feat(terminal): trigger TermRequest autocommand events (#22159)

This commit implements a new TermRequest autocommand event and has Neovim
emit this event when children of terminal buffers emit an OSC or DCS sequence
libvterm does not handle.

The TermRequest autocommand event has additional data in the
v:termrequest variable.

Co-authored-by: Gregory Anders <greg@gpanders.com>

11 files changed, 156 insertions, 4 deletions

diff --git a/runtime/doc/autocmd.txt b/runtime/doc/autocmd.txt
index 45d7a4244c..ce3af01073 100644
--- a/runtime/doc/autocmd.txt
+++ b/runtime/doc/autocmd.txt
@@ -986,6 +986,11 @@ TermLeave			After leaving |Terminal-mode|.
 TermClose			When a |terminal| job ends.
 				Sets these |v:event| keys:
 				    status
+							*TermRequest*
+TermRequest			When a |terminal| job emits an OSC or DCS
+				sequence. Sets |v:termrequest|. When used from
+				Lua, the request string is included in the
+				"data" field of the autocommand callback.
 							*TermResponse*
 TermResponse			When Nvim receives an OSC or DCS response from
 				the terminal.  Sets |v:termresponse|. When used
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index d3ace5f33b..98b782a105 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -284,6 +284,9 @@ The following new APIs and features were added.
 
 • |vim.deepcopy()| has a `noref` argument to avoid hashing table values.
 
+• Terminal buffers emit a |TermRequest| autocommand event when the child
+  process emits an OSC or DCS control sequence.
+
 ==============================================================================
 CHANGED FEATURES                                                 *news-changed*
 
diff --git a/runtime/doc/vvars.txt b/runtime/doc/vvars.txt
index 7e96d356d0..e71e31abf8 100644
--- a/runtime/doc/vvars.txt
+++ b/runtime/doc/vvars.txt
@@ -647,9 +647,16 @@ v:t_number	Value of |Number| type.  Read-only.  See: |type()|
 					*v:t_string* *t_string-variable*
 v:t_string	Value of |String| type.  Read-only.  See: |type()|
 
+				*v:termrequest* *termrequest-variable*
+v:termrequest
+		The value of the most recent OSC or DCS control sequence
+		sent from a process running in the embedded |terminal|.
+		This can be read in a |TermRequest| event handler to respond
+		to queries from embedded applications.
+
 				*v:termresponse* *termresponse-variable*
 v:termresponse
-		The value of the most recent OSC or DCS escape sequence
+		The value of the most recent OSC or DCS control sequence
 		received by Nvim from the terminal. This can be read in a
 		|TermResponse| event handler after querying the terminal using
 		another escape sequence.
diff --git a/runtime/lua/vim/_meta/vvars.lua b/runtime/lua/vim/_meta/vvars.lua
index e3b89aeff7..ca87fb9d15 100644
--- a/runtime/lua/vim/_meta/vvars.lua
+++ b/runtime/lua/vim/_meta/vvars.lua
@@ -687,11 +687,18 @@ vim.v.t_number = ...
 --- @type integer
 vim.v.t_string = ...
 
---- The value of the most recent OSC or DCS escape sequence
+--- The value of the most recent OSC or DCS control sequence
+--- sent from a process running in the embedded `terminal`.
+--- This can be read in a `TermRequest` event handler to respond
+--- to queries from embedded applications.
+--- @type string
+vim.v.termrequest = ...
+
+--- The value of the most recent OSC or DCS control sequence
 --- received by Nvim from the terminal. This can be read in a
 --- `TermResponse` event handler after querying the terminal using
 --- another escape sequence.
---- @type any
+--- @type string
 vim.v.termresponse = ...
 
