Add log_filter: pluggable per-unit output filter subprocess

Each unit can specify log_filter: <path> pointing to any executable
that reads stdin and writes filtered output to stdout. The daemon
spawns it once at startup (and on SIGHUP), pipes raw log data through
it before ring-buffering and broadcasting to attached clients.

filter.c handles spawn (fork/exec with pipes), per-chunk apply
(write + poll with 250ms timeout, pass-through on silence/timeout),
and stop (SIGTERM + fd cleanup). Filters are stopped and restarted
cleanly on SIGHUP alongside log_tail.

Bundled filters in filters/:
  source.py    — TF2, GMod: strips server_cvar/stuck/path_goal spam,
                 strips the L MM/DD/YYYY - HH:MM:SS: prefix
  minecraft.py — vanilla/Paper/Spigot: strips keepAlive, autosave,
                 internal class logs, strips [HH:MM:SS] [thread] prefix
  terraria.py  — vanilla/tModLoader: strips blank lines and mod
                 loading noise during startup

Any executable reading stdin/writing stdout works as a custom filter.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 25 insertions, 0 deletions

diff --git a/src/filter.h b/src/filter.h
new file mode 100644
index 0000000..60a176f
--- /dev/null
+++ b/src/filter.h
@@ -0,0 +1,25 @@
+#ifndef FILTER_H
+#define FILTER_H
+
+#include "umbrella.h"
+
+/*
+ * filter_start  — spawn the unit's log_filter executable as a persistent
+ *                 subprocess with stdin/stdout pipes. No-op if log_filter
+ *                 is empty. Safe to call on every unit unconditionally.
+ *
+ * filter_apply  — write buf to the filter's stdin, read back the
+ *                 transformed output into buf. Updates *len. If the filter
+ *                 suppresses all output (returns 0 bytes), *len is set to 0.
+ *                 Falls back to pass-through on timeout or error.
+ *
+ * filter_stop   — signal and close the filter subprocess for one unit.
+ * filter_stop_all — stop filters for all loaded units.
+ */
+
+void filter_start(Unit *u);
+void filter_apply(Unit *u, char *buf, int bufsize, int *len);
+void filter_stop(Unit *u);
+void filter_stop_all(void);
+
+#endif /* FILTER_H */