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lib: AgentX: use threads instead of eventloop hack

AgentX fd/timeout handling is rather hackishly monkeyed into thread.c.
Replace with code that uses plain thread_* functions.

NB: Net-SNMP's API rivals Quagga's in terms of age and absence of
documentation.  netsnmp_check_outstanding_agent_requests() in particular
seems to be unused and is therefore untested.

The most useful documentation on this is actually the blog post Vincent
Bernat wrote when he originally integrated this into lldpd and Quagga:
https://vincent.bernat.im/en/blog/2012-snmp-event-loop.html

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
David Lamparter 3 years ago
parent
commit
cfb4826340
2 changed files with 104 additions and 50 deletions
  1. 103 1
      lib/agentx.c
  2. 1 49
      lib/thread.c

+ 103 - 1
lib/agentx.c

@@ -24,11 +24,108 @@
 #if defined HAVE_SNMP && defined SNMP_AGENTX
 #include <net-snmp/net-snmp-config.h>
 #include <net-snmp/net-snmp-includes.h>
+#include <net-snmp/agent/net-snmp-agent-includes.h>
+#include <net-snmp/agent/snmp_vars.h>
 
 #include "command.h"
 #include "smux.h"
 
-int agentx_enabled = 0;
+static int agentx_enabled = 0;
+
+static struct thread_master *agentx_tm;
+static struct thread *timeout_thr = NULL;
+static struct list *events = NULL;
+
+static void agentx_events_update(void);
+
+static int
+agentx_timeout(struct thread *t)
+{
+  timeout_thr = NULL;
+
+  snmp_timeout ();
+  run_alarms ();
+  netsnmp_check_outstanding_agent_requests ();
+  agentx_events_update ();
+  return 0;
+}
+
+static int
+agentx_read(struct thread *t)
+{
+  fd_set fds;
+  struct listnode *ln = THREAD_ARG (t);
+  list_delete_node (events, ln);
+
+  FD_ZERO (&fds);
+  FD_SET (THREAD_FD (t), &fds);
+  snmp_read (&fds);
+
+  netsnmp_check_outstanding_agent_requests ();
+  agentx_events_update ();
+  return 0;
+}
+
+static void
+agentx_events_update(void)
+{
+  int maxfd = 0;
+  int block = 1;
+  struct timeval timeout = { .tv_sec = 0, .tv_usec = 0 };
+  fd_set fds;
+  struct listnode *ln;
+  struct thread *thr;
+  int fd, thr_fd;
+
+  THREAD_OFF (timeout_thr);
+
+  FD_ZERO (&fds);
+  snmp_select_info (&maxfd, &fds, &timeout, &block);
+
+  if (!block)
+    timeout_thr = thread_add_timer_tv (agentx_tm, agentx_timeout, NULL, &timeout);
+
+  ln = listhead (events);
+  thr = ln ? listgetdata (ln) : NULL;
+  thr_fd = thr ? THREAD_FD (thr) : -1;
+
+  /* "two-pointer" / two-list simultaneous iteration
+   * ln/thr/thr_fd point to the next existing event listener to hit while
+   * fd counts to catch up */
+  for (fd = 0; fd < maxfd; fd++)
+    {
+      /* caught up */
+      if (thr_fd == fd)
+        {
+          struct listnode *nextln = listnextnode (ln);
+          if (!FD_ISSET (fd, &fds))
+            {
+              thread_cancel (thr);
+              list_delete_node (events, ln);
+            }
+          ln = nextln;
+          thr = ln ? listgetdata (ln) : NULL;
+          thr_fd = thr ? THREAD_FD (thr) : -1;
+        }
+      /* need listener, but haven't hit one where it would be */
+      else if (FD_ISSET (fd, &fds))
+        {
+          struct listnode *newln;
+          thr = thread_add_read (agentx_tm, agentx_read, NULL, fd);
+          newln = listnode_add_before (events, ln, thr);
+          thr->arg = newln;
+        }
+    }
+
+  /* leftover event listeners at this point have fd > maxfd, delete them */
+  while (ln)
+    {
+      struct listnode *nextln = listnextnode (ln);
+      thread_cancel (listgetdata (ln));
+      list_delete_node (events, ln);
+      ln = nextln;
+    }
+}
 
