Add JSON-parsing presubmit check

This is followup from the malformed-json bug which took down
Sheriff-o-Matic on 2019-05-16. This CL just adds the canned check;
future CLs can either add it to PanProjectChecks or add it to
other PRESUBMIT files directly.

Change-Id: I4a445193c1744966a448b12c7eb2915873e484d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1617941
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Sean McCullough <seanmccullough@chromium.org>

presubmit_canned_checks.py

@@ -1468,3 +1468,21 @@ def CheckLucicfgGenOutput(input_api, output_api, entry_script):
         },
         output_api.PresubmitError)
   ]
+
+# TODO(agable): Add this to PanProjectChecks.
+def CheckJsonParses(input_api, output_api):
+  """Verifies that all JSON files at least parse as valid JSON."""
+  import json
+  affected_files = input_api.AffectedFiles(
+      include_deletes=False,
+      file_filter=lambda x: x.LocalPath().endswith('.json'))
+  warnings = []
+  for f in affected_files:
+    with open(f.AbsoluteLocalPath()) as j:
+      try:
+        json.load(j)
+      except ValueError:
+        # Just a warning for now, in case people are using JSON5 somewhere.
+        warnings.append(output_api.PresubmitPromptWarning(
+            '%s does not appear to be valid JSON.' % f.LocalPath()))
+  return warnings

tests/presubmit_unittest.py

@@ -1670,7 +1670,7 @@ class CannedChecksUnittest(PresubmitTestsBase):
       'GetPythonUnitTests', 'GetPylint',
       'GetUnitTests', 'GetUnitTestsInDirectory', 'GetUnitTestsRecursively',
       'CheckCIPDManifest', 'CheckCIPDPackages', 'CheckCIPDClientDigests',
-      'CheckChangedLUCIConfigs', 'CheckLucicfgGenOutput',
+      'CheckChangedLUCIConfigs', 'CheckLucicfgGenOutput', 'CheckJsonParses',
       'print_function',
     ]
     # If this test fails, you should add the relevant test.