api: change nvim_command_output behavior

Implement nvim_command_output with `execute({cmd},"silent")`.

Behavior changes:
- does not provoke any hit-enter prompt
- no longer prepends a newline char
- does not capture some noise (like the "[New File]" message, see the
  change to tabnewentered_spec.lua)

Technically ("bug-for-bug") this a breaking change.  But the previous
behavior of nvim_command_output meant that it probably wasn't used for
anything outside of tests.

Also remove the undocumented `v:command_output` variable which was
a hack introduced only for the purposes of nvim_command_output.

closes #7726

9 files changed, 105 insertions, 36 deletions

diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index 172f2ce18e..8929bc5b9b 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -43,14 +43,15 @@
 #endif
 
 /// Executes an ex-command.
-/// On VimL error: Returns the VimL error; v:errmsg is not updated.
+///
+/// On parse error: forwards the Vim error; does not update v:errmsg.
+/// On runtime error: forwards the Vim error; does not update v:errmsg.
 ///
 /// @param command  Ex-command string
-/// @param[out] err Error details (including actual VimL error), if any
+/// @param[out] err Error details (Vim error), if any
 void nvim_command(String command, Error *err)
   FUNC_API_SINCE(1)
 {
-  // Run the command
   try_start();
   do_cmdline_cmd(command.data);
   update_screen(VALID);
@@ -207,18 +208,39 @@ String nvim_replace_termcodes(String str, Boolean from_part, Boolean do_lt,
   return cstr_as_string(ptr);
 }
 
-String nvim_command_output(String str, Error *err)
+/// Executes an ex-command and returns its (non-error) output.
+/// Shell |:!| output is not captured.
+///
+/// On parse error: forwards the Vim error; does not update v:errmsg.
+/// On runtime error: forwards the Vim error; does not update v:errmsg.
+///
+/// @param command  Ex-command string
+/// @param[out] err Error details (Vim error), if any
+String nvim_command_output(String command, Error *err)
   FUNC_API_SINCE(1)
 {
-  do_cmdline_cmd("redir => v:command_output");
-  nvim_command(str, err);
-  do_cmdline_cmd("redir END");
-
+  Array args = ARRAY_DICT_INIT;
+  ADD(args, STRING_OBJ(copy_string(command)));
+  ADD(args, STRING_OBJ(cstr_to_string("silent")));
+  String fn = cstr_to_string("execute");
+  Object rv = nvim_call_function(fn, args, err);
+  api_free_string(fn);
+  api_free_array(args);
   if (ERROR_SET(err)) {
+    assert(rv.type == kObjectTypeNil);
     return (String)STRING_INIT;
   }
 
-  return cstr_to_string((char *)get_vim_var_str(VV_COMMAND_OUTPUT));
+  assert(rv.type == kObjectTypeString);
+  // execute() always(?) prepends a newline; remove it.
+  if (rv.data.string.size > 1) {
+    assert(rv.data.string.data[0] == '\n');
+    String *s = &rv.data.string;
+    s->size--;
+    memmove(s->data, s->data + 1, s->size);
+    s->data[s->size] = '\0';
+  }
+  return rv.data.string;
 }
 
 /// Evaluates a VimL expression (:help expression).
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index f34b6db8d8..875f323a28 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -402,7 +402,6 @@ static struct vimvar {
   VV(VV_OLDFILES,       "oldfiles",         VAR_LIST, 0),
   VV(VV_WINDOWID,       "windowid",         VAR_NUMBER, VV_RO_SBX),
   VV(VV_PROGPATH,       "progpath",         VAR_STRING, VV_RO),
-  VV(VV_COMMAND_OUTPUT, "command_output",   VAR_STRING, 0),
   VV(VV_COMPLETED_ITEM, "completed_item",   VAR_DICT, VV_RO),
   VV(VV_OPTION_NEW,     "option_new",       VAR_STRING, VV_RO),
   VV(VV_OPTION_OLD,     "option_old",       VAR_STRING, VV_RO),
diff --git a/src/nvim/eval.h b/src/nvim/eval.h
index 0c0a6881f6..b798eae187 100644
--- a/src/nvim/eval.h
+++ b/src/nvim/eval.h
@@ -86,7 +86,6 @@ typedef enum {
     VV_OLDFILES,
     VV_WINDOWID,
     VV_PROGPATH,
-    VV_COMMAND_OUTPUT,
     VV_COMPLETED_ITEM,
     VV_OPTION_NEW,
     VV_OPTION_OLD,
diff --git a/test/functional/api/vim_spec.lua b/test/functional/api/vim_spec.lua
index ff28e3d133..d213c3c34e 100644
--- a/test/functional/api/vim_spec.lua
+++ b/test/functional/api/vim_spec.lua
@@ -37,7 +37,7 @@ describe('api', function()
       os.remove(fname)
     end)
 
