Distinguish redirect loop and redirect chain timeout errors

[heydemoura]

Summary

Add detection for two distinct redirect failure modes that were previously conflated:

1. Redirect Loop (ErrorRedirectLoop = 9): Detects when a redirect chain visits the same URL twice via URL deduplication. Maps to "redirect" status type.

2. Redirect Chain Timeout (ErrorRedirectTimeout = 10): Detects when following a redirect chain exceeds the site's timeout limit (same as regular check timeout). Maps to "intermittent" status type.

Rationale

Long-running redirect chains present an exploitable attack surface for Jetmon's veriflier:

• CPU exhaustion: Each veriflier goroutine following a redirect chain can consume significant CPU time
• Resource starvation: A malicious site serving slow or infinite redirects can stall veriflier's check queue
• Observability: Previously, redirect-induced timeouts were indistinguishable from regular network timeouts, making it harder to diagnose redirect-specific issues

Implementation Details

New Error Constants

• ErrorRedirectLoop = 9 — redirect chain cycle detected
• ErrorRedirectTimeout = 10 — redirect chain exceeded timeout

Detection Mechanism

• Maintains a seenURLs map in the CheckRedirect hook
• Tracks elapsed time since check start (checkStart := time.Now())
• Returns sentinel errors (errRedirectLoop, errRedirectChainTimeout) that survive *url.Error wrapping

Error Classification

Checks for sentinel errors via errors.Is() before generic checks:

1. Loop detection (specific)
2. Chain timeout (specific)
3. Context timeout (generic)
4. Substring heuristics (fallback)

Time Limit

Uses the same timeout as regular checks — NET_COMMS_TIMEOUT or per-site TimeoutSeconds. No new config key required.

Test Coverage

• TestCheckRedirectLoop: Verifies A→B→A cycle detection
• TestCheckRedirectChainTimeout: Verifies slow redirects timeout correctly
• TestResultStatusType: Updated to test both new error codes

Metrics

New StatsD counter: scheduler.page.check.redirect_timeout.count

Both loop and timeout cases are tracked separately in orchestrator for debugging.

Files Changed

• internal/checker/checker.go — error constants, sentinel errors, detection logic, error classification
• internal/orchestrator/orchestrator.go — tracking, metrics, logging
• internal/checker/checker_test.go — new test cases

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