From 2dbbc4c36f223f87b88b16f8c3dd473fa509295b Mon Sep 17 00:00:00 2001
From: Maria Matejka <mq@ucw.cz>
Date: Thu, 24 Aug 2023 17:00:54 +0200
Subject: [PATCH] Logging: fixed size logfiles behaving as mmapped ringbuffers

This variant of logging avoids calling write() for every log line,
allowing for waitless logging. This makes heavy logging less heavy
and more useful for race condition debugging.
---
 doc/bird.sgml        |  5 +++--
 sysdep/unix/config.Y | 10 +++++++++-
 sysdep/unix/io.c     | 45 ++++++++++++++++++++++++++++++++++++++++++--
 sysdep/unix/unix.h   |  3 ++-
 4 files changed, 57 insertions(+), 6 deletions(-)

diff --git a/doc/bird.sgml b/doc/bird.sgml
index 7c8e9895d..0da616c70 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -471,7 +471,7 @@ ipv6 table
 include "tablename.conf";;
 </code>
 
-	<tag><label id="opt-log">log "<m/filename/" [<m/limit/ "<m/backup/"] | syslog [name <m/name/] | stderr all|{ <m/list of classes/ }</tag>
+	<tag><label id="opt-log">log "<m/filename/" [<m/limit/ "<m/backup/"] | fixed "<m/filename/" <m/size/ | syslog [name <m/name/] | stderr all|{ <m/list of classes/ }</tag>
 	Set logging of messages having the given class (either <cf/all/ or <cf>{
 	error|trace [, <m/.../] }</cf> etc.) into selected destination - a file
 	specified as a filename string (with optional log rotation information),
@@ -488,7 +488,8 @@ include "tablename.conf";;
 	Logging directly to file supports basic log rotation -- there is an
 	optional log file limit and a backup filename, when log file reaches the
 	limit, the current log file is renamed to the backup filename and a new
-	log file is created.
+	log file is created. It's also possible to log to a single file behaving
+	as a ring buffer with a fixed size.
 
 	You may specify more than one <cf/log/ line to establish logging to
 	multiple destinations. Default: log everything to the system log, or
diff --git a/sysdep/unix/config.Y b/sysdep/unix/config.Y
index a9feec021..a4aed8474 100644
--- a/sysdep/unix/config.Y
+++ b/sysdep/unix/config.Y
@@ -19,7 +19,7 @@ CF_DECLS
 
 CF_KEYWORDS(LOG, SYSLOG, ALL, DEBUG, TRACE, INFO, REMOTE, WARNING, ERROR, AUTH, FATAL, BUG, STDERR, SOFT)
 CF_KEYWORDS(NAME, CONFIRM, UNDO, CHECK, TIMEOUT, DEBUG, LATENCY, LIMIT, WATCHDOG, WARNING, STATUS)
-CF_KEYWORDS(GRACEFUL, RESTART)
+CF_KEYWORDS(GRACEFUL, RESTART, FIXED)
 
 %type <i> log_mask log_mask_list log_cat cfg_timeout
 %type <t> cfg_name
@@ -57,6 +57,14 @@ log_file:
      }
      this_log->filename = $1;
    }
+ | FIXED text expr {
+     if (!parse_and_exit)
+     {
+       this_log->rf = rf_open(new_config->pool, $2, RF_FIXED, this_log->limit = $3);
+       if (!this_log->rf) cf_error("Unable to open log file '%s': %m", $2);
+     }
+     this_log->filename = $2;
+   }
  | SYSLOG syslog_name { this_log->rf = NULL; new_config->syslog_name = $2; }
  | STDERR { this_log->rf = &rf_stderr; }
  ;
diff --git a/sysdep/unix/io.c b/sysdep/unix/io.c
index faf759a58..6076d3f4b 100644
--- a/sysdep/unix/io.c
+++ b/sysdep/unix/io.c
@@ -16,6 +16,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
+#include <sys/mman.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -69,6 +70,7 @@ struct rfile {
   int fd;
   off_t limit;
   _Atomic off_t pos;
+  void *mapping;
 };
 
 struct rfile rf_stderr = {
@@ -80,6 +82,9 @@ rf_free(resource *r)
 {
   struct rfile *a = (struct rfile *) r;
 
+  if (a->mapping)
+    munmap(a->mapping, a->limit);
+
   close(a->fd);
 }
 
@@ -112,6 +117,10 @@ rf_open_get_fd(const char *name, enum rf_mode mode)
       flags = O_WRONLY | O_CREAT | O_APPEND;
       break;
 
+    case RF_FIXED:
+      flags = O_RDWR | O_CREAT;
+      break;
+
     default:
       bug("rf_open() must have the mode set");
   }
@@ -145,6 +154,19 @@ rf_open(pool *p, const char *name, enum rf_mode mode, off_t limit)
       r->pos = S_ISREG(r->stat.st_mode) ? r->stat.st_size : 0;
       break;
 
+    case RF_FIXED:
+      if ((ftruncate(fd, limit) < 0)
+	  || ((r->mapping = mmap(NULL, limit, PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED))
+      {
+	bug("Failed: %m");
+	int erf = errno;
+	r->mapping = NULL;
+	rfree(r);
+	errno = erf;
+	return NULL;
+      }
+      break;
+
     default:
       bug("rf_open() must have the mode set");
   }
@@ -165,15 +187,34 @@ rf_same(struct rfile *a, struct rfile *b)
   rf_stat(b);
 
   return
+    (a->limit == b->limit) &&
     (a->stat.st_mode == b->stat.st_mode) &&
     (a->stat.st_dev == b->stat.st_dev) &&
     (a->stat.st_ino == b->stat.st_ino);
 }
 
 int
-rf_write(struct rfile *r, const void *buf, size_t count)
+rf_write(struct rfile *r, const void *buf, size_t _count)
 {
-  if (r->limit && (atomic_fetch_add_explicit(&r->pos, count, memory_order_relaxed) + (off_t) count > r->limit))
+  off_t count = _count;
+
+  if (r->mapping)
+  {
+    /* Update the pointer */
+    off_t target = atomic_fetch_add_explicit(&r->pos, count, memory_order_relaxed) % r->limit;
+
+    /* Take care of wrapping */
+    if (target + count > r->limit)
+    {
+      memcpy(r->mapping, buf + (r->limit - target), target + count - r->limit);
+      count = r->limit - target;
+    }
+
+    /* Write the line */
+    memcpy(r->mapping + target, buf, count);
+    return 1;
+  }
+  else if (r->limit && (atomic_fetch_add_explicit(&r->pos, count, memory_order_relaxed) + count > r->limit))
   {
     atomic_fetch_sub_explicit(&r->pos, count, memory_order_relaxed);
     return 0;
diff --git a/sysdep/unix/unix.h b/sysdep/unix/unix.h
index f193a8db8..7e1ef97c9 100644
--- a/sysdep/unix/unix.h
+++ b/sysdep/unix/unix.h
@@ -116,7 +116,8 @@ void io_log_dump(void);
 int sk_open_unix(struct birdsock *s, struct birdloop *, char *name);
 
 enum rf_mode {
-  RF_APPEND,
+  RF_APPEND = 1,
+  RF_FIXED,
 };
 
 struct rfile *rf_open(struct pool *, const char *name, enum rf_mode mode, off_t limit);
-- 
2.47.2