-    it("VimL error: fails (VimL error), does NOT update v:errmsg", function()
+    it("parse error: fails (specific error), does NOT update v:errmsg", function()
       -- Most API methods return generic errors (or no error) if a VimL
       -- expression fails; nvim_command returns the VimL error details.
       local status, rv = pcall(nvim, "command", "bogus_command")
@@ -45,6 +45,57 @@ describe('api', function()
       eq("E492:", string.match(rv, "E%d*:"))  -- VimL error was returned.
       eq("", nvim("eval", "v:errmsg"))        -- v:errmsg was not updated.
     end)
+
+    it("runtime error: fails (specific error)", function()
+      local status, rv = pcall(nvim, "command_output", "buffer 23487")
+      eq(false, status)                 -- nvim_command() failed.
+      eq("E86: Buffer 23487 does not exist", string.match(rv, "E%d*:.*"))
+      eq("", nvim("eval", "v:errmsg"))  -- v:errmsg was not updated.
+    end)
+  end)
+
+  describe('nvim_command_output', function()
+    it('does not induce hit-enter prompt', function()
+      -- Induce a hit-enter prompt use nvim_input (non-blocking).
+      nvim('command', 'set cmdheight=1')
+      nvim('input', [[:echo "hi\nhi2"<CR>]])
+
+      -- Verify hit-enter prompt.
+      eq({mode='r', blocking=true}, nvim("get_mode"))
+      nvim('input', [[<C-c>]])
+
+      -- Verify NO hit-enter prompt.
+      nvim('command_output', [[echo "hi\nhi2"]])
+      eq({mode='n', blocking=false}, nvim("get_mode"))
+    end)
+
+    it('returns command output', function()
+      eq('this is\nspinal tap',
+         nvim('command_output', [[echo "this is\nspinal tap"]]))
+    end)
+
+    it('does not return shell |:!| output', function()
+      eq(':!echo "foo"\r\n', nvim('command_output', [[!echo "foo"]]))
+    end)
+
+    it("parse error: fails (specific error), does NOT update v:errmsg", function()
+      local status, rv = pcall(nvim, "command_output", "bogus commannnd")
+      eq(false, status)                 -- nvim_command_output() failed.
+      eq("E492: Not an editor command: bogus commannnd",
+         string.match(rv, "E%d*:.*"))
+      eq("", nvim("eval", "v:errmsg"))  -- v:errmsg was not updated.
+      -- Verify NO hit-enter prompt.
+      eq({mode='n', blocking=false}, nvim("get_mode"))
+    end)
+
+    it("runtime error: fails (specific error)", function()
+      local status, rv = pcall(nvim, "command_output", "buffer 42")
+      eq(false, status)                 -- nvim_command_output() failed.
+      eq("E86: Buffer 42 does not exist", string.match(rv, "E%d*:.*"))
+      eq("", nvim("eval", "v:errmsg"))  -- v:errmsg was not updated.
+      -- Verify NO hit-enter prompt.
+      eq({mode='n', blocking=false}, nvim("get_mode"))
+    end)
   end)
 
