Exim Observer Plugin

Experimental native C integration for Exim Observer. The first milestone provides source analysis, a loadable misc module, a generic native event hook, a fail-open Unix socket handshake, and a dual-version build/integration harness.

The inspected targets are Exim 4.99.5 and 4.100.1. This is not yet a production observer: tracking headers, application event serialization, the control listener, queue operations and message inspection are not implemented. The module advertises zero application capabilities. See compatibility.

Exim → native C plugin → Exim Observer Protocol → separate observer agent. No Go runtime, HTTP service, database, WAL, or central processing runs inside Exim. The protocol codec is a reviewed external C dependency, not a private wire format.

Development

Place this checkout beside exim-observer-protocol at the revision recorded in protocol.lock.json. Install the tools listed in building. The provided read-only references are expected under the protocol checkout's .tools; set EXIM_SOURCE_ROOT to change their location.

make test                 # local unit tests and shared protocol vectors
make check                # formatting, analysis, tests and patch application
make build integration EXIM_VERSION=4.99.5 CC=gcc
make build integration EXIM_VERSION=4.100.1 CC=clang
make check-all            # full checks, both releases and both compilers
make test-compilers       # unit/module warnings as errors and SMTP integration
make test-sanitize        # standalone ASan + UBSan

Ordinary builds and tests do not download anything. For a new developer/CI source cache, explicitly run make fetch-sources EXIM_SOURCE_ROOT=.tools. Archives and complete extracted manifests are verified before use. Supplied references are never modified; patches apply to disposable copies.

The native loader requires observer_miscmod.so, exporting observer_module_info. build/<version>-<compiler>/exim-observer.so is an alias. Each binary is specific to the patched Exim build and its generated headers.

Current behavior

observer_enabled defaults to false. Configuring it loads the module through Exim's native module option mechanism. observer_events_socket defaults to /run/exim-observer/events.sock. When enabled, callbacks attempt a nonblocking SOCK_SEQPACKET HELLO/HELLO_ACK handshake using protocol draft 0.1. Failure never changes an SMTP result. There is no listener in this milestone: the agent owns the event socket, and no control socket is created yet.

Native event_raise calls notify the module even without event_action. With -d+load, the module logs native event names only. Existing event_action expansion continues unchanged. Some upstream call sites have additional guards, documented in events; this is not complete lifecycle coverage.

References

• Exact source analysis and required patch sites
• Architecture, protocol boundary, security, contribution process
• Exim Observer Protocol
• Exim Observer

Python is used only for build/test tooling. Runtime plugin and codec code are C.