 --- Must be set before using `test_garbagecollect_now()`.
diff --git a/src/nvim/auevents.lua b/src/nvim/auevents.lua
index 11d1597236..b931907fe3 100644
--- a/src/nvim/auevents.lua
+++ b/src/nvim/auevents.lua
@@ -109,6 +109,7 @@ return {
     'TermEnter', -- after entering Terminal mode
     'TermLeave', -- after leaving Terminal mode
     'TermOpen', -- after opening a terminal buffer
+    'TermRequest', -- after an unhandled OSC sequence is emitted
     'TermResponse', -- after setting "v:termresponse"
     'TextChanged', -- text was modified
     'TextChangedI', -- text was modified in Insert mode(no popup)
@@ -166,6 +167,7 @@ return {
     TabNewEntered = true,
     TermClose = true,
     TermOpen = true,
+    TermRequest = true,
     UIEnter = true,
     UILeave = true,
   },
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 5069f00df0..451e258356 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -185,6 +185,7 @@ static struct vimvar {
   VV(VV_VERSION,          "version",          VAR_NUMBER, VV_COMPAT + VV_RO),
   VV(VV_LNUM,             "lnum",             VAR_NUMBER, VV_RO_SBX),
   VV(VV_TERMRESPONSE,     "termresponse",     VAR_STRING, VV_RO),
+  VV(VV_TERMREQUEST,      "termrequest",      VAR_STRING, VV_RO),
   VV(VV_FNAME,            "fname",            VAR_STRING, VV_RO),
   VV(VV_LANG,             "lang",             VAR_STRING, VV_RO),
   VV(VV_LC_TIME,          "lc_time",          VAR_STRING, VV_RO),
diff --git a/src/nvim/eval.h b/src/nvim/eval.h
index c46a895c06..abe2685424 100644
--- a/src/nvim/eval.h
+++ b/src/nvim/eval.h
@@ -86,6 +86,7 @@ typedef enum {
   VV_THIS_SESSION,
   VV_VERSION,
   VV_LNUM,
+  VV_TERMREQUEST,
   VV_TERMRESPONSE,
   VV_FNAME,
   VV_LANG,
diff --git a/src/nvim/terminal.c b/src/nvim/terminal.c
index c6a2cb3354..ee482f7104 100644
--- a/src/nvim/terminal.c
+++ b/src/nvim/terminal.c
@@ -169,6 +169,54 @@ static VTermScreenCallbacks vterm_screen_callbacks = {
 
 static Set(ptr_t) invalidated_terminals = SET_INIT;
 
+static void emit_term_request(void **argv)
+{
+  char *payload = argv[0];
+  size_t payload_length = (size_t)argv[1];
+
+  String termrequest = { .data = payload, .size = payload_length };
+  Object data = STRING_OBJ(termrequest);
+  set_vim_var_string(VV_TERMREQUEST, payload, (ptrdiff_t)payload_length);
+  apply_autocmds_group(EVENT_TERMREQUEST, NULL, NULL, false, AUGROUP_ALL, curbuf, NULL, &data);
+  xfree(payload);
+}
+
+static int on_osc(int command, VTermStringFragment frag, void *user)
+{
+  if (frag.str == NULL) {
+    return 0;
+  }
+
+  StringBuilder request = KV_INITIAL_VALUE;
+  kv_printf(request, "\x1b]%d;", command);
+  kv_concat_len(request, frag.str, frag.len);
+  multiqueue_put(main_loop.events, emit_term_request, request.items, (void *)request.size);
+  return 1;
+}
+
+static int on_dcs(const char *command, size_t commandlen, VTermStringFragment frag, void *user)
+{
+  if ((command == NULL) || (frag.str == NULL)) {
+    return 0;
+  }
+
+  StringBuilder request = KV_INITIAL_VALUE;
+  kv_printf(request, "\x1bP%*s", (int)commandlen, command);
+  kv_concat_len(request, frag.str, frag.len);
+  multiqueue_put(main_loop.events, emit_term_request, request.items, (void *)request.size);
+  return 1;
+}
+
+static VTermStateFallbacks vterm_fallbacks = {
+  .control = NULL,
+  .csi = NULL,
+  .osc = on_osc,
+  .dcs = on_dcs,
+  .apc = NULL,
+  .pm = NULL,
+  .sos = NULL,
+};
+
 void terminal_init(void)
 {
   time_watcher_init(&main_loop, &refresh_timer, NULL);
@@ -222,6 +270,7 @@ void terminal_open(Terminal **termpp, buf_T *buf, TerminalOptions opts)
   vterm_screen_enable_reflow(rv->vts, true);
   // delete empty lines at the end of the buffer
   vterm_screen_set_callbacks(rv->vts, &vterm_screen_callbacks, rv);
+  vterm_screen_set_unrecognised_fallbacks(rv->vts, &vterm_fallbacks, rv);
   vterm_screen_set_damage_merge(rv->vts, VTERM_DAMAGE_SCROLL);
   vterm_screen_reset(rv->vts, 1);
   vterm_output_set_callback(rv->vt, term_output_callback, rv);
diff --git a/src/nvim/vvars.lua b/src/nvim/vvars.lua
index d1b5e13401..6410df1a57 100644
--- a/src/nvim/vvars.lua
+++ b/src/nvim/vvars.lua
@@ -770,13 +770,23 @@ M.vars = {
     desc = 'Value of |String| type.  Read-only.  See: |type()|',
   },
   termresponse = {
+    type = 'string',
     desc = [=[
-      The value of the most recent OSC or DCS escape sequence
+      The value of the most recent OSC or DCS control sequence
       received by Nvim from the terminal. This can be read in a
       |TermResponse| event handler after querying the terminal using
       another escape sequence.
     ]=],
   },
+  termrequest = {
+    type = 'string',
+    desc = [=[
+      The value of the most recent OSC or DCS control sequence
+      sent from a process running in the embedded |terminal|.
+      This can be read in a |TermRequest| event handler to respond
+      to queries from embedded applications.
+    ]=],
+  },
   testing = {
     desc = [=[
       Must be set before using `test_garbagecollect_now()`.
diff --git a/test/functional/terminal/buffer_spec.lua b/test/functional/terminal/buffer_spec.lua
index 480ca96289..d0462b5619 100644
--- a/test/functional/terminal/buffer_spec.lua
+++ b/test/functional/terminal/buffer_spec.lua
@@ -317,6 +317,18 @@ describe(':terminal buffer', function()
       pcall_err(command, 'write test/functional/fixtures/tty-test.c')
     )
   end)
+
+  it('emits TermRequest events', function()
+    command('split')
+    command('enew')
+    local term = meths.open_term(0, {})
+    -- cwd will be inserted in a file URI, which cannot contain backs
+    local cwd = funcs.getcwd():gsub('\\', '/')
+    local parent = cwd:match('^(.+/)')
+    local expected = '\027]7;file://host' .. parent
+    meths.chan_send(term, string.format('%s\027\\', expected))
+    eq(expected, eval('v:termrequest'))
+  end)
 end)
 