   describe('nvim_eval', function()
diff --git a/test/functional/autocmd/tabclose_spec.lua b/test/functional/autocmd/tabclose_spec.lua
index fb777e7eea..b7c33dc3d8 100644
--- a/test/functional/autocmd/tabclose_spec.lua
+++ b/test/functional/autocmd/tabclose_spec.lua
@@ -11,10 +11,10 @@ describe('TabClosed', function()
         repeat
           nvim('command', 'tabnew')
         until nvim('eval', 'tabpagenr()') == 6 -- current tab is now 6
-        eq("\ntabclosed:6:6:5", nvim('command_output', 'tabclose')) -- close last 6, current tab is now 5
-        eq("\ntabclosed:5:5:4", nvim('command_output', 'close')) -- close last window on tab, closes tab
-        eq("\ntabclosed:2:2:3", nvim('command_output', '2tabclose')) -- close tab 2, current tab is now 3
-        eq("\ntabclosed:1:1:2\ntabclosed:1:1:1", nvim('command_output', 'tabonly')) -- close tabs 1 and 2
+        eq("tabclosed:6:6:5", nvim('command_output', 'tabclose')) -- close last 6, current tab is now 5
+        eq("tabclosed:5:5:4", nvim('command_output', 'close')) -- close last window on tab, closes tab
+        eq("tabclosed:2:2:3", nvim('command_output', '2tabclose')) -- close tab 2, current tab is now 3
+        eq("tabclosed:1:1:2\ntabclosed:1:1:1", nvim('command_output', 'tabonly')) -- close tabs 1 and 2
       end)
 
       it('is triggered when closing a window via bdelete from another tab', function()
@@ -23,7 +23,7 @@ describe('TabClosed', function()
         nvim('command', '1tabedit Xtestfile')
         nvim('command', 'normal! 1gt')
         eq({1, 3}, nvim('eval', '[tabpagenr(), tabpagenr("$")]'))
-        eq("\ntabclosed:2:2:1\ntabclosed:2:2:1", nvim('command_output', 'bdelete Xtestfile'))
+        eq("tabclosed:2:2:1\ntabclosed:2:2:1", nvim('command_output', 'bdelete Xtestfile'))
         eq({1, 1}, nvim('eval', '[tabpagenr(), tabpagenr("$")]'))
       end)
 
@@ -35,7 +35,7 @@ describe('TabClosed', function()
 
         -- Only one tab is closed, and the alternate file is used for the other.
         eq({2, 3}, nvim('eval', '[tabpagenr(), tabpagenr("$")]'))
-        eq("\ntabclosed:2:2:2", nvim('command_output', 'bdelete Xtestfile2'))
+        eq("tabclosed:2:2:2", nvim('command_output', 'bdelete Xtestfile2'))
         eq('Xtestfile1', nvim('eval', 'bufname("")'))
       end)
     end)
@@ -48,9 +48,9 @@ describe('TabClosed', function()
           nvim('command',  'tabnew')
         until nvim('eval', 'tabpagenr()') == 7 -- current tab is now 7
         -- sanity check, we shouldn't match on tabs with numbers other than 2
-        eq("\ntabclosed:7:7:6", nvim('command_output', 'tabclose'))
+        eq("tabclosed:7:7:6", nvim('command_output', 'tabclose'))
         -- close tab page 2, current tab is now 5
-        eq("\ntabclosed:2:2:5\ntabclosed:match", nvim('command_output', '2tabclose'))
+        eq("tabclosed:2:2:5\ntabclosed:match", nvim('command_output', '2tabclose'))
       end)
     end)
 
