Improve ninja-to-siso switch detection

autoninja tries to detect if a user switches an output directory from
ninja to siso without doing a "gn clean" between. Initiallly this
detection looked for .ninja_deps, but this file doesn't always exist
after a ninja build. This change switches to looking for .ninja_log.

However autosiso creates a .ninja_log file (currently zero length) so
the new check is:

a) If .siso_deps exists then a siso build has been done and only siso
builds are allowed.
b) If .siso_deps doesn't exist but .ninja_log does then a ninja build
has been done and only ninja builds are allowed.
c) If neither file exists then the directory was cleaned and any type of
build is allowed.

Bug: b/293657720
Change-Id: I2cdba74f0894e62aa0e68f91e225f497b2425f45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4784103
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Junji Watanabe <jwata@google.com>
Reviewed-by: Philipp Wollermann <philwo@google.com>

autoninja.py

@@ -97,9 +97,13 @@ def main(args):
           use_siso = True
           continue
 
+    siso_marker = os.path.join(output_dir, '.siso_deps')
     if use_siso:
-      ninja_marker = os.path.join(output_dir, '.ninja_deps')
-      if os.path.exists(ninja_marker):
+      ninja_marker = os.path.join(output_dir, '.ninja_log')
+      # autosiso generates a .ninja_log file so the mere existence of a
+      # .ninja_log file doesn't imply that a ninja build was done. However if
+      # there is a .ninja_log but no .siso_deps then that implies a ninja build.
+      if os.path.exists(ninja_marker) and not os.path.exists(siso_marker):
         return ('echo Run gn clean before switching from ninja to siso in %s' %
                 output_dir)
       siso = ['autosiso'] if use_remoteexec else ['siso', 'ninja']
@@ -109,7 +113,6 @@ def main(args):
         siso = ['call'] + siso
       return ' '.join(siso + input_args[1:])
 
-    siso_marker = os.path.join(output_dir, '.siso_deps')
     if os.path.exists(siso_marker):
       return ('echo Run gn clean before switching from siso to ninja in %s' %
               output_dir)