win: libuv_process_spawn(): special-case cmd.exe

Disable CommandLineToArgvW-standard quoting for cmd.exe.

libuv assumes spawned processes follow the convention expected by
CommandLineToArgvW(). But cmd.exe is non-conformant, so for cmd.exe:
- With system([]), the caller has full control (and responsibility) to
  quote arguments correctly.
- With system(''), shell* options are used.

libuv quoting is disabled if argv[0] is:
- cmd.exe
- cmd
- $COMSPEC resolving to a path with filename cmd.exe

Closes #6329
References #6387

runtime/doc/eval.txt

@@ -4852,16 +4852,18 @@ jobstart({cmd}[, {opts}])				{Nvim} *jobstart()*
 		Spawns {cmd} as a job.  If {cmd} is a |List| it is run
 		directly.  If {cmd} is a |String| it is processed like this: >
 		  :call jobstart(split(&shell) + split(&shellcmdflag) + ['{cmd}'])
-<		NOTE: This only shows the idea; see |shell-unquoting| before
+<		(Only shows the idea; see |shell-unquoting| for full details.)
-		constructing lists with 'shell' or 'shellcmdflag'.
 
-		NOTE: On Windows if {cmd} is a List, cmd[0] must be a valid
+		NOTE: on Windows if {cmd} is a List:
-		executable (.exe, .com).  If the executable is in $PATH it can
+		  - cmd[0] must be executable. If it is in $PATH it can be
-		be called by name, with or without an extension: >
+		    called by name, with or without an extension: >
 		      :call jobstart(['ping', 'neovim.io'])
-<		If it is a path (not a name), it must include the extension: >
+<		    If it is a path (not a name), extension is required: >
 		      :call jobstart(['System32\ping.exe', 'neovim.io'])
-<
+<		  - {cmd} is quoted per the convention expected by
+		    CommandLineToArgvW https://msdn.microsoft.com/bb776391
+		    unless the first argument is some form of "cmd.exe".
+
 		{opts} is a dictionary with these keys:
 		  on_stdout: stdout event handler (function name or |Funcref|)
 		  on_stderr: stderr event handler (function name or |Funcref|)

src/nvim/event/libuv_process.c

@@ -8,6 +8,8 @@
 #include "nvim/event/process.h"
 #include "nvim/event/libuv_process.h"
 #include "nvim/log.h"
+#include "nvim/path.h"
+#include "nvim/os/os.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "event/libuv_process.c.generated.h"
@@ -24,6 +26,26 @@ int libuv_process_spawn(LibuvProcess *uvproc)
   if (proc->detach) {
       uvproc->uvopts.flags |= UV_PROCESS_DETACHED;
   }
+#ifdef WIN32
+  // libuv assumes spawned processes follow the convention from
+  // CommandLineToArgvW(), cmd.exe does not. Disable quoting since it will
+  // result in unexpected behaviour, the caller is left with the responsibility
+  // to quote arguments accordingly. system('') has shell* options for this.
+  //
+  // Disable quoting for cmd, cmd.exe and $COMSPEC with a cmd.exe filename
+  bool is_cmd = STRICMP(proc->argv[0], "cmd.exe") == 0
+                || STRICMP(proc->argv[0], "cmd") == 0;
+  if (!is_cmd) {
+    const char_u *comspec = (char_u *)os_getenv("COMSPEC");
+    const char_u *comspecshell = path_tail((char_u *)proc->argv[0]);
+    is_cmd = comspec != NULL && STRICMP(proc->argv[0], comspec) == 0
+             && STRICMP("cmd.exe", (char *)comspecshell) == 0;
+  }
+
+  if (is_cmd) {
+    uvproc->uvopts.flags |= UV_PROCESS_WINDOWS_VERBATIM_ARGUMENTS;
+  }
+#endif
   uvproc->uvopts.exit_cb = exit_cb;
   uvproc->uvopts.cwd = proc->cwd;
   uvproc->uvopts.env = NULL;

test/functional/eval/system_spec.lua

@@ -129,15 +129,39 @@ describe('system()', function()
         screen:detach()
     end)
 
-    it('escapes inner double quotes #6329', function()
     if helpers.os_name() == 'windows' then
-        -- In Windows cmd.exe's echo prints the quotes
+      it('with the default cmd.exe shell', function()
         eq('""\n', eval([[system('echo ""')]]))
+        eq('"a b"\n', eval([[system('echo "a b"')]]))
+        -- TODO: consistent with Vim, but it should be fixed
+        eq('a & echo b\n', eval([[system('echo a & echo b')]]))
+        eval([[system('cd "C:\Program Files"')]])
+        eq(0, eval('v:shell_error'))
+      end)
+
+      it('with set shell="cmd"', function()
+        helpers.source('let &shell="cmd"')
+        eq('"a b"\n', eval([[system('echo "a b"')]]))
+      end)
+
+      it('works with cmd.exe from $COMSPEC', function()
+        local comspecshell = eval("fnamemodify($COMSPEC, ':t')")
+        if comspecshell == 'cmd.exe' then
+          helpers.source('let &shell=$COMSPEC')
+          eq('"a b"\n', eval([[system('echo "a b"')]]))
         else
-        eq('\n', eval([[system('echo ""')]]))
+          pending('$COMSPEC is not cmd.exe ' .. comspecshell)
         end
       end)
 
+      it('works with powershell', function()
+        helpers.set_shell_powershell()
+        eq('a\nb\n', eval([[system('echo a b')]]))
+        eq('C:\\\n', eval([[system('cd c:\; (Get-Location).Path')]]))
+        eq('a b\n', eval([[system('echo "a b"')]]))
+      end)
+    end
+
     it('`echo` and waits for its return', function()
       feed(':call system("echo")<cr>')
       screen:expect([[