[autosiso] suggest using siso command direclty for non build commands

`autosiso` doesn't handle non build commands. e.g. help, version, login
We should encourage users to use `siso` command directly,
and get rid of  `autosiso` eventually at some point.

Bug: b/278675516
Change-Id: I651e537d8d03674f5d1996983083e25ab5d149c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4452374
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Junji Watanabe <jwata@google.com>

autosiso.py

@@ -4,16 +4,41 @@
 # found in the LICENSE file.
 """
 Developers invoke this script via autosiso or autosiso.bat to simply run
-Siso builds.
+Siso/Reclient builds.
 """
+# TODO(b/278976196): `siso ninja` command should handle the reclient and
+# authentication accordingly.
 
+import os
+import re
 import sys
 
 import reclient_helper
 import siso
 
 
+def _use_remoteexec(argv):
+  out_dir = reclient_helper.find_ninja_out_dir(argv)
+  gn_args_path = os.path.join(out_dir, 'args.gn')
+  if not os.path.exists(gn_args_path):
+    return False
+  with open(gn_args_path) as f:
+    for line in f:
+      line_without_comment = line.split('#')[0]
+      if re.search(r'(^|\s)use_remoteexec\s*=\s*true($|\s)',
+                   line_without_comment):
+        return True
+  return False
+
+
 def main(argv):
+  if not _use_remoteexec(argv):
+    print(
+        "`use_remoteexec=true` is not detected.\n"
+        "Please run `siso` command directly.",
+        file=sys.stderr)
+    return 1
+
   with reclient_helper.build_context(argv) as ret_code:
     if ret_code:
       return ret_code