[recipe_modules/gsutil] Fix docstring and potential error in name derivation.

This caused a bit of confusion for a customer who thought that they were
forbidden from passing Path objects in cmd.

R=tandrii@chromium.org

Change-Id: I352a6d5be75f48dcc0c8a8c92d6921359585ed48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1536493
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Auto-Submit: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>

recipes/README.recipes.md

@@ -615,7 +615,8 @@ Generates a Gitiles repo URL. See also parse_repo_url.
 
 A step to run arbitrary gsutil commands.
 
-Note that this assumes that gsutil authentication environment variables
+On LUCI this should automatically use the ambient task account credentials.
+On Buildbot, this assumes that gsutil authentication environment variables
 (AWS_CREDENTIAL_FILE and BOTO_CONFIG) are already set, though if you want to
 set them to something else you can always do so using the env={} kwarg.
 
@@ -623,28 +624,29 @@ Note also that gsutil does its own wildcard processing, so wildcards are
 valid in file-like portions of the cmd. See 'gsutil help wildcards'.
 
 Arguments:
-  cmd: list of (string) arguments to pass to gsutil.
-       Include gsutil-level options first (see 'gsutil help options').
-  name: the (string) name of the step to use.
-        Defaults to the first non-flag token in the cmd.
 
-— **def [cat](/recipes/recipe_modules/gsutil/api.py#99)(self, url, args=None, \*\*kwargs):**
+  * cmd (List[str|Path]) - Arguments to pass to gsutil. Include gsutil-level
+      options first (see 'gsutil help options').
+  * name (str) - Name of the step to use. Defaults to the first non-flag
+      token in the cmd.
 
-— **def [copy](/recipes/recipe_modules/gsutil/api.py#106)(self, source_bucket, source, dest_bucket, dest, args=None, link_name='gsutil.copy', metadata=None, unauthenticated_url=False, \*\*kwargs):**
+— **def [cat](/recipes/recipe_modules/gsutil/api.py#108)(self, url, args=None, \*\*kwargs):**
 
-— **def [download](/recipes/recipe_modules/gsutil/api.py#85)(self, bucket, source, dest, args=None, \*\*kwargs):**
+— **def [copy](/recipes/recipe_modules/gsutil/api.py#115)(self, source_bucket, source, dest_bucket, dest, args=None, link_name='gsutil.copy', metadata=None, unauthenticated_url=False, \*\*kwargs):**
 
-— **def [download\_url](/recipes/recipe_modules/gsutil/api.py#92)(self, url, dest, args=None, \*\*kwargs):**
+— **def [download](/recipes/recipe_modules/gsutil/api.py#94)(self, bucket, source, dest, args=None, \*\*kwargs):**
+
+— **def [download\_url](/recipes/recipe_modules/gsutil/api.py#101)(self, url, dest, args=None, \*\*kwargs):**
 
 &emsp; **@property**<br>&mdash; **def [gsutil\_py\_path](/recipes/recipe_modules/gsutil/api.py#10)(self):**
 
-&mdash; **def [list](/recipes/recipe_modules/gsutil/api.py#123)(self, url, args=None, \*\*kwargs):**
+&mdash; **def [list](/recipes/recipe_modules/gsutil/api.py#132)(self, url, args=None, \*\*kwargs):**
 
-&mdash; **def [remove\_url](/recipes/recipe_modules/gsutil/api.py#137)(self, url, args=None, \*\*kwargs):**
+&mdash; **def [remove\_url](/recipes/recipe_modules/gsutil/api.py#146)(self, url, args=None, \*\*kwargs):**
 
-&mdash; **def [signurl](/recipes/recipe_modules/gsutil/api.py#130)(self, private_key_file, bucket, dest, args=None, \*\*kwargs):**
+&mdash; **def [signurl](/recipes/recipe_modules/gsutil/api.py#139)(self, private_key_file, bucket, dest, args=None, \*\*kwargs):**
 
-&mdash; **def [upload](/recipes/recipe_modules/gsutil/api.py#69)(self, source, bucket, dest, args=None, link_name='gsutil.upload', metadata=None, unauthenticated_url=False, \*\*kwargs):**
+&mdash; **def [upload](/recipes/recipe_modules/gsutil/api.py#78)(self, source, bucket, dest, args=None, link_name='gsutil.upload', metadata=None, unauthenticated_url=False, \*\*kwargs):**
 ### *recipe_modules* / [infra\_paths](/recipes/recipe_modules/infra_paths)
 
 [DEPS](/recipes/recipe_modules/infra_paths/__init__.py#1): [recipe\_engine/path][recipe_engine/recipe_modules/path], [recipe\_engine/properties][recipe_engine/recipe_modules/properties]

recipes/recipe_modules/gsutil/api.py

@@ -16,7 +16,8 @@ class GSUtilApi(recipe_api.RecipeApi):
                **kwargs):
     """A step to run arbitrary gsutil commands.
 
-    Note that this assumes that gsutil authentication environment variables
+    On LUCI this should automatically use the ambient task account credentials.
+    On Buildbot, this assumes that gsutil authentication environment variables
     (AWS_CREDENTIAL_FILE and BOTO_CONFIG) are already set, though if you want to
     set them to something else you can always do so using the env={} kwarg.
 
@@ -24,14 +25,22 @@ class GSUtilApi(recipe_api.RecipeApi):
     valid in file-like portions of the cmd. See 'gsutil help wildcards'.
 
     Arguments:
-      cmd: list of (string) arguments to pass to gsutil.
-           Include gsutil-level options first (see 'gsutil help options').
-      name: the (string) name of the step to use.
-            Defaults to the first non-flag token in the cmd.
+
+      * cmd (List[str|Path]) - Arguments to pass to gsutil. Include gsutil-level
+          options first (see 'gsutil help options').
+      * name (str) - Name of the step to use. Defaults to the first non-flag
+          token in the cmd.
     """
-    if not name:
-      name = (t for t in cmd if not t.startswith('-')).next()
-    full_name = 'gsutil ' + name
+    if name:
+      full_name = 'gsutil ' + name
+    else:
+      full_name = 'gsutil'  # our fall-through name
+      # Find first cmd token not starting with '-'
+      for itm in cmd:
+        token = str(itm)   # it could be a Path
+        if not token.startswith('-'):
+          full_name = 'gsutil ' + token
+          break
 
     gsutil_path = self.gsutil_py_path
     cmd_prefix = []