[cpplint] add stdin support in stdin

Cpplint can already process input from stdin (when the filename is '-'),
but `git cl lint` currently lacks this capability.

Adding stdin support to `git cl lint` would enable seamless integration
with IDEs like Visual Studio. Linters could then operate directly on
unsaved code within the editor.

Bug: 372288610
Change-Id: Ib2d5a3534ac0c5afcfcac64b708becde41612881
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5917767
Commit-Queue: Scott Lee <ddoman@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>

cpplint.py

@@ -6399,10 +6399,9 @@ def ProcessFile(filename, vlevel, extra_check_functions=[]):
         # If after the split a trailing '\r' is present, it is removed
         # below.
         if filename == '-':
-            lines = codecs.StreamReaderWriter(sys.stdin,
-                                              codecs.getreader('utf8'),
-                                              codecs.getwriter('utf8'),
-                                              'replace').read().split('\n')
+            lines = codecs.StreamReaderWriter(
+                sys.stdin.read().encode('utf8'), codecs.getreader('utf8'),
+                codecs.getwriter('utf8'), 'replace').decode('utf8').split('\n')
         else:
             with codecs.open(filename, 'r', 'utf8', 'replace') as stream:
                 lines = stream.read().split('\n')

git_cl.py

@@ -4795,7 +4795,10 @@ def FindFilesForLint(options, args):
     """Returns the base folder and a list of files to lint."""
     files = []
     cwd = os.getcwd()
-    if len(args) > 0:
+    if len(args) == 1 and args[0] == '-':
+        # the input is from stdin.
+        files.append(args[0])
+    elif len(args) > 0:
         # If file paths are given in positional args, run the lint tools
         # against them without using git commands. This allows git_cl.py
         # runnable against any files even out of git checkouts.
@@ -4836,10 +4839,8 @@ def CMDlint(parser, args):
     """Runs cpplint on the current changelist or given files.
 
     positional arguments:
-      files           Files to lint. If omitted in the current git checkout, it
-                      will run cpplint against all the affected files. If it is
-                      not executed in git checkouts, files to lint must be
-                      provided.
+      files           Files to lint. If omitted, lint all the affected files.
+                      If -, lint stdin.
     """
     parser.add_option(
         '--filter',