[depot_tools] Support both gn paths in gn.py

Build in support for both buildtools/<platform>/gn and
buildtools/<platform>/gn/gn preemptively.
This will allow the libfuzzer builders in https://chromium-review.googlesource.com/c/chromium/src/+/5474162 to succeed.

Bug: b/328065301
Change-Id: I97b401cb1b3339cfa7962f60b891be05baac75d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5479888
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Stephanie Kim <kimstephanie@google.com>

gn.py

@@ -33,22 +33,33 @@ def PruneVirtualEnv():
     ])
 
 
-def findGnInPath():
+# TODO(b/328065301): Once chromium/src CL has landed to migrate
+# buildtools/<platform>/gn to buildtools/<platform>/gn/gn, only return
+# gn/gn path.
+def findGnOptionsInPath():
     env_path = os.getenv('PATH')
     if not env_path:
         return
     exe = 'gn'
     if sys.platform in ('win32', 'cygwin'):
         exe += '.exe'
+    path_options = []
     for bin_dir in env_path.split(os.pathsep):
         if bin_dir.rstrip(os.sep).endswith('depot_tools'):
             # skip depot_tools to avoid calling gn.py infinitely.
             continue
-        gn_path = os.path.join(bin_dir, exe)
-        if os.path.isfile(gn_path):
-            return gn_path
+        for path in [
+                os.path.join(bin_dir, 'gn', exe),
+                os.path.join(bin_dir, exe),
+        ]:
+            if os.path.isfile(path):
+                path_options.append(path)
+        return path_options
 
 
+# TODO(b/328065301): Once chromium/src CL has landed to migrate
+# buildtools/<platform>/gn to buildtools/<platform>/gn/gn, only return
+# gn/gn path.
 def main(args):
     # Prune all evidence of VPython/VirtualEnv out of the environment. This
     # means that we 'unwrap' vpython VirtualEnv path/env manipulation.
@@ -63,29 +74,35 @@ def main(args):
     # downloaded by cipd in the projects DEPS.
     primary_solution_path = gclient_paths.GetPrimarySolutionPath()
     if primary_solution_path:
-        gn_path = os.path.join(primary_solution_path, 'third_party', 'gn',
-                               'gn' + gclient_paths.GetExeSuffix())
-        if os.path.exists(gn_path):
-            return subprocess.call([gn_path] + args[1:])
+        old_gn_path = os.path.join(primary_solution_path, 'third_party',
+                                   'gn' + gclient_paths.GetExeSuffix())
+        new_gn_path = os.path.join(primary_solution_path, 'third_party', 'gn',
+                                   'gn' + gclient_paths.GetExeSuffix())
+        for path in [new_gn_path, old_gn_path]:
+            if os.path.exists(path):
+                return subprocess.call([path] + args[1:])
 
     # Otherwise try the old .sha1 and download_from_google_storage locations
     # inside of buildtools.
     bin_path = gclient_paths.GetBuildtoolsPlatformBinaryPath()
     if not bin_path:
-        gn_path = findGnInPath()
-        if gn_path:
-            return subprocess.call([gn_path] + args[1:])
+        gn_path_options = findGnOptionsInPath()
+        for path in gn_path_options:
+            if os.path.exists(path):
+                return subprocess.call([path] + args[1:])
         print(
             'gn.py: Could not find checkout in any parent of the current '
             'path.\nThis must be run inside a checkout.',
             file=sys.stderr)
         return 1
-    gn_path = os.path.join(bin_path, 'gn' + gclient_paths.GetExeSuffix())
-    if not os.path.exists(gn_path):
-        print('gn.py: Could not find gn executable at: %s' % gn_path,
-              file=sys.stderr)
-        return 2
-    return subprocess.call([gn_path] + args[1:])
+    old_gn_path = os.path.join(bin_path, gclient_paths.GetExeSuffix())
+    new_gn_path = os.path.join(bin_path, 'gn', gclient_paths.GetExeSuffix())
+    paths = [new_gn_path, old_gn_path]
+    for path in paths:
+        if os.path.exists(path):
+            return subprocess.call([path] + args[1:])
+    print('gn.py: Could not find gn executable at: %s' % paths, file=sys.stderr)
+    return 2
 
 
 if __name__ == '__main__':