win: jobstart(), system(): $PATHEXT-resolve exe

Windows: In order for jobstart(['foo']), system(['foo']) to find
"foo.cmd", we must replace "foo" with "foo.cmd" before sending `argv` to
process_spawn().

Rationale: jobstart([…]), system([…]) "executable" semantics should be
consistent with the VimL executable() function.

fix #9569
related: #10554

2 files changed, 17 insertions, 7 deletions

diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 6774000ae4..f27e73a7a2 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -12084,10 +12084,11 @@ static void f_jobresize(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 /// @param[out] executable  Returns `false` if argv[0] is not executable.
 ///
 /// @returns Result of `shell_build_argv()` if `cmd_tv` is a String.
-///          Else, string values of `cmd_tv` copied to a (char **) list.
+///          Else, string values of `cmd_tv` copied to a (char **) list with
+///          argv[0] resolved to full path ($PATHEXT-resolved on Windows).
 static char **tv_to_argv(typval_T *cmd_tv, const char **cmd, bool *executable)
 {
-  if (cmd_tv->v_type == VAR_STRING) {
+  if (cmd_tv->v_type == VAR_STRING) {  // String => "shell semantics".
     const char *cmd_str = tv_get_string(cmd_tv);
     if (cmd) {
       *cmd = cmd_str;
@@ -12107,16 +12108,17 @@ static char **tv_to_argv(typval_T *cmd_tv, const char **cmd, bool *executable)
     return NULL;
   }
 
-  const char *exe = tv_get_string_chk(TV_LIST_ITEM_TV(tv_list_first(argl)));
-  if (!exe || !os_can_exe((const char_u *)exe, NULL, true)) {
-    if (exe && executable) {
+  const char *arg0 = tv_get_string_chk(TV_LIST_ITEM_TV(tv_list_first(argl)));
+  char_u *exe_resolved = NULL;
+  if (!arg0 || !os_can_exe((const char_u *)arg0, &exe_resolved, true)) {
+    if (arg0 && executable) {
       *executable = false;
     }
     return NULL;
   }
 
   if (cmd) {
-    *cmd = exe;
+    *cmd = exe_resolved;
   }
 
   // Build the argument vector
@@ -12127,10 +12129,15 @@ static char **tv_to_argv(typval_T *cmd_tv, const char **cmd, bool *executable)
     if (!a) {
       // Did emsg in tv_get_string_chk; just deallocate argv.
       shell_free_argv(argv);
+      xfree(exe_resolved);
       return NULL;
     }
     argv[i++] = xstrdup(a);
   });
+  // Replace argv[0] with absolute path. The only reason for this is to make
+  // $PATHEXT work on Windows with jobstart([…]). #9569
+  xfree(argv[0]);
+  argv[0] = exe_resolved;
 
   return argv;
 }
diff --git a/src/nvim/os/fs.c b/src/nvim/os/fs.c
index 4a10b5199c..f34592d107 100644
--- a/src/nvim/os/fs.c
+++ b/src/nvim/os/fs.c
@@ -230,7 +230,7 @@ int os_exepath(char *buffer, size_t *size)
 /// Checks if the file `name` is executable.
 ///
 /// @param[in]  name     Filename to check.
-/// @param[out] abspath  Returns resolved executable path, if not NULL.
+/// @param[out,allocated] abspath  Returns resolved exe path, if not NULL.
 /// @param[in] use_path  Also search $PATH.
 ///
 /// @return true if `name` is executable and
@@ -271,6 +271,9 @@ bool os_can_exe(const char_u *name, char_u **abspath, bool use_path)
 }
 
 /// Returns true if `name` is an executable file.
+///
+/// @param[in]            name     Filename to check.
+/// @param[out,allocated] abspath  Returns full exe path, if not NULL.
 static bool is_executable(const char *name, char_u **abspath)
   FUNC_ATTR_NONNULL_ARG(1)
 {