feat(--headless): add on_print callback to stdioopen

This commit adds an on_print callback to stdioopen's dictionary
argument which lets the caller specify a function called each time
neovim will try to output something to stdout (e.g. on "echo" or
"echoerr" in --headless mode).

src/nvim/channel.h

@@ -96,6 +96,8 @@ struct Channel {
 
 EXTERN PMap(uint64_t) channels INIT(= MAP_INIT);
 
+EXTERN Callback on_print INIT(= CALLBACK_INIT);
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "channel.h.generated.h"
 #endif

src/nvim/eval/funcs.c

@@ -10213,6 +10213,10 @@ static void f_stdioopen(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   if (!tv_dict_get_callback(opts, S_LEN("on_stdin"), &on_stdin.cb)) {
     return;
   }
+  if (!tv_dict_get_callback(opts, S_LEN("on_print"), &on_print)) {
+    return;
+  }
+
   on_stdin.buffered = tv_dict_get_number(opts, "stdin_buffered");
   if (on_stdin.buffered && on_stdin.cb.type == kCallbackNone) {
     on_stdin.self = opts;

src/nvim/eval/typval.h

@@ -81,7 +81,8 @@ typedef struct {
   } data;
   CallbackType type;
 } Callback;
-#define CALLBACK_NONE ((Callback){ .type = kCallbackNone })
+#define CALLBACK_INIT { .type = kCallbackNone }
+#define CALLBACK_NONE ((Callback)CALLBACK_INIT)
 
 /// Structure holding dictionary watcher
 typedef struct dict_watcher {

src/nvim/message.c

@@ -2647,6 +2647,17 @@ static void msg_puts_printf(const char *str, const ptrdiff_t maxlen)
   char buf[7];
   char *p;
 
+  if (on_print.type != kCallbackNone) {
+    typval_T argv[1];
+    argv[0].v_type = VAR_STRING;
+    argv[0].v_lock = VAR_UNLOCKED;
+    argv[0].vval.v_string = (char_u *)str;
+    typval_T rettv = TV_INITIAL_VALUE;
+    callback_call(&on_print, 1, argv, &rettv);
+    tv_clear(&rettv);
+    return;
+  }
+
   while ((maxlen < 0 || s - str < maxlen) && *s != NUL) {
     int len = utf_ptr2len((const char_u *)s);
     if (!(silent_mode && p_verbose == 0)) {