Stress Test the Email Deduplication Fix

Problem/Feature Description

The marketing platform's email sending service experienced an incident last month where a campaign email was delivered multiple times to some recipients due to a race condition in the deduplication logic. A senior engineer has now submitted a fix and says it prevents duplicate sends by checking a Redis-backed "sent" set before dispatching each email.

Before the fix is deployed to production, the team wants an adversarial review of the claim. The concern is not just whether the fix works in the happy path, but whether there are other conditions under which it could fail — including situations that are hard to reproduce in local testing. You have been asked to analyze the fix and produce a thorough adversarial assessment.

Output Specification

Write your assessment to verification_report.md. Document what you looked for, what you found, and what verdict you assign.

Input Files

The following files are provided as inputs. Extract them before beginning.

=============== FILE: src/email_sender.py =============== import hashlib import time

class EmailSender: def init(self, redis_client, smtp_client): self.redis = redis_client self.smtp = smtp_client self.sent_key_prefix = "email:sent:" self.ttl_seconds = 86400 # 24 hours

def _dedup_key(self, recipient: str, campaign_id: str) -> str:
    raw = f"{recipient}:{campaign_id}"
    return self.sent_key_prefix + hashlib.sha256(raw.encode()).hexdigest()

def send(self, recipient: str, campaign_id: str, body: str) -> bool:
    """
    Send an email only if it has not been sent before.
    Returns True if the email was sent, False if it was a duplicate.
    """
    key = self._dedup_key(recipient, campaign_id)

    # Check if already sent
    if self.redis.exists(key):
        return False

    # Send the email
    self.smtp.send(recipient, body)

    # Mark as sent
    self.redis.setex(key, self.ttl_seconds, "1")
    return True