fix(pii): Prevent cycles in rule collection

[jjbayer]

When collecting rules from a PII config, temporarily insert non-leaf nodes into the set of seen nodes to prevent cycles.

Fixes INGEST-1127

[linear-code]

INGEST-1127

[sentry-warden]

collect_rules has a path-only cycle guard and no traversal bound

An attacker-controlled PII rule graph can define nested Multiple nodes that repeatedly reference the same non-leaf child. Because collect_rules removes intermediates after each branch, the cycle set provides no memoization or work bound: a linear-size graph can trigger exponential traversal, while a long Alias chain can exhaust the worker stack and abort processing.

Evidence

• PiiConfig deserializes attacker-controlled rules and applications (relay-pii/src/config.rs:236-247); production project-event processing compiles the project PII config at relay-server/src/processing/utils/event.rs:424-430, and relay_validate_pii_config also compiles externally supplied JSON at relay-cabi/src/processing.rs:314-316.
• collect_rules checks rules.contains and inserts each Multiple/Alias only while it is active (relay-pii/src/compiledconfig.rs:95-118), then removes the intermediate at lines 109 and 119; completed non-leaf nodes are not memoized.
• A graph where each Multiple node contains rules: ["next", "next"] re-walks the same child twice at every level, producing Θ(2^N) traversal from O(N) rule definitions. A linear Alias chain still creates one recursive frame per rule because the path-only set does not bound acyclic depth.
• Compilation occurs on the shared envelope processor pool (relay-server/src/service.rs:113-127), and PiiConfig::compiled caches only after the first unbounded compilation (relay-pii/src/config.rs:274-283); there is no depth, step, or graph-size guard before the traversal.

Also found at 2 additional locations

• relay-pii/src/compiledconfig.rs:105-119
• relay-pii/src/compiledconfig.rs:206-242

_{Identified by Warden · wrdn-dos-review · Z2S-FSJ}

[Dav1dde]

Would it be possible to insert before the match and remove with a drop impl?

[loewenheim]

See mstange/pdb-addr2line#71 for some prior art for this, though it might be overkill.