Fix modbus segfault / New strategy for connecting

The previous implementation had a reconnection loop which should not
really be necessary. Idea tried here is to check if connection is alive,
and establish it if not. Change was begun because previously connection
would only be attempted as a retry, so on first pass a NULL was handed to
libmodbus which caused segmentation faults reproducibly.

src/modbus.c

@@ -115,7 +115,6 @@ struct mb_host_s /* {{{ */
   modbus_t *connection;
 #endif
   _Bool is_connected;
-  _Bool have_reconnected;
 }; /* }}} */
 typedef struct mb_host_s mb_host_t;
 
@@ -260,6 +259,7 @@ static float mb_register_to_float (uint16_t hi, uint16_t lo) /* {{{ */
   union
   {
     uint8_t b[4];
+    uint16_t s[2];
     float f;
   } conv;
 
@@ -288,13 +288,6 @@ static int mb_init_connection (mb_host_t *host) /* {{{ */
   if (host == NULL)
     return (EINVAL);
 
-  if (host->is_connected)
-    return (0);
-
-  /* Only reconnect once per interval. */
-  if (host->have_reconnected)
-    return (-1);
-
   modbus_set_debug (&host->connection, 1);
 
   /* We'll do the error handling ourselves. */
@@ -319,7 +312,6 @@ static int mb_init_connection (mb_host_t *host) /* {{{ */
   }
 
   host->is_connected = 1;
-  host->have_reconnected = 1;
   return (0);
 } /* }}} int mb_init_connection */
 /* #endif LEGACY_LIBMODBUS */
@@ -336,10 +328,6 @@ static int mb_init_connection (mb_host_t *host) /* {{{ */
   if (host->connection != NULL)
     return (0);
 
-  /* Only reconnect once per interval. */
-  if (host->have_reconnected)
-    return (-1);
-
   if ((host->port < 1) || (host->port > 65535))
     host->port = MODBUS_TCP_DEFAULT_PORT;
 
@@ -349,7 +337,6 @@ static int mb_init_connection (mb_host_t *host) /* {{{ */
   host->connection = modbus_new_tcp (host->node, host->port);
   if (host->connection == NULL)
   {
-    host->have_reconnected = 1;
     ERROR ("Modbus plugin: Creating new Modbus/TCP object failed.");
     return (-1);
   }
@@ -369,7 +356,6 @@ static int mb_init_connection (mb_host_t *host) /* {{{ */
     return (status);
   }
 
-  host->have_reconnected = 1;
   return (0);
 } /* }}} int mb_init_connection */
 #endif /* !LEGACY_LIBMODBUS */
@@ -391,8 +377,9 @@ static int mb_read_data (mb_host_t *host, mb_slave_t *slave, /* {{{ */
   uint16_t values[2];
   int values_num;
   const data_set_t *ds;
+  struct sockaddr sockaddr;
+  socklen_t saddrlen = sizeof(sockaddr);
   int status;
-  int i;
 
   if ((host == NULL) || (slave == NULL) || (data == NULL))
     return (EINVAL);
@@ -429,6 +416,35 @@ static int mb_read_data (mb_host_t *host, mb_slave_t *slave, /* {{{ */
   else
     values_num = 1;
 
+  if(host->connection == NULL)
+    goto try_connect;
+
+  if(getpeername(modbus_get_socket(host->connection),
+                 &sockaddr, &saddrlen) != 0)
+    switch(errno){
+    case EBADF:
+    case ENOTSOCK:
+    case ENOTCONN:
+    try_connect:
+      status = mb_init_connection (host);
+      if (status != 0)
+      {
+        ERROR ("Modbus plugin: mb_init_connection (%s/%s) failed. ",
+           host->host, host->node);
+        host->is_connected = 0;
+        host->connection = NULL;
+        return (-1);
+      }
+    break;
+    default:
+#if LEGACY_LIBMODBUS
+      modbus_close (&host->connection);
+#else
+      modbus_close (host->connection);
+      modbus_free (host->connection);
+#endif
+  }
+
 #if LEGACY_LIBMODBUS
   /* Version 2.0.3: Pass the connection struct as a pointer and pass the slave
    * id to each call of "read_holding_registers". */
@@ -445,51 +461,15 @@ static int mb_read_data (mb_host_t *host, mb_slave_t *slave, /* {{{ */
   }
 #endif
 
-  for (i = 0; i < 2; i++)
-  {
-    status = modbus_read_registers (host->connection,
+  status = modbus_read_registers (host->connection,
         /* start_addr = */ data->register_base,
         /* num_registers = */ values_num, /* buffer = */ values);
-    if (status > 0)
-      break;
-
-    if (host->is_connected)
-    {
-#if LEGACY_LIBMODBUS
-      modbus_close (&host->connection);
-      host->is_connected = 0;
-#else
-      modbus_close (host->connection);
-      modbus_free (host->connection);
-      host->connection = NULL;
-#endif
-    }
-
-    /* If we already tried reconnecting this round, give up. */
-    if (host->have_reconnected)
-    {
-      ERROR ("Modbus plugin: modbus_read_registers (%s) failed. "
-          "Reconnecting has already been tried. Giving up.", host->host);
-      return (-1);
-    }
-
-    /* Maybe the device closed the connection during the waiting interval.
-     * Try re-establishing the connection. */
-    status = mb_init_connection (host);
-    if (status != 0)
-    {
-      ERROR ("Modbus plugin: modbus_read_registers (%s) failed. "
-          "While trying to reconnect, connecting to \"%s\" failed. "
-          "Giving up.",
-          host->host, host->node);
-      return (-1);
-    }
-
-    DEBUG ("Modbus plugin: Re-established connection to %s", host->host);
-
-    /* try again */
-    continue;
-  } /* for (i = 0, 1) */
+  if (status != values_num)
+  {
+    ERROR ("Modbus plugin: modbus_read_registers (%s) failed. "
+        "Giving up.", host->host);
+    return (-1);
+  }
 
   DEBUG ("Modbus plugin: mb_read_data: Success! "
       "modbus_read_registers returned with status %i.", status);
@@ -602,9 +582,6 @@ static int mb_read (user_data_t *user_data) /* {{{ */
 
   host = user_data->data;
 
-  /* Clear the reconnect flag. */
-  host->have_reconnected = 0;
-
   success = 0;
   for (i = 0; i < host->slaves_num; i++)
   {