Refactor GN executable.

Changes:
* Refactored FindGnInPath to FindInPath
* Added function FindGnTool like we have for all other tools.
* Improved error messages
* Switched subprocess to execv for better handling of signals.

Bug: chromium:471053914
Change-Id: I58ca4d153974dd50a64c6651917a9fb96a6a6964

gclient_paths.py

@@ -122,6 +122,8 @@ def _GetBuildtoolsPathInternal(cwd, override):
 
     # buildtools may be in the gclient root.
     gclient_root = FindGclientRoot(os.getcwd())
+    if not gclient_root:
+        return None
     buildtools_path = os.path.join(gclient_root, 'buildtools')
     if os.path.exists(buildtools_path):
         return buildtools_path
@@ -185,3 +187,17 @@ def GetGClientPrimarySolutionURL(gclient_root_dir_path):
     if solutions:
         return solutions[0].get('url')
     return None
+
+
+def FindInPath(name: str):
+    """Shutil.which replacement that skips depot_tools in PATH."""
+    env_path = os.getenv('PATH')
+    if not env_path:
+        return
+    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
+        path = os.path.join(bin_dir, name + GetExeSuffix())
+        if os.path.isfile(path):
+            return path

gn.py

@@ -33,18 +33,42 @@ def PruneVirtualEnv():
     ])
 
 
-def findGnInPath():
-    env_path = os.getenv('PATH')
-    if not env_path:
-        return
-    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, 'gn' + gclient_paths.GetExeSuffix())
-        if os.path.isfile(gn_path):
+def FindGnTool():
+    # Try in primary solution location first, with the gn binary having been
+    # 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 gn_path
 
+    # 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 = gclient_paths.FindInPath('gn')
+        if gn_path:
+            return gn_path
+        print('gn.py: Unable to find gn in your $PATH')
+        print('Hint: `which -a gn` should output two entries')
+        return None
+    # TODO(b/328065301): Once chromium/src CL has landed to migrate
+    # buildtools/<platform>/gn to buildtools/<platform>/gn/gn, only return
+    # gn/gn path.
+    old_gn_path = os.path.join(bin_path, 'gn' + gclient_paths.GetExeSuffix())
+    new_gn_path = os.path.join(bin_path, 'gn',
+                               'gn' + gclient_paths.GetExeSuffix())
+    paths = [new_gn_path, old_gn_path]
+    for path in paths:
+        if os.path.isfile(path):
+            return path
+    print('gn.py: Could not find gn executable at: %s' % paths, file=sys.stderr)
+    print(
+        "Either GN isn't installed on your system, or you're not running in " +
+        "a checkout with a preinstalled gn binary.",
+        file=sys.stderr)
+
 
 def main(args):
     # Prune all evidence of VPython/VirtualEnv out of the environment. This
@@ -55,45 +79,11 @@ def main(args):
     # .vpython3 file (or lack thereof), but instead reference the default python
     # from the PATH.
     PruneVirtualEnv()
-
-    # Try in primary solution location first, with the gn binary having been
-    # 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:])
-
-    # 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:])
-        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
-    # TODO(b/328065301): Once chromium/src CL has landed to migrate
-    # buildtools/<platform>/gn to buildtools/<platform>/gn/gn, only return
-    # gn/gn path.
-    old_gn_path = os.path.join(bin_path, 'gn' + gclient_paths.GetExeSuffix())
-    new_gn_path = os.path.join(bin_path, 'gn',
-                               'gn' + gclient_paths.GetExeSuffix())
-    paths = [new_gn_path, old_gn_path]
-    for path in paths:
-        if os.path.isfile(path):
-            return subprocess.call([path] + args[1:])
-    print('gn.py: Could not find gn executable at: %s' % paths, file=sys.stderr)
+    gn = FindGnTool()
+    if gn:
+        os.execv(gn, [gn] + args[1:])
     return 2
 
 
 if __name__ == '__main__':
-    try:
-        sys.exit(main(sys.argv))
-    except KeyboardInterrupt:
-        sys.stderr.write('interrupted\n')
-        sys.exit(1)
+    sys.exit(main(sys.argv))