qemu-nbd: Improve error reporting

- use err(3) instead of errx(3) if errno is available
  to report why failed
- let fail prior to daemon(3) if opening a nbd file
  is likely to fail after daemonizing to avoid silent
  failure exit
- add missing 'ret = 1' when unix_socket_outgoing failed
Signed-off-by: NRyota Ozaki <ozaki.ryota@gmail.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qemu-nbd.c

@@ -112,9 +112,12 @@ static int find_partition(BlockDriverState *bs, int partition,
     uint8_t data[512];
     int i;
     int ext_partnum = 4;
+    int ret;
 
-    if (bdrv_read(bs, 0, data, 1))
-        errx(EXIT_FAILURE, "error while reading");
+    if ((ret = bdrv_read(bs, 0, data, 1)) < 0) {
+        errno = -ret;
+        err(EXIT_FAILURE, "error while reading");
+    }
 
     if (data[510] != 0x55 || data[511] != 0xaa) {
         errno = -EINVAL;
@@ -132,8 +135,10 @@ static int find_partition(BlockDriverState *bs, int partition,
             uint8_t data1[512];
             int j;
 
-            if (bdrv_read(bs, mbr[i].start_sector_abs, data1, 1))
-                errx(EXIT_FAILURE, "error while reading");
+            if ((ret = bdrv_read(bs, mbr[i].start_sector_abs, data1, 1)) < 0) {
+                errno = -ret;
+                err(EXIT_FAILURE, "error while reading");
+            }
 
             for (j = 0; j < 4; j++) {
                 read_partition(&data1[446 + 16 * j], &ext[j]);
@@ -316,7 +321,7 @@ int main(int argc, char **argv)
     if (disconnect) {
         fd = open(argv[optind], O_RDWR);
         if (fd == -1)
-            errx(EXIT_FAILURE, "Cannot open %s", argv[optind]);
+            err(EXIT_FAILURE, "Cannot open %s", argv[optind]);
 
         nbd_disconnect(fd);
 
@@ -333,23 +338,30 @@ int main(int argc, char **argv)
     if (bs == NULL)
         return 1;
 
-    if (bdrv_open(bs, argv[optind], flags, NULL) < 0)
-        return 1;
+    if ((ret = bdrv_open(bs, argv[optind], flags, NULL)) < 0) {
+        errno = -ret;
+        err(EXIT_FAILURE, "Failed to bdrv_open '%s'", argv[optind]);
+    }
 
     fd_size = bs->total_sectors * 512;
 
     if (partition != -1 &&
         find_partition(bs, partition, &dev_offset, &fd_size))
-        errx(EXIT_FAILURE, "Could not find partition %d", partition);
+        err(EXIT_FAILURE, "Could not find partition %d", partition);
 
     if (device) {
         pid_t pid;
         int sock;
 
+        /* want to fail before daemonizing */
+        if (access(device, R_OK|W_OK) == -1) {
+            err(EXIT_FAILURE, "Could not access '%s'", device);
+        }
+
         if (!verbose) {
             /* detach client and server */
             if (daemon(0, 0) == -1) {
-                errx(EXIT_FAILURE, "Failed to daemonize");
+                err(EXIT_FAILURE, "Failed to daemonize");
             }
         }
 
@@ -372,8 +384,10 @@ int main(int argc, char **argv)
             do {
                 sock = unix_socket_outgoing(socket);
                 if (sock == -1) {
-                    if (errno != ENOENT && errno != ECONNREFUSED)
+                    if (errno != ENOENT && errno != ECONNREFUSED) {
+                        ret = 1;
                         goto out;
+                    }
                     sleep(1);	/* wait children */
                 }
             } while (sock == -1);