vim-patch:8.1.2115: MS-Windows: shell commands fail if &shell contains a space

Problem:    MS-Windows: shell commands fail if &shell contains a space.
Solution:   Use quotes instead of escaping. (closes vim/vim#4920)
https://github.com/vim/vim/commit/2efc44b3f0b6bd8307cb281af095e08e15ab1c24

Always double-quote &shell if it contains a space.
Neovim does not support escaping space with backslash, unlike Vim.

N/A patches for version.c:

vim-patch:8.0.1455: if $SHELL contains a space then 'shell' is incorrect

Problem:    If $SHELL contains a space then the default value of 'shell' is
            incorrect. (Matthew Horan)
Solution:   Escape spaces in $SHELL. (Christian Brabandt, closes vim/vim#459)
https://github.com/vim/vim/commit/4bfa8af14142e54f509048239f4e8596911f56aa

vim-patch:8.2.1194: test failure because shell prompt differs

Problem:    Test failure because shell prompt differs.
Solution:   Set the shell prompt.
https://github.com/vim/vim/commit/a4dc6f92bb29b00783f8945bbe1101e837b6ad3c

3 files changed, 86 insertions, 5 deletions

diff --git a/src/nvim/option.c b/src/nvim/option.c
index 146bce8cc0..6149331763 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -652,7 +652,14 @@ void set_init_1(bool clean_arg)
   {
     const char *shell = os_getenv("SHELL");
     if (shell != NULL) {
-      set_string_default("sh", (char *) shell, false);
+      if (vim_strchr((const char_u *)shell, ' ') != NULL) {
+        const size_t len = strlen(shell) + 3;  // two quotes and a trailing NUL
+        char *const cmd = xmalloc(len);
+        snprintf(cmd, len, "\"%s\"", shell);
+        set_string_default("sh", cmd, true);
+      } else {
+        set_string_default("sh", (char *)shell, false);
+      }
     }
   }
 
@@ -987,10 +994,9 @@ static void set_string_default(const char *name, char *val, bool allocated)
       xfree(options[opt_idx].def_val[VI_DEFAULT]);
     }
 
-    options[opt_idx].def_val[VI_DEFAULT] = (char_u *) (
-        allocated
-        ? (char_u *) val
-        : (char_u *) xstrdup(val));
+    options[opt_idx].def_val[VI_DEFAULT] = allocated
+        ? (char_u *)val
+        : (char_u *)xstrdup(val);
     options[opt_idx].flags |= P_DEF_ALLOCED;
   }
 }
diff --git a/src/nvim/testdir/test_startup.vim b/src/nvim/testdir/test_startup.vim
index 7fe0168356..6214975ef5 100644
--- a/src/nvim/testdir/test_startup.vim
+++ b/src/nvim/testdir/test_startup.vim
@@ -581,6 +581,27 @@ func Test_read_stdin()
   call delete('Xtestout')
 endfunc
 
+func Test_set_shell()
+  let after =<< trim [CODE]
+    call writefile([&shell], "Xtestout")
+    quit!
+  [CODE]
+
+  if has('win32')
+    let $SHELL = 'C:\with space\cmd.exe'
+    let expected = '"C:\with space\cmd.exe"'
+  else
+    let $SHELL = '/bin/with space/sh'
+    let expected = '"/bin/with space/sh"'
+  endif
+
+  if RunVimPiped([], after, '', '')
+    let lines = readfile('Xtestout')
+    call assert_equal(expected, lines[0])
+  endif
+  call delete('Xtestout')
+endfunc
+
 func Test_progpath()
   " Tests normally run with "./vim" or "../vim", these must have been expanded
   " to a full path.
diff --git a/src/nvim/testdir/test_system.vim b/src/nvim/testdir/test_system.vim
index d3c0594c03..424cb4abd0 100644
--- a/src/nvim/testdir/test_system.vim
+++ b/src/nvim/testdir/test_system.vim
@@ -1,5 +1,8 @@
 " Tests for system() and systemlist()
 
+source shared.vim
+source check.vim
+
 function! Test_System()
   if !executable('echo') || !executable('cat') || !executable('wc')
     return
@@ -88,3 +91,54 @@ function! Test_system_exmode()
   let a = system(v:progpath. cmd)
   call assert_notequal(0, v:shell_error)
 endfunc
+
+func Test_system_with_shell_quote()
+  throw 'skipped: enable after porting method patches'
+  CheckMSWindows
+
+  call mkdir('Xdir with spaces', 'p')
+  call system('copy "%COMSPEC%" "Xdir with spaces\cmd.exe"')
+
+  let shell_save = &shell
+  let shellxquote_save = &shellxquote
+  try
+    " Set 'shell' always needs noshellslash.
+    let shellslash_save = &shellslash
+    set noshellslash
+    let shell_tests = [
+          \ expand('$COMSPEC'),
+          \ '"' . fnamemodify('Xdir with spaces\cmd.exe', ':p') . '"',
+          \]
+    let &shellslash = shellslash_save
+
+    let sxq_tests = ['', '(', '"']
+
+    " Matrix tests: 'shell' * 'shellxquote'
+    for shell in shell_tests
+      let &shell = shell
+      for sxq in sxq_tests
+        let &shellxquote = sxq
+
+        let msg = printf('shell=%s shellxquote=%s', &shell, &shellxquote)
+
+        try
+          let out = 'echo 123'->system()
+        catch
+          call assert_report(printf('%s: %s', msg, v:exception))
+          continue
+        endtry
+
+        " On Windows we may get a trailing space and CR.
+        if out != "123 \n"
+          call assert_equal("123\n", out, msg)
+        endif
+
+      endfor
+    endfor
+
+  finally
+    let &shell = shell_save
+    let &shellxquote = shellxquote_save
+    call delete('Xdir with spaces', 'rf')
+  endtry
+endfunc