storage: zfs: flexible use of 'volmode' option

There are slight differences in various ZFS implementations.
Specifically, ZFS on FreeBSD requires to set value of 'volmode'
option to 'dev' to expose volumes as raw disk device (that's what
we need) rather than geom provides, for example.

With ZFS on Linux, however, such option is not available and
volumes exposed like we need by default.

To make our implementation more flexible, only pass 'volmode'
when it's supported. Support is checked by parsing usage
information of the 'zfs get' command.

src/storage/storage_backend_zfs.c

@@ -39,6 +39,47 @@ VIR_LOG_INIT("storage.storage_backend_zfs");
  *       for size, show just a number instead of 2G etc
  */
 
+/**
+ * virStorageBackendZFSVolModeNeeded:
+ *
+ * Checks if it's necessary to specify 'volmode' (i.e. that
+ * we're working with BSD ZFS implementation).
+ *
+ * Returns 1 if 'volmode' is need, 0 if not needed, -1 on error
+ */
+static int
+virStorageBackendZFSVolModeNeeded(void)
+{
+    virCommandPtr cmd = NULL;
+    int ret = -1, exit = -1;
+    char *error = NULL;
+
+    /* 'zfs get' without arguments prints out
+     * usage information to stderr, including
+     * list of supported options, and exits with
+     * exit code 2
+     */
+    cmd = virCommandNewArgList(ZFS, "get", NULL);
+    virCommandAddEnvString(cmd, "LC_ALL=C");
+    virCommandSetErrorBuffer(cmd, &error);
+
+    ret = virCommandRun(cmd, &exit);
+    if ((ret < 0) || (exit != 2)) {
+        VIR_WARN("Command 'zfs get' either failed "
+                 "to run or exited with unexpected status");
+        goto cleanup;
+    }
+
+    if (strstr(error, " volmode "))
+        ret = 1;
+    else
+        ret = 0;
+
+ cleanup:
+    virCommandFree(cmd);
+    VIR_FREE(error);
+    return ret;
+}
 
 static int
 virStorageBackendZFSCheckPool(virStoragePoolObjPtr pool ATTRIBUTE_UNUSED,
@@ -258,6 +299,7 @@ virStorageBackendZFSCreateVol