 /* AgentX node. */
 static struct cmd_node agentx_node =
@@ -77,6 +174,8 @@ DEFUN (agentx_enable,
   if (!agentx_enabled)
     {
       init_snmp("quagga");
+      events = list_new();
+      agentx_events_update ();
       agentx_enabled = 1;
       return CMD_SUCCESS;
     }
@@ -99,6 +198,8 @@ DEFUN (no_agentx,
 void
 smux_init (struct thread_master *tm)
 {
+  agentx_tm = tm;
+
   netsnmp_enable_subagent ();
   snmp_disable_log ();
   snmp_enable_calllog ();
@@ -207,6 +308,7 @@ smux_trap (struct variable *vp, size_t vp_len,
 
   send_v2trap (notification_vars);
   snmp_free_varbind (notification_vars);
+  agentx_events_update ();
   return 1;
 }
 

+ 1 - 49
lib/thread.c

@@ -32,15 +32,6 @@
 #include "command.h"
 #include "sigevent.h"
 
-#if defined HAVE_SNMP && defined SNMP_AGENTX
-#include <net-snmp/net-snmp-config.h>
-#include <net-snmp/net-snmp-includes.h>
-#include <net-snmp/agent/net-snmp-agent-includes.h>
-#include <net-snmp/agent/snmp_vars.h>
-
-extern int agentx_enabled;
-#endif
-
 #if defined(__APPLE__)
 #include <mach/mach.h>
 #include <mach/mach_time.h>
@@ -1174,12 +1165,7 @@ thread_fetch (struct thread_master *m, struct thread *fetch)
   while (1)
     {
       int num = 0;
-#if defined HAVE_SNMP && defined SNMP_AGENTX
-      struct timeval snmp_timer_wait;
-      int snmpblock = 0;
-      int fdsetsize;
-#endif
-      
+
       /* Signals pre-empt everything */
       quagga_sigevent_process ();
        
@@ -1214,26 +1200,6 @@ thread_fetch (struct thread_master *m, struct thread *fetch)
             timer_wait = timer_wait_bg;
         }
       
-#if defined HAVE_SNMP && defined SNMP_AGENTX
-      /* When SNMP is enabled, we may have to select() on additional
-	 FD. snmp_select_info() will add them to `readfd'. The trick
-	 with this function is its last argument. We need to set it to
-	 0 if timer_wait is not NULL and we need to use the provided
-	 new timer only if it is still set to 0. */
-      if (agentx_enabled)
-        {
-          fdsetsize = FD_SETSIZE;
-          snmpblock = 1;
-          if (timer_wait)
-            {
-              snmpblock = 0;
-              memcpy(&snmp_timer_wait, timer_wait, sizeof(struct timeval));
-            }
-          snmp_select_info(&fdsetsize, &readfd, &snmp_timer_wait, &snmpblock);
-          if (snmpblock == 0)
-            timer_wait = &snmp_timer_wait;
-        }
-#endif
       num = fd_select (FD_SETSIZE, &readfd, &writefd, &exceptfd, timer_wait);
       
       /* Signals should get quick treatment */
@@ -1245,20 +1211,6 @@ thread_fetch (struct thread_master *m, struct thread *fetch)
           return NULL;
         }
 
-#if defined HAVE_SNMP && defined SNMP_AGENTX
-      if (agentx_enabled)
-        {
-          if (num > 0)
-            snmp_read(&readfd);
-          else if (num == 0)
-            {
-              snmp_timeout();
-              run_alarms();
-            }
-          netsnmp_check_outstanding_agent_requests();
-        }
-#endif
-
       /* Check foreground timers.  Historically, they have had higher
          priority than I/O threads, so let's push them onto the ready
 	 list in front of the I/O threads. */