ADR 0005 — A traced process writes its traces once, at exit

Status: Accepted (2026-09-26)

Context

A woven test process attributes every probe hit to the current xUnit v3 test (or a fixture, collection, assembly, pool or static-init scope) and keeps one bitset of probe ids per scope. Those bitsets have to leave the process. Writing each test's trace as the test finishes needs a per-test hook, and there is none without a compile-time reference in every test project: xUnit v3 publishes a test's TestState only on a new TestContext object, not through an event a loaded assembly can subscribe to, and Microsoft Testing Platform registers its extensions at build time.

Decision

The recorder writes one testprune-trace/1 NDJSON dump per process, from a ProcessExit handler, to trace-<pid>.ndjson.tmp and renames it to trace-<pid>.ndjson. The last line is {"end":true}; a dump without it is rejected as truncated. Ingestion merges every process's dump by scope after the run.

Consequences

A continuation that runs after its test has finished, but still in the test's async flow, is attributed to that test. That over-attributes, which is sound: a trace may name code the test did not need, never omit code it ran. A process that is killed (a timeout, a signal, a runtime crash) leaves no dump, so its tests keep no trace: a missing main-process dump stores the run as failed, and a missing child dump marks the starting test incomplete. Environment.Exit still runs the handler. Memory is one bit per probe id per scope, O(scopes × ids / 64) words for the whole run, held until exit. If the phase-1 dogfood measurement of peak RSS (test-prune-traces overhead reports it) shows that to bite, the fallback is a two-level sparse bitset per scope; nothing else changes.