symbol_xkb_keymap_unref, dl, "xkb_keymap_unref");
+        if (r < 0)
+                goto finish;
+
+        ctx = symbol_xkb_context_new(XKB_CONTEXT_NO_ENVIRONMENT_NAMES);
         if (!ctx) {
                 r = -ENOMEM;
-                goto exit;
+                goto finish;
         }
 
-        xkb_context_set_log_fn(ctx, log_xkb);
+        symbol_xkb_context_set_log_fn(ctx, log_xkb);
 
-        km = xkb_keymap_new_from_names(ctx, &rmlvo, XKB_KEYMAP_COMPILE_NO_FLAGS);
+        km = symbol_xkb_keymap_new_from_names(ctx, &rmlvo, XKB_KEYMAP_COMPILE_NO_FLAGS);
         if (!km) {
                 r = -EINVAL;
-                goto exit;
+                goto finish;
         }
 
         r = 0;
 
-exit:
-        xkb_keymap_unref(km);
-        xkb_context_unref(ctx);
+finish:
+        if (symbol_xkb_keymap_unref && km)
+                symbol_xkb_keymap_unref(km);
+
+        if (symbol_xkb_context_unref && ctx)
+                symbol_xkb_context_unref(ctx);
+
+        (void) dlclose(dl);
         return r;
 }
+
 #else
+
 static int verify_xkb_rmlvo(const char *model, const char *layout, const char *variant, const char *options) {
         return 0;
 }
+
 #endif
 
 static int method_set_x11_keyboard(sd_bus_message *m, void *userdata, sd_bus_error *error) {
@@ -1203,7 +1255,11 @@ static int method_set_x11_keyboard(sd_bus_message *m, void *userdata, sd_bus_err
                 if (r < 0) {
                         log_error_errno(r, "Cannot compile XKB keymap for new x11 keyboard layout ('%s' / '%s' / '%s' / '%s'): %m",
                                         strempty(model), strempty(layout), strempty(variant), strempty(options));
-                        return sd_bus_error_set(error, SD_BUS_ERROR_INVALID_ARGS, "Cannot compile XKB keymap, refusing");
+
+                        if (r == -EOPNOTSUPP)
+                                return sd_bus_error_setf(error, SD_BUS_ERROR_NOT_SUPPORTED, "Local keyboard configuration not supported on this system.");
+
+                        return sd_bus_error_set(error, SD_BUS_ERROR_INVALID_ARGS, "Specified keymap cannot be compiled, refusing as invalid.");
                 }
 
                 if (free_and_strdup(&c->x11_layout, layout) < 0 ||