command: add msg-prefix command

This may be useful for IPC clients and libmpv users, which do not have
descriptive client names (`ipc_%d`, `main`), or may want to log using
a different prefixes for different components.

Author: layercak3

DOCS/interface-changes/msg-prefix.txt

@@ -0,0 +1 @@
+add `msg-prefix` command

DOCS/man/input.rst

@@ -824,6 +824,10 @@ Text Manipulation
 
     .. note:: Lua and JS code should use the provided ``mp.msg`` modules.
 
+``msg-prefix <level> <prefix> <message> [...]``
+    Same as ``msg``, but allows writing messages using ``prefix`` as the prefix
+    instead of the name of the client that sent the command.
+
 Configuration Commands
 ~~~~~~~~~~~~~~~~~~~~~~
 

player/command.c

@@ -7123,6 +7123,25 @@ static void cmd_msg(void *p)
     talloc_free(log);
 }
 
+static void cmd_msg_prefix(void *p)
+{
+    struct mp_cmd_ctx *cmd = p;
+    struct MPContext *mpctx = cmd->mpctx;
+
+    if (cmd->num_args < 3)
+        return;
+
+    int level = mp_msg_find_level(cmd->args[0].v.s);
+    if (level < 0)
+        return;
+
+    struct mp_log *log = mp_log_new(NULL, mpctx->log, cmd->args[1].v.s);
+    for (int i = 2; i < cmd->num_args; i++)
+        mp_msg(log, level, (i == 2 ? "%s" : " %s"), cmd->args[i].v.s);
+    mp_msg(log, level, "\n");
+    talloc_free(log);
+}
+
 /* This array defines all known commands.
  * The first field the command name used in libmpv and input.conf.
  * The second field is the handler function (see mp_cmd_def.handler and
@@ -7644,6 +7663,10 @@ const struct mp_cmd_def mp_cmds[] = {
 
     { "msg", cmd_msg, { {"level", OPT_STRING(v.s)}, {"message", OPT_STRING(v.s)} },
         .is_noisy = true, .vararg = true },
+    { "msg-prefix", cmd_msg_prefix, { {"level", OPT_STRING(v.s)},
+                                      {"prefix", OPT_STRING(v.s)},
+                                      {"message", OPT_STRING(v.s)} },
+        .is_noisy = true, .vararg = true },
 
     {0}
 };