jobstart(): Fix hang on non-executable cwd #9204

* os/fs.c: add os_isdir_executable()
* eval.c: fix hang on job start caused by non-executable cwd option
* channel.c: assert cwd is an executable directory
* test: jobstart() produces error when using non-executable cwd

src/nvim/channel.c

@@ -284,6 +284,8 @@ Channel *channel_job_start(char **argv, CallbackReader on_stdout,
                            uint16_t pty_width, uint16_t pty_height,
                            char *term_name, varnumber_T *status_out)
 {
+  assert(cwd == NULL || os_isdir_executable(cwd));
+
   Channel *chan = channel_alloc(kChannelStreamProc);
   chan->on_stdout = on_stdout;
   chan->on_stderr = on_stderr;

src/nvim/eval.c

@@ -11724,7 +11724,7 @@ static void f_jobstart(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     if (new_cwd && strlen(new_cwd) > 0) {
       cwd = new_cwd;
       // The new cwd must be a directory.
-      if (!os_isdir((char_u *)cwd)) {
+      if (!os_isdir_executable((const char *)cwd)) {
         EMSG2(_(e_invarg2), "expected valid directory");
         shell_free_argv(argv);
         return;
@@ -16769,7 +16769,7 @@ static void f_termopen(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     if (new_cwd && *new_cwd != NUL) {
       cwd = new_cwd;
       // The new cwd must be a directory.
-      if (!os_isdir((const char_u *)cwd)) {
+      if (!os_isdir_executable((const char *)cwd)) {
         EMSG2(_(e_invarg2), "expected valid directory");
         shell_free_argv(argv);
         return;

src/nvim/os/fs.c

@@ -110,7 +110,7 @@ bool os_isrealdir(const char *name)
 
 /// Check if the given path is a directory or not.
 ///
-/// @return `true` if `fname` is a directory.
+/// @return `true` if `name` is a directory.
 bool os_isdir(const char_u *name)
   FUNC_ATTR_NONNULL_ALL
 {
@@ -126,6 +126,25 @@ bool os_isdir(const char_u *name)
   return true;
 }
 
+/// Check if the given path is a directory and is executable.
+/// Gives the same results as `os_isdir()` on Windows.
+///
+/// @return `true` if `name` is a directory and executable.
+bool os_isdir_executable(const char *name)
+  FUNC_ATTR_NONNULL_ALL
+{
+  int32_t mode = os_getperm((const char *)name);
+  if (mode < 0) {
+    return false;
+  }
+
+#ifdef WIN32
+  return (S_ISDIR(mode));
+#else
+  return (S_ISDIR(mode) && (S_IXUSR & mode));
+#endif
+}
+
 /// Check what `name` is:
 /// @return NODE_NORMAL: file or directory (or doesn't exist)
 ///         NODE_WRITABLE: writable device, socket, fifo, etc.