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).

5 files changed, 51 insertions, 1 deletions

diff --git a/src/nvim/channel.h b/src/nvim/channel.h
index 81b75e2d31..50d6b3600a 100644
--- a/src/nvim/channel.h
+++ b/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
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 5252c940f7..6d43097ddd 100644
--- a/src/nvim/eval/funcs.c
+++ b/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;
diff --git a/src/nvim/eval/typval.h b/src/nvim/eval/typval.h
index d1275d6512..ad01c01499 100644
--- a/src/nvim/eval/typval.h
+++ b/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 {
diff --git a/src/nvim/message.c b/src/nvim/message.c
index befca8c76b..76aa863bde 100644
--- a/src/nvim/message.c
+++ b/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)) {
diff --git a/test/functional/core/channels_spec.lua b/test/functional/core/channels_spec.lua
index 93dec9fb35..c28300f0f4 100644
--- a/test/functional/core/channels_spec.lua
+++ b/test/functional/core/channels_spec.lua
@@ -100,6 +100,38 @@ describe('channels', function()
     eq({"notification", "exit", {3,0}}, next_msg())
   end)
 
+  it('can use stdio channel and on_print callback', function()
+    source([[
+      let g:job_opts = {
+      \ 'on_stdout': function('OnEvent'),
+      \ 'on_stderr': function('OnEvent'),
+      \ 'on_exit': function('OnEvent'),
+      \ }
+    ]])
+    meths.set_var("nvim_prog", nvim_prog)
+    meths.set_var("code", [[
+      function! OnStdin(id, data, event) dict
+        echo string([a:id, a:data, a:event])
+        if a:data == ['']
+          quit
+        endif
+      endfunction
+      function! OnPrint(text) dict
+        call chansend(g:x, ['OnPrint:' .. a:text])
+      endfunction
+      let g:x = stdioopen({'on_stdin': funcref('OnStdin'), 'on_print':'OnPrint'})
+      call chansend(x, "hello")
+    ]])
+    command("let g:id = jobstart([ g:nvim_prog, '-u', 'NONE', '-i', 'NONE', '--cmd', 'set noswapfile', '--headless', '--cmd', g:code], g:job_opts)")
+    local id = eval("g:id")
+    ok(id > 0)
+
+    eq({ "notification", "stdout", {id, { "hello" } } }, next_msg())
+
+    command("call chansend(id, 'howdy')")
+    eq({"notification", "stdout", {id, {"OnPrint:[1, ['howdy'], 'stdin']"}}}, next_msg())
+  end)
+
   local function expect_twoline(id, stream, line1, line2, nobr)
     local msg = next_msg()
     local joined = nobr and {line1..line2} or {line1, line2}