sd-bus: add custom return code when $XDG_RUNTIME_DIR is not set

We would return ENOENT, which is extremely confusing. Strace is not helpful because
no *file* is actually missing. So let's add some logs at debug level and also use
a custom return code. Let all user-facing utilities print a custom error message
in that case.

man/sd_bus_default.xml

@@ -276,13 +276,20 @@
       <para>Returned errors may indicate the following problems:</para>
 
       <variablelist>
-
         <varlistentry>
           <term><constant>-EINVAL</constant></term>
 
           <listitem><para>The specified parameters are invalid.</para></listitem>
         </varlistentry>
 
+        <varlistentry>
+          <term><constant>-ENOMEDIUM</constant></term>
+
+          <listitem><para>The requested bus type is not available because of invalid environment (for example
+          the user session bus is not available because <varname>$XDG_RUNTIME_DIR</varname> is not set).
+          </para></listitem>
+        </varlistentry>
+
         <varlistentry>
           <term><constant>-ENOMEM</constant></term>
 

src/busctl/busctl.c

@@ -129,11 +129,11 @@ static int acquire_bus(bool set_monitor, sd_bus **ret) {
                 }
         }
         if (r < 0)
-                return log_error_errno(r, "Failed to set address: %m");
+                return bus_log_address_error(r);
 
         r = sd_bus_start(bus);
         if (r < 0)
-                return log_error_errno(r, "Failed to connect to bus: %m");
+                return bus_log_connect_error(r);
 
         *ret = TAKE_PTR(bus);
 

src/home/homectl.c

@@ -99,7 +99,7 @@ static int acquire_bus(sd_bus **bus) {
 
         r = bus_connect_transport(arg_transport, arg_host, false, bus);
         if (r < 0)
-                return log_error_errno(r, "Failed to connect to bus: %m");
+                return bus_log_connect_error(r);
 
         (void) sd_bus_set_allow_interactive_authorization(*bus, arg_ask_password);
 

src/libsystemd/sd-bus/sd-bus.c

@@ -1340,7 +1340,8 @@ int bus_set_address_user(sd_bus *b) {
 
                 e = secure_getenv("XDG_RUNTIME_DIR");
                 if (!e)
-                        return -ENOENT;
+                        return log_debug_errno(SYNTHETIC_ERRNO(ENOMEDIUM),
+                                               "sd-bus: $XDG_RUNTIME_DIR not set, cannot connect to user bus.");
 
                 ee = bus_address_escape(e);
                 if (!ee)

src/libsystemd/sd-bus/test-bus-cleanup.c

@@ -23,7 +23,7 @@ static int test_bus_open(void) {
         int r;
 
         r = sd_bus_open_user(&bus);
-        if (IN_SET(r, -ECONNREFUSED, -ENOENT)) {
+        if (IN_SET(r, -ECONNREFUSED, -ENOENT, -ENOMEDIUM)) {
                 r = sd_bus_open_system(&bus);
                 if (IN_SET(r, -ECONNREFUSED, -ENOENT))
                         return r;

src/libsystemd/sd-bus/test-bus-track.c

@@ -55,7 +55,7 @@ int main(int argc, char *argv[]) {
         assert_se(r >= 0);
 
         r = sd_bus_open_user(&a);
-        if (IN_SET(r, -ECONNREFUSED, -ENOENT)) {
+        if (IN_SET(r, -ECONNREFUSED, -ENOENT, -ENOMEDIUM)) {
                 r = sd_bus_open_system(&a);
                 if (IN_SET(r, -ECONNREFUSED, -ENOENT))
                         return log_tests_skipped("Failed to connect to bus");

src/login/inhibit.c

@@ -282,7 +282,7 @@ static int run(int argc, char *argv[]) {
 
         r = sd_bus_default_system(&bus);
         if (r < 0)
-                return log_error_errno(r, "Failed to connect to bus: %m");
+                return bus_log_connect_error(r);
 
         if (arg_action == ACTION_LIST)
                 return print_inhibitors(bus);

src/portable/portablectl.c

@@ -178,7 +178,7 @@ static int acquire_bus(sd_bus **bus) {
 
         r = bus_connect_transport(arg_transport, arg_host, false, bus);
         if (r < 0)
-                return log_error_errno(r, "Failed to connect to bus: %m");
+                return bus_log_connect_error(r);
 
         (void) sd_bus_set_allow_interactive_authorization(*bus, arg_ask_password);
 

src/shared/bus-util.h

@@ -38,8 +38,14 @@ int bus_connect_user_systemd(sd_bus **_bus);
 int bus_connect_transport(BusTransport transport, const char *host, bool user, sd_bus **bus);
 int bus_connect_transport_systemd(BusTransport transport, const char *host, bool user, sd_bus **bus);
 
+#define bus_log_address_error(r)                                        \
+        log_error_errno(r,                                              \
+                r == -ENOMEDIUM ? "Failed to set bus address: $DBUS_SESSION_BUS_ADDRESS and $XDG_RUNTIME_DIR not defined" : \
+                                  "Failed to set bus address: %m")
 #define bus_log_connect_error(r)                                        \
-        log_error_errno(r, "Failed to create bus connection: %m")
+        log_error_errno(r,                                              \
+                r == -ENOMEDIUM ? "Failed to connect to bus: $DBUS_SESSION_BUS_ADDRESS and $XDG_RUNTIME_DIR not defined" : \
+                                  "Failed to connect to bus: %m")
 
 #define bus_log_parse_error(r)                                  \
         log_error_errno(r, "Failed to parse bus message: %m")

src/systemctl/systemctl-util.c

@@ -52,7 +52,7 @@ int acquire_bus(BusFocus focus, sd_bus **ret) {
                 else
                         r = bus_connect_transport(arg_transport, arg_host, user, &buses[focus]);
                 if (r < 0)
-                        return log_error_errno(r, "Failed to connect to bus: %m");
+                        return bus_log_connect_error(r);
 
                 (void) sd_bus_set_allow_interactive_authorization(buses[focus], arg_ask_password);
         }