 describe('No heap-buffer-overflow when using', function()
diff --git a/test/functional/terminal/tui_spec.lua b/test/functional/terminal/tui_spec.lua
index f6ee0f9c77..e65e57bc7f 100644
--- a/test/functional/terminal/tui_spec.lua
+++ b/test/functional/terminal/tui_spec.lua
@@ -28,6 +28,7 @@ local new_pipename = helpers.new_pipename
 local spawn_argv = helpers.spawn_argv
 local set_session = helpers.set_session
 local write_file = helpers.write_file
+local eval = helpers.eval
 
 if helpers.skip(is_os('win')) then
   return
@@ -2736,6 +2737,42 @@ describe('TUI', function()
       unchanged = true,
     }
   end)
+
+  it('queries the terminal for truecolor support', function()
+    clear()
+    exec_lua([[
+      vim.api.nvim_create_autocmd('TermRequest', {
+        callback = function(args)
+          local req = args.data
+          local payload = req:match('^\027P%+q([%x;]+)$')
+          if payload then
+            vim.g.xtgettcap = true
+            return true
+          end
+        end,
+      })
+    ]])
+    screen = thelpers.setup_child_nvim({
+      '-u',
+      'NONE',
+      '-i',
+      'NONE',
+    }, {
+      env = {
+        VIMRUNTIME = os.getenv('VIMRUNTIME'),
+
+        -- Force COLORTERM to be unset and use a TERM that does not contain Tc or RGB in terminfo.
+        -- This will force the nested nvim instance to query with XTGETTCAP
+        COLORTERM = '',
+        TERM = 'xterm-256colors',
+      },
+    })
+
+    retry(nil, 1000, function()
+      eq(true, eval("get(g:, 'xtgettcap', v:false)"))
+      eq(1, eval('&termguicolors'))
+    end)
+  end)
 end)
 
 describe('TUI bg color', function()
@@ -2743,6 +2780,18 @@ describe('TUI bg color', function()
 
   local function setup_bg_test()
     clear()
+    exec_lua([[
+      vim.api.nvim_create_autocmd('TermRequest', {
+        callback = function(args)
+          local req = args.data
+          if req == '\027]11;?' then
+            vim.g.oscrequest = true
+            return true
+          end
+        end,
+      })
+    ]])
+
     screen = thelpers.setup_child_nvim({
       '-u',
       'NONE',
@@ -2759,6 +2808,12 @@ describe('TUI bg color', function()
 
   before_each(setup_bg_test)
 
+  it('queries the terminal for background color', function()
+    retry(nil, 1000, function()
+      eq(true, eval("get(g:, 'oscrequest', v:false)"))
+    end)
+  end)
+
   it('triggers OptionSet event on unsplit terminal-response', function()
     screen:expect([[
       {1: }                                                 |