@@ -59,7 +59,7 @@ describe('TabClosed', function()
         nvim('command', 'au! TabClosed * echom "tabclosed:".expand("<afile>").":".expand("<amatch>").":".tabpagenr()')
         nvim('command',  'tabedit Xtestfile')
         eq({2, 2}, nvim('eval', '[tabpagenr(), tabpagenr("$")]'))
-        eq("\ntabclosed:2:2:1", nvim('command_output', 'close'))
+        eq("tabclosed:2:2:1", nvim('command_output', 'close'))
         eq({1, 1}, nvim('eval', '[tabpagenr(), tabpagenr("$")]'))
       end)
     end)
diff --git a/test/functional/autocmd/tabnewentered_spec.lua b/test/functional/autocmd/tabnewentered_spec.lua
index bdbe677132..59cac07b34 100644
--- a/test/functional/autocmd/tabnewentered_spec.lua
+++ b/test/functional/autocmd/tabnewentered_spec.lua
@@ -7,14 +7,14 @@ describe('TabNewEntered', function()
       it('matches when entering any new tab', function()
         clear()
         nvim('command', 'au! TabNewEntered * echom "tabnewentered:".tabpagenr().":".bufnr("")')
-        eq("\ntabnewentered:2:2", nvim('command_output', 'tabnew'))
-        eq("\n\"test.x2\" [New File]\ntabnewentered:3:3", nvim('command_output', 'tabnew test.x2'))
+        eq("tabnewentered:2:2", nvim('command_output', 'tabnew'))
+        eq("tabnewentered:3:3", nvim('command_output', 'tabnew test.x2'))
      end)
     end)
     describe('with FILE as <afile>', function()
       it('matches when opening a new tab for FILE', function()
         nvim('command', 'au! TabNewEntered Xtest-tabnewentered echom "tabnewentered:match"')
-        eq('\n"Xtest-tabnewentered" [New File]\ntabnewentered:4:4\ntabnewentered:match',
+        eq('tabnewentered:4:4\ntabnewentered:match',
           nvim('command_output', 'tabnew Xtest-tabnewentered'))
      end)
     end)
@@ -24,7 +24,7 @@ describe('TabNewEntered', function()
         nvim('command', 'au! TabNewEntered * echom "entered"')
         nvim('command', 'tabnew test.x2')
         nvim('command', 'split')
-        eq('\nentered', nvim('command_output', 'execute "normal \\<C-W>T"'))
+        eq('entered', nvim('command_output', 'execute "normal \\<C-W>T"'))
       end)
     end)
   end)
diff --git a/test/functional/ex_cmds/sign_spec.lua b/test/functional/ex_cmds/sign_spec.lua
index b37e6e8563..df0f5db860 100644
--- a/test/functional/ex_cmds/sign_spec.lua
+++ b/test/functional/ex_cmds/sign_spec.lua
@@ -16,8 +16,8 @@ describe('sign', function()
         nvim('command', 'sign place 34 line=3 name=Foo buffer='..buf2)
         -- now unplace without specifying a buffer
         nvim('command', 'sign unplace 34')
-        eq("\n--- Signs ---\n", nvim('command_output', 'sign place buffer='..buf1))
-        eq("\n--- Signs ---\n", nvim('command_output', 'sign place buffer='..buf2))
+        eq("--- Signs ---\n", nvim('command_output', 'sign place buffer='..buf1))
+        eq("--- Signs ---\n", nvim('command_output', 'sign place buffer='..buf2))
       end)
     end)
   end)
diff --git a/test/functional/helpers.lua b/test/functional/helpers.lua
index 31a2c3b3ff..dfc4694272 100644
--- a/test/functional/helpers.lua
+++ b/test/functional/helpers.lua
@@ -644,7 +644,7 @@ local function alter_slashes(obj)
 end
 
 local function hexdump(str)
-  local len = string.len( str )
+  local len = string.len(str)
   local dump = ""
   local hex = ""
   local asc = ""
@@ -652,22 +652,20 @@ local function hexdump(str)
   for i = 1, len do
     if 1 == i % 8 then
       dump = dump .. hex .. asc .. "\n"
-      hex = string.format( "%04x: ", i - 1 )
+      hex = string.format("%04x: ", i - 1)
       asc = ""
     end
 
-    local ord = string.byte( str, i )
-    hex = hex .. string.format( "%02x ", ord )
+    local ord = string.byte(str, i)
+    hex = hex .. string.format("%02x ", ord)
     if ord >= 32 and ord <= 126 then
-      asc = asc .. string.char( ord )
+      asc = asc .. string.char(ord)
     else
       asc = asc .. "."
     end
   end
 
-  return dump .. hex
-  .. string.rep( "   ", 8 - len % 8 ) .. asc
-
+  return dump .. hex .. string.rep("   ", 8 - len % 8) .. asc
 end
 
 local module = {
diff --git a/test/functional/ui/cmdline_highlight_spec.lua b/test/functional/ui/cmdline_highlight_spec.lua
index 73fe94c056..ffb6a26aef 100644
--- a/test/functional/ui/cmdline_highlight_spec.lua
+++ b/test/functional/ui/cmdline_highlight_spec.lua
@@ -849,7 +849,7 @@ describe('Ex commands coloring support', function()
       {EOB:~                                       }|
                                               |
     ]])
-    eq('\nError detected while processing :\nE605: Exception not caught: 42',
+    eq('Error detected while processing :\nE605: Exception not caught: 42',
        meths.command_output('messages'))
   end)
   it('errors out when failing to get callback', function()