at_send_command: call write(2) in a loop

write(2) might return less than expected due to various reasons.
Therefore, unless we are dealing with a critical error, it must be
called in a loop until all bytes are written.

src/at.c

@@ -9,6 +9,7 @@
 #include "gpio.h"
 #include "gnss.h"
 
+#include <errno.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -50,7 +51,7 @@ gboolean at_send_command(struct EG25Manager *manager)
 {
     char command[256];
     struct AtCommand *at_cmd = manager->at_cmds ? g_list_nth_data(manager->at_cmds, 0) : NULL;
-    int ret, len = 0;
+    int ret, len = 0, pos = 0;
 
     if (at_cmd) {
     	/* Wake up the modem from soft sleep before sending an AT command */
@@ -82,9 +83,27 @@ gboolean at_send_command(struct EG25Manager *manager)
 
         manager->at_callback = at_cmd->callback;
 
-        ret = write(manager->at_fd, command, len);
-        if (ret < len)
-            g_warning("Couldn't write full AT command: wrote %d/%d bytes", ret, len);
+        do {
+            ret = write(manager->at_fd, &command[pos], len);
+
+            if (ret < 0) {
+                switch (errno) {
+                case EAGAIN:
+                case EINTR:
+                    /* Try again. */
+                    break;
+
+                default:
+                    g_warning("write(2): %s", strerror(errno));
+                    at_next_command(manager);
+                    return FALSE;
+                }
+            }
+            else {
+                len -= ret;
+                pos += ret;
+            }
+        } while (len > 0);
 
         g_message("Sending command: %s", g_strstrip(command));
     } else {