ChannelsAPI silently accepts ?start=0 / ?end=0 and returns null milestone data with HTTP 200

[lucasccordeiro]

Describe the bug

When the /api/v0/channels endpoint receives ?start=0 or ?end=0, it
returns an HTTP 200 OK response containing null dates
(stable_date: null, earliest_beta: null, etc.) instead of rejecting the
request with HTTP 400. Milestone 0 is not a real Chrome milestone.

The root cause is the same as in the ?num=0 bug in FeaturesAPI:
basehandlers.get_int_arg() only rejects values < 0, so zero passes through.
The start > end guard at line 146 does not fire for start = end = 0.
construct_specified_milestones_details(0, 0) then calls
fetch_chrome_release_info(0), which returns placeholder null data.

To Reproduce

Steps to reproduce the behavior:

1. Send a GET request with start=0 and end=0:

   GET /api/v0/channels?start=0&end=0
   

2. Observe the response with HTTP 200:

   {"0": {"stable_date": null, "earliest_beta": null, "latest_beta": null,
          "mstone": 0, "version": 0}}

Minimal standalone reproducer (no dependencies beyond Python stdlib):

# get_int_arg admits 0 (inline of basehandlers.py:182-197)
def get_int_arg(val_str):
    val = val_str
    num = int(val)
    if num < 0:
        return 'ABORT_400'
    return num

start, end = get_int_arg('0'), get_int_arg('0')
print(start, end)           # 0 0 — both admitted
print(start > end)          # False — guard not triggered

# fetch_chrome_release_info fallback for mstone=0 (fetchchannels.py:144-151)
response = {0: {'stable_date': None, 'earliest_beta': None,
                'latest_beta': None, 'mstone': 0, 'version': 0}}
print(response)             # null dates, HTTP 200

Expected behavior

A request with ?start=0 or ?end=0 should be rejected with HTTP 400 Bad
Request. Chrome milestone numbers start at 1; milestone 0 has no schedule
data.

Additional context

• Affected files:

  • api/channels_api.py, lines 138–139
  • framework/basehandlers.py, line 193

• The fix is to add a minimum-value check after the get_int_arg calls:

  if start < 1 or end < 1:
      self.abort(400, msg='Milestone numbers must be >= 1')

• Shares the same root cause as the FeaturesAPI ?num=0 bug
  (get_int_arg lacking a positivity guard); could be filed as a single issue.

• Found by ESBMC v8.3.0 bounded model
  checking on a symbolic harness
  (harness/channels_milestone_zero_silent_acceptance.py). ESBMC assigned
  start = 0 and produced the following counterexample (1 VCC, solver: Bitwuzla):

  State 1  start = 0
  
  Violated property:
    file channels_milestone_zero_silent_acceptance.py line 55 function main
    assertion start >= 1
  
  VERIFICATION FAILED
  

  Confirmed empirically with the standalone reproducer above.