structured-error-logging

name	structured-error-logging
description	Add or review structured error logging in EdenFS daemon (C++) — the ErrorLogger / EdenErrorInfo system that feeds the edenfs_errors table. Use when writing a catch block or handling a boxed/async exception (folly::Try / exception_wrapper / .thenError), instrumenting a failure path, or debugging a wrong stack trace or wrong component. Covers throw-site trace rules, component choice, errorType, noise filtering, gating, and testing.
metadata	{"oncalls":"source_control","strict":true,"apply_to_path":"eden/fs/.*\\.(cpp|h)$","apply_to_user_prompt":"(?i)(error logging|structured error|errorlogger|edenerrorinfo|edenfs_errors|log this error|fromexceptionwithouttrace)"}

EdenFS Structured Error Logging

EdenFS records daemon-side failures as structured rows in the edenfs_errors table (Hive + Scuba) via the ErrorLogger. Unlike XLOG(ERR, ...) — which only writes to the local log file — structured logging gives every error queryable columns (component, error code/name, mount point, inode, stack trace, …) so we can aggregate failures across the fleet, alert on them, and debug them after the fact.

Logging correctly matters: the two most common mistakes — a wrong stack trace and a mislabeled component — make the data actively misleading rather than just incomplete. The rules below prevent both. (If you'd XLOG(ERR, ...) an unexpected failure, consider a structured error too.)

Key files

Component	Location
Logger entry point	eden/fs/telemetry/ErrorLogger.h (log(), isEnabled())
Builder + factories	eden/fs/telemetry/EdenErrorInfo.h, EdenErrorInfoBuilder.h
Exception wrapper	eden/fs/telemetry/ErrorArg.h
Field → column mapping	eden/fs/telemetry/DaemonError.h (populate())
Component enum	eden/fs/telemetry/EdenComponent.h
Access from a handler	serverState_->getErrorLogger()

The basic shape

try {
  doSomethingThatMayThrow();
} catch (const std::exception& ex) {
  serverState_->getErrorLogger().log(
      EdenErrorInfo::backingStore(ex)        // 1. pick the component, pass the error
          .withMountPoint(mountPath.asString())  // 2. chain optional context
          .withErrorType("blob_import_failed")); // 3. a stable, queryable subtype
}

log() consumes the builder — you never call create() yourself.

Rule 1 (most important): stack-trace provenance

How the trace is captured

EdenFS hooks the C++ throw path so that every throw records a backtrace into a thread-local slot. Two properties of that slot decide correctness:

1. It is per-thread — only the thread that threw has the backtrace.
2. It holds only the most recent throw on that thread, and reading it (when log() runs) consumes it.

ErrorArg(const std::exception&) opts into reading that slot. So the trace attached to your error matches your exception only if the most recent throw on this thread was the one you're logging, and nothing has thrown since.

Where the trace is valid: an inline catch on the throwing thread

The one valid case — pass ex directly:

try {
  importBlob(id);                      // throws here, on this thread
} catch (const std::exception& ex) {
  // nothing else throws between the catch and the log
  logger.log(EdenErrorInfo::backingStore(ex).withErrorType("blob_import"));
}

Watch the "nothing thrown since" part: any throw between the catch and the log() — even one thrown-and-caught internally by a helper, a folly::tryTo, a map/fmt op — overwrites the slot, so do the log() before such work.

Where ex's trace is wrong → use fromExceptionWithoutTrace

In all these cases the slot holds a different throw (or none), so passing ex attaches a confidently wrong trace (worse than none) — use ErrorArg::fromExceptionWithoutTrace(ex):

• Boxed — folly::Try / exception_wrapper, or .thenError / .thenTry / with_exception.
• Cross-thread — the continuation runs on a different thread than the throw.
• Rethrown/reconstructed — newEdenError(ex), std::rethrow_exception (a new throw overwrites the slot).
• Deferred — logged after other work that may have thrown.

// Visiting a boxed exception (here a folly::exception_wrapper) away from its
// throw site — strip the mismatched trace:
ew.with_exception([&](const std::exception& e) {
  logger.log(EdenErrorInfo::objectStore(ErrorArg::fromExceptionWithoutTrace(e))
                 .withErrorType("checkout_update_error"));
});

Rule 2: pick the component that matches where the failure happened

The component column is how errors are bucketed. EdenErrorInfo has one factory per component — see EdenErrorInfo.h for the factories/signatures and EdenComponent.h for the component list.

The non-obvious part is which to pick: use the factory for the subsystem where the failure actually originated, which is not always the subsystem whose code you're editing. E.g. an inode load failure surfaces in InodeMap, but the data came from the object store, so it's objectStore(...), not overlay(...). Ask "where did the thing that failed live?", not "what file am I editing?".

Rule 3: errorType is a stable, queryable subtype

withErrorType("...") distinguishes failure modes within a component so you can filter/alert on them. Use descriptive, stable lower_snake_case strings (e.g. blob_import_failed). Treat them like enum values — don't reword an existing one casually; dashboards and alerts key off them.

Builder context methods

Chain whatever .with*() context is useful (mount point, inode, file path, …); all are optional and return the builder — see EdenErrorInfoBuilder.h. One thing not visible from the signatures: .withErrorCode() / .withErrorName() are auto-filled from std::system_error, so you don't set them yourself.

Rule 4: log genuine faults, not expected errors

Skip errors that are normal control flow (e.g. a FUSE/NFS ENOENT on a missing path) — logging them buries the real failures. Where an errno is available, filter on it.

Gating

Behavior is controlled by config flags (see EdenConfig.h); log() no-ops when disabled, so just call it from the catch:

• telemetry:enable-error-logging — master on/off.
• telemetry:error-scribe-category — Scribe category for the legacy path.
• telemetry:enable-stack-trace-upload — upload captured stack traces to Manifold.
• telemetry:enable-xplatlogger-errors — route errors via XplatLogger to the Logger (Hive).

Testing

Components that take an injectable ErrorLogger&/ErrorLogger* (e.g. SaplingBackingStore, FuseChannel, TestMount) are unit-testable directly: inject a capturing logger and assert on what was logged.

• Use eden/fs/telemetry/test/CapturingScribeLogger.h to capture emitted events.
• TestMount accepts an injectable ErrorLogger.
• For an end-to-end smoke test, the debugLogError Thrift endpoint throws and logs a test error: eden debug thrift debugLogError.