unix/dgram: return EAGAIN instead of ENOBUFS when O_NONBLOCK set

[glebius]

Summary

This is behavior what some programs expect and what Linux does. For
example nginx expects EAGAIN when sending messages to /var/run/log,
which it connects to with O_NONBLOCK. Particularly with nginx the
problem is magnified by the fact that a ENOBUFS on send(2) is also
logged, so situation creates a log-bomb - a failed log message
triggers another log message.

[markj]

I think this is probably ok. EAGAIN is the usual error value for this case. I thought this would break our syslog(3), which explicitly checks for ENOBUFS, but it uses a blocking socket.

sys/kern/uipc_usrreq.c

cap_openlog(cap_channel_t *chan, const char *ident, int logopt, int facility)
 
         nvl = nvlist_create(0);
         nvlist_add_string(nvl, "cmd", "openlog");
         if (ident != NULL) {
                 nvlist_add_string(nvl, "ident", ident);
         }
         nvlist_add_number(nvl, "logopt", logopt);
         nvlist_add_number(nvl, "facility", facility);
+        if (logopt & LOG_PERROR) {
+                nvlist_add_descriptor(nvl, "stderr", STDERR_FILENO);
+        }
         nvl = cap_xfer_nvlist(chan, nvl);
         if (nvl == NULL) {
                 return;
         }
         nvlist_destroy(nvl);
 }
 
 void
cap_setlogmask(cap_channel_t *chan, int maskpri)
         return (omask);
 }
 
 /*
  * Service functions.
  */
 
 static char *LogTag;
+static int prev_stderr = -1;
 
 static void
 slog_vsyslog(const nvlist_t *limits __unused, const nvlist_t *nvlin,
     nvlist_t *nvlout __unused)
 {
 
         syslog(nvlist_get_number(nvlin, "priority"), "%s",
             nvlist_get_string(nvlin, "message"));
 }
 
 static void
 slog_openlog(const nvlist_t *limits __unused, const nvlist_t *nvlin,
     nvlist_t *nvlout __unused)
 {
         const char *ident;
+        uint64_t logopt;
+        int stderr_fd;
 
         ident = dnvlist_get_string(nvlin, "ident", NULL);
         if (ident != NULL) {
                 free(LogTag);
                 LogTag = strdup(ident);
         }
 
-        openlog(LogTag, nvlist_get_number(nvlin, "logopt"),
-            nvlist_get_number(nvlin, "facility"));
+        logopt = nvlist_get_number(nvlin, "logopt");
+        if (logopt & LOG_PERROR) {
+                stderr_fd = dnvlist_get_descriptor(nvlin, "stderr", -1);
+                if (prev_stderr == -1)
+                        prev_stderr = dup(STDERR_FILENO);
+                if (prev_stderr != -1)
+                        (void)dup2(stderr_fd, STDERR_FILENO);
+        } else if (prev_stderr != -1) {
+                (void)dup2(prev_stderr, STDERR_FILENO);
+                close(prev_stderr);
+                prev_stderr = -1;
         }
+        openlog(LogTag, logopt, nvlist_get_number(nvlin, "facility"));
+}
 
 static void
 slog_closelog(const nvlist_t *limits __unused, const nvlist_t *nvlin __unused,
     nvlist_t *nvlout __unused)
 {
 
         closelog();
 
         free(LogTag);
         LogTag = NULL;
+
+        if (prev_stderr != -1) {
+                (void)dup2(prev_stderr, STDERR_FILENO);
+                close(prev_stderr);
+                prev_stderr = -1;
         }
+}
 
 static void
 slog_setlogmask(const nvlist_t *limits __unused, const nvlist_t *nvlin,
     nvlist_t *nvlout)
 {
         int omask;
 
         omask = setlogmask(nvlist_get_number(nvlin, "maskpri"));
if (strcmp(cmd, "vsyslog") == 0) {
                 slog_setlogmask(limits, nvlin, nvlout);
         } else {
                 return (EINVAL);
         }
 
         return (0);
 }
 
-CREATE_SERVICE("system.syslog", NULL, syslog_command, CASPER_SERVICE_STDIO);
+CREATE_SERVICE("system.syslog", NULL, syslog_command, 0);