resolve: allow setting the log level dynamically as in pid1

This is useful to raise the log level for a single transaction or a few,
without affecting other state of the resolved as a restart would.
The log level can only be set, I didn't bother with having the ability
to restore the original as in pid1.

man/org.freedesktop.resolve1.xml

@@ -145,6 +145,9 @@ node /org/freedesktop/resolve1 {
       readonly as DNSSECNegativeTrustAnchors = ['...', ...];
       @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
       readonly s DNSStubListener = '...';
+      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
+      @org.freedesktop.systemd1.Privileged("true")
+      readwrite s LogLevel = '...';
   };
   interface org.freedesktop.DBus.Peer { ... };
   interface org.freedesktop.DBus.Introspectable { ... };
@@ -460,6 +463,10 @@ node /org/freedesktop/resolve1 {
       which DNS is configured and for the system-wide settings if there are any. Note that <filename>systemd-resolved</filename> assumes
       DNSSEC is supported by DNS servers until it verifies that this is not the case. Thus, the reported
       value may initially be true, until the first transactions are executed.</para>
+
+      <para>The <varname>LogLevel</varname> property shows the (maximum) log level of the manager, with the
+      same values as the <option>--log-level=</option> option described in
+      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
     </refsect2>
   </refsect1>
 

man/resolvectl.xml

@@ -175,6 +175,7 @@
         automatically, an explicit reverting is not necessary in that case.</para></listitem>
       </varlistentry>
 
+      <xi:include href="systemctl.xml" xpointer="log-level" />
     </variablelist>
   </refsect1>
 

man/systemctl.xml

@@ -1067,7 +1067,7 @@ Jan 12 10:46:45 example.com bluetoothd[8900]: gatt-time-server: Input/output err
           </listitem>
         </varlistentry>
 
-        <varlistentry>
+        <varlistentry id='log-level'>
           <term><command>log-level</command> [<replaceable>LEVEL</replaceable>]</term>
 
           <listitem><para>If no argument is given, print the current log level of the manager. If an

src/resolve/resolvectl.c

@@ -2516,6 +2516,48 @@ static int verb_revert_link(int argc, char **argv, void *userdata) {
         return 0;
 }
 
+static int verb_log_level(int argc, char *argv[], void *userdata) {
+        _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
+        sd_bus *bus = userdata;
+        int r;
+
+        assert(bus);
+
+        if (argc == 1) {
+                _cleanup_free_ char *level = NULL;
+
+                r = sd_bus_get_property_string(
+                                bus,
+                                "org.freedesktop.resolve1",
+                                "/org/freedesktop/resolve1",
+                                "org.freedesktop.resolve1.Manager",
+                                "LogLevel",
+                                &error,
+                                &level);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to get log level: %s", bus_error_message(&error, r));
+
+                puts(level);
+
+        } else {
+                assert(argc == 2);
+
+                r = sd_bus_set_property(
+                                bus,
+                                "org.freedesktop.resolve1",
+                                "/org/freedesktop/resolve1",
+                                "org.freedesktop.resolve1.Manager",
+                                "LogLevel",
+                                &error,
+                                "s",
+                                argv[1]);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to set log level: %s", bus_error_message(&error, r));
+        }
+
+        return 0;
+}
+
 static void help_protocol_types(void) {
         if (arg_legend)
                 puts("Known protocol types:");
@@ -3190,6 +3232,7 @@ static int native_main(int argc, char *argv[], sd_bus *bus) {
                 { "dnssec",                VERB_ANY, 3,        0,            verb_dnssec           },
                 { "nta",                   VERB_ANY, VERB_ANY, 0,            verb_nta              },
                 { "revert",                VERB_ANY, 2,        0,            verb_revert_link      },
+                { "log-level",             VERB_ANY, 2,        0,            verb_log_level        },
                 {}
         };
 

src/resolve/resolved-bus.c

@@ -16,6 +16,7 @@
 #include "socket-netlink.h"
 #include "stdio-util.h"
 #include "strv.h"
+#include "syslog-util.h"
 #include "user-util.h"
 #include "utf8.h"
 
@@ -1835,6 +1836,57 @@ static int bus_method_unregister_service(sd_bus_message *message, void *userdata
         return call_dnssd_method(m, message, bus_dnssd_method_unregister, error);
 }
 
+static int property_get_log_level(
+                sd_bus *bus,
+                const char *path,
+                const char *interface,
+                const char *property,
+                sd_bus_message *reply,
+                void *userdata,
+                sd_bus_error *error) {
+
+        _cleanup_free_ char *t = NULL;
+        int r;
+
+        assert(bus);
+        assert(reply);
+
+        r = log_level_to_string_alloc(log_get_max_level(), &t);
+        if (r < 0)
+                return r;
+
+        return sd_bus_message_append(reply, "s", t);
+}
+
+static int property_set_log_level(
+                sd_bus *bus,
+                const char *path,
+                const char *interface,
+                const char *property,
+                sd_bus_message *value,
+                void *userdata,
+                sd_bus_error *error) {
+
+        const char *t;
+        int r;
+
+        assert(bus);
+        assert(value);
+
+        r = sd_bus_message_read(value, "s", &t);
+        if (r < 0)
+                return r;
+
+        r = log_level_from_string(t);
+        if (r < 0)
+                return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Invalid log level '%s'", t);
+
+        log_info("Setting log level to %s.", t);
+        log_set_max_level(r);
+
+        return 0;
+}
+
 static const sd_bus_vtable resolve_vtable[] = {
         SD_BUS_VTABLE_START(0),
         SD_BUS_PROPERTY("LLMNRHostname", "s", NULL, offsetof(Manager, llmnr_hostname), SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
@@ -1853,7 +1905,7 @@ static const sd_bus_vtable resolve_vtable[] = {
         SD_BUS_PROPERTY("DNSSECNegativeTrustAnchors", "as", bus_property_get_ntas, 0, 0),
         SD_BUS_PROPERTY("DNSStubListener", "s", bus_property_get_dns_stub_listener_mode, offsetof(Manager, dns_stub_listener_mode), 0),
 
-
+        SD_BUS_WRITABLE_PROPERTY("LogLevel", "s", property_get_log_level, property_set_log_level, 0, 0),
 
         SD_BUS_METHOD_WITH_NAMES("ResolveHostname",
                                  "isit",