# Crash Fixes Reference

This document catalogues crashes that have been diagnosed and fixed in Didactyl.
Use it as a reference when investigating future crashes — the symptoms, root causes,
and investigation techniques described here may save significant debugging time.

---

## 1. Silent process exit on relay disconnect — SIGPIPE (v0.0.24)

**Date**: 2026-03-02  
**Version**: v0.0.23 → fixed in v0.0.24  
**Severity**: Critical — process terminates without any log output  
**Files changed**: `src/main.c`

### Symptoms

- The process exits cleanly to the shell prompt with **no error message**.
- The `[didactyl] shutting down` log line does **not** appear.
- No coredump is generated.
- The last log lines show all relays transitioning from `connected → disconnected`
  and then immediately `disconnected → connected`.
- The crash typically follows a period of network instability or DNS resolution
  failure — for example, an LLM HTTP request failing with
  `curl=Could not resolve hostname`.

### Root cause

`SIGPIPE` was never handled. The default OS action for `SIGPIPE` is to terminate
the process immediately — no signal handler runs, no cleanup occurs, no coredump
is written.

The TLS write path in `nostr_core_lib` uses `SSL_write()`, which internally calls
`write()` on the underlying socket. When a `wss://` relay drops its TCP connection
and the relay pool attempts to write to that socket, the kernel delivers `SIGPIPE`.

The plain-TCP path for `ws://` connections was already protected by using
`MSG_NOSIGNAL` on `send()` calls, but the TLS path had no equivalent protection.

### How it was diagnosed

1. **No crash message or coredump** ruled out `SIGSEGV`, `SIGABRT`, and OOM.
2. **No shutdown log** ruled out a graceful exit via the signal handler.
3. `dmesg` and `journalctl` showed no OOM-kill or segfault entries.
4. Searching the entire codebase for `SIGPIPE` or `SIG_IGN` returned zero results.
5. The websocket TLS transport was confirmed to use `SSL_write()` without
   `MSG_NOSIGNAL` or any per-thread signal mask.
6. The timeline matched: relay disconnects occurred, the next poll cycle attempted
   a write to a dead TLS socket, and the process vanished.

### Fix

Added `signal(SIGPIPE, SIG_IGN)` in `main()` alongside the existing `SIGINT` and
`SIGTERM` handlers:

```c
signal(SIGINT, signal_handler);
signal(SIGTERM, signal_handler);
signal(SIGPIPE, SIG_IGN);          /* ← added */
```

This causes `SSL_write()` and any other write to a broken pipe to return `-1` with
`errno = EPIPE` instead of killing the process. The relay pool and libcurl already
handle write errors gracefully.

### How to recognise this class of bug in the future

- Process disappears without any log output or coredump.
- Happens after network disruption or relay disconnects.
- `ulimit -c` may be 0, but even with unlimited core size, `SIGPIPE` does not
  produce a coredump by default.
- Any new network transport layer added to the project should be audited for
  `SIGPIPE` protection.

---

## 2. Use-after-free in `execute_nostr_list_manage` — SIGSEGV (v0.0.24)

**Date**: 2026-03-01 (coredump), fixed 2026-03-02  
**Version**: v0.0.23 → fixed in v0.0.24  
**Severity**: Critical — segmentation fault during tool execution  
**Files changed**: `src/tools.c`

### Symptoms

- `SIGSEGV` crash during execution of the `nostr_list_manage` tool.
- Coredump shows the crash at `src/tools.c:29` inside `json_error()`, but with
  heavily corrupted stack frames — the real crash site is in
  `execute_nostr_list_manage()`.
- The corrupted stack is characteristic of heap corruption from use-after-free.

### Root cause

In `execute_nostr_list_manage()`, the `action` pointer was obtained from the
`args` cJSON tree:

```c
cJSON* action = cJSON_GetObjectItemCaseSensitive(args, "action");
```

Later, `args` was freed:

```c
cJSON_Delete(args);          /* frees the entire tree including action */
```

But `action->valuestring` was still accessed afterwards:

```c
cJSON_AddStringToObject(out, "action", action->valuestring);  /* dangling pointer */
```

After `cJSON_Delete(args)`, the `action` pointer is dangling. Accessing
`action->valuestring` reads freed heap memory, which may contain arbitrary data
or may have been reallocated for another purpose.

### How it was diagnosed

1. The coredump was extracted from systemd-coredump storage:
   ```
   zstd -d /var/lib/systemd/coredump/core.didactyl_static.*.zst -o /tmp/core.bin
   gdb ./didactyl_static_x86_64_debug /tmp/core.bin -batch -ex "bt"
   ```
2. The backtrace showed `execute_nostr_list_manage` with corrupted frames.
3. Code review of the function identified the `cJSON_Delete(args)` call occurring
   before the last use of `action->valuestring`.

### Fix

Deferred `cJSON_Delete(args)` until after `action->valuestring` has been consumed
by `cJSON_AddStringToObject()`. The free now occurs immediately after the last use:

```c
cJSON_AddStringToObject(out, "action", action->valuestring);
cJSON_Delete(args);          /* ← moved here, after last use of action */
```

All early-return error paths before this point also received their own
`cJSON_Delete(args)` call to prevent leaks.

