# `Mob.Diag`
[🔗](https://github.com/genericjam/mob/blob/master/lib/mob/diag.ex#L1)

Runtime diagnostics that run inside a Mob app's BEAM. Designed to be
invoked via Erlang RPC from a developer's machine to inspect the
actual state of a deployed app.

Pairs with mob_dev's tooling — `mix mob.verify_strip` calls into
`verify_loaded_modules/0`. Kept in the `mob` library (not `mob_dev`)
so the functions are present in every shipped app, not just at build
time on the developer's machine.

Don't expand the API surface here without thinking — anything added
is permanently shipped to every Mob app and a permanent target for
remote-execution if dist credentials leak.

# `load_failure`

```elixir
@type load_failure() :: %{module: module(), reason: term()}
```

# `load_report`

```elixir
@type load_report() :: %{
  total: non_neg_integer(),
  loaded: non_neg_integer(),
  failed: [load_failure()],
  elapsed_us: non_neg_integer(),
  otp_root: String.t() | nil
}
```

# `loaded_snapshot`

```elixir
@type loaded_snapshot() :: %{
  loaded: [module()],
  loaded_count: non_neg_integer(),
  shipped_count: non_neg_integer(),
  unloaded_in_bundle: [module()],
  otp_root: String.t() | nil,
  captured_at: DateTime.t()
}
```

# `loaded_snapshot`

```elixir
@spec loaded_snapshot() :: loaded_snapshot()
```

Snapshot of what's currently loaded in the running BEAM, plus
what's shipped-but-never-loaded (the empirical strip candidates).

In interactive mode (Mob's default), a module is loaded only when
something calls into it. So the loaded set after a representative
user session is "what the app actually needs." Anything in the
bundle but not in the loaded set is a strong strip candidate.

Better than tracing for our purposes: zero overhead, no rate-limit
worries, no risk of mailbox-overflowing a busy app.

Workflow:

  1. Deploy the app
  2. User exercises every flow they care about
  3. RPC `Mob.Diag.loaded_snapshot/0` from a Mix task
  4. Cross-reference `:unloaded_in_bundle` with the static audit:
     shipped + statically-reachable + never-loaded = high-confidence
     strip candidates.

Caveats: a flow that wasn't exercised won't show up. Run after a
thorough session, not after just opening the app.

# `mfa_trace`

```elixir
@spec mfa_trace(non_neg_integer()) :: %{
  mfas: [{module(), atom(), arity()}],
  modules: [module()],
  mfa_count: non_neg_integer(),
  module_count: non_neg_integer(),
  duration_ms: non_neg_integer(),
  captured_at: DateTime.t()
}
```

Captures unique MFAs called during a tracing window from a running app.

Wraps `:erlang.trace_pattern/3` + `:erlang.trace/3` for `duration_ms`,
then collects the unique `{mod, fun, arity}` set into ETS for retrieval.

Useful for empirical reachability beyond what `loaded_snapshot/0`
shows — `loaded_snapshot/0` answers "which modules are loaded";
`mfa_trace/1` answers "which functions actually got called during
this window." The MFA grain matters for Pass 4 (OpenSSL feature
surgery) where the question is "does the app call `crypto:rsa_*` at
all" not just "is the `:crypto` module loaded."

Returns:

    %{
      mfas: [{:crypto, :crypto_one_time_aead, 6}, ...],
      modules: [:crypto, :ssl, ...],
      mfa_count: 1247,
      module_count: 89,
      duration_ms: 30_000,
      captured_at: ~U[...]
    }

Limits:

- `:erlang.trace/3` is process-global. **One trace at a time** —
  overlapping calls clobber each other.
- Holds an ETS table during the window. ~100k events / 60s on an
  active app, dedup keeps the unique set small.
- Tracing has a measurable runtime cost (~1.5–2× slowdown). Don't
  leave a trace running indefinitely.

# `verify_loaded_modules`

```elixir
@spec verify_loaded_modules() :: load_report()
```

Force-load every `.beam` file under the running app's OTP tree and
report any that fail. Used by `mix mob.verify_strip` to validate
that an aggressive strip didn't remove a module something else
needed.

Walks all entries in `:code.get_path/0`, finds the OTP root from
the first matching `.../otp/lib/...` path, and enumerates `.beam`
files under it.

Returns `t:load_report/0`. Failures usually mean a stripped lib
contained a transitive dependency of a kept module.

---

*Consult [api-reference.md](api-reference.md) for complete listing*