### How to recognise this class of bug in the future

- `SIGSEGV` with corrupted or nonsensical stack frames.
- Crash location reported by GDB does not match the actual buggy code — the
  corruption happened earlier.
- Any function that calls `cJSON_Delete()` on a parent object should be audited
  to ensure no child pointers are used afterwards.
- Pattern to watch for:
  ```c
  cJSON* child = cJSON_GetObjectItemCaseSensitive(parent, "key");
  /* ... */
  cJSON_Delete(parent);
  /* ... */
  use(child->valuestring);   /* BUG: child is dangling */
  ```

---

## 3. DM subscription not receiving incoming kind 4 events — INVESTIGATION (v0.0.24)

**Date**: 2026-03-02
**Version**: v0.0.24
**Severity**: High — agent does not respond to incoming DMs
**Status**: Under investigation — diagnostic logging added
**Files changed**: `src/nostr_handler.c`

### Symptoms

- The agent starts up normally, connects to relays, and can **send** kind 4 DMs.
- Incoming kind 4 messages posted to the same relays are never processed.
- No error messages appear in the log — the agent simply sits idle after sending
  its startup DM.
- The last log lines show successful outbound DM publishing but no inbound event
  processing.

### Possible root causes (under investigation)

1. **Events never reach `on_event()` callback** — The relay pool library only
   calls `on_event()` when it receives an `EVENT` message matching the
   subscription ID. If the subscription was silently closed, errored, or not
   re-established after a relay reconnect, no events would be delivered. The
   relay could also be rejecting the subscription filter.

2. **`since` filter timing mismatch** — `g_start_time` is set to `time(NULL)`
   during `nostr_handler_init()` (early in startup), but the DM subscription
   is created much later (after admin context subscription, startup event
   reconciliation, and startup DM sending). If the sender's clock is behind
   the server's clock, their `created_at` timestamp would be before
   `g_start_time` and the relay would filter them out.

3. **Sender tier filtering** — If the sender's pubkey doesn't match the admin
   pubkey and isn't in the WoT contact list, the message is classified as
   `DIDACTYL_SENDER_STRANGER` and silently dropped (only a DEBUG_LOG at
   level 4).

4. **Signature verification failure** — If `verify_signatures` is enabled and
   the event has an invalid signature, it is dropped with only a WARN log.

5. **Pool-level deduplication** — The relay pool's `is_event_seen()` cache is
   shared across all subscriptions. If an event ID was somehow marked as seen
   by another subscription, it would be silently dropped.

### Diagnostic logging added

TRACE-level (level 5 / `--debug 5`) logs were added at every decision point
in the `on_event()` callback and the `nostr_handler_subscribe_dms()` setup:

| Log message prefix | What it tells you |
|---|---|
| `DEBUG on_event ENTRY` | Callback was called — events are reaching didactyl |
| `DEBUG on_event NULL guard` | Event, config, or callback pointer is NULL |
| `DEBUG on_event: missing required fields` | Event JSON is malformed |
| `DEBUG on_event: kind=N id=... from=...` | Event kind, ID, and sender pubkey |
| `DEBUG on_event: ignoring non-kind4` | Event is not kind 4 |
| `DEBUG on_event: no p-tag found` | Kind 4 event has no `p` tag |
| `DEBUG on_event: p-tag mismatch` | `p` tag doesn't match agent's pubkey |
| `DEBUG on_event: sender=... tier=N` | Sender tier classification |
| `DEBUG on_eose called` | EOSE received from relays |
| `DEBUG DM subscription filter` | Full subscription filter JSON |
| `DEBUG DM subscription g_start_time=...` | `since` timestamp and time delta |
| `DEBUG DM subscription sub=...` | Subscription pointer (confirms creation) |

### How to use the diagnostic logs

1. Build with `./build_static.sh --debug`
2. Run with `--debug 5` to enable TRACE output
3. Send a kind 4 DM to the agent
4. Check the output:
   - **No `on_event ENTRY` lines** → problem is at the relay/subscription level
   - **`on_event ENTRY` appears but processing stops** → the specific drop-point
     log identifies the exact filter that rejected the event
5. Check the `DM subscription filter` log to verify the `since` timestamp and
   `#p` tag are correct

### How to recognise this class of bug in the future

- Agent can send but not receive — asymmetric connectivity.
- No error messages in the log — silent event filtering.
- The subscription filter (`since`, `#p`, `kinds`) may not match what the
  sender is actually publishing.
- Clock skew between sender and receiver can cause `since` filter mismatches.
- Relay reconnections may not automatically re-subscribe.

---

## General debugging checklist

When investigating a crash in Didactyl, work through these steps:

1. **Check for coredumps**: `ls /var/lib/systemd/coredump/ | grep didactyl`
2. **Check dmesg/journalctl**: `dmesg | grep -i 'didactyl\|oom\|segfault'`
3. **Check for the shutdown log line**: If `[didactyl] shutting down` is missing,
   the process was killed by a signal that bypassed the handler.
4. **Check ulimit**: `ulimit -c` — if 0, coredumps are disabled.
5. **Extract and analyse coredumps**:
   ```bash
   zstd -d /var/lib/systemd/coredump/core.didactyl*.zst -o /tmp/core.bin
   gdb ./didactyl_static_x86_64_debug /tmp/core.bin -batch -ex "bt full"
   ```
6. **Common silent killers**:
   - `SIGPIPE` — process writes to a broken socket/pipe
   - `SIGKILL` — OOM killer or external kill
   - `SIGBUS` — memory-mapped file issues
7. **Common crash causes**:
   - Use-after-free on cJSON child pointers after parent deletion
   - Buffer overflows in fixed-size stack buffers
   - NULL pointer dereference on failed allocations
   - Re-entrancy in callbacks during internal `nostr_relay_pool_poll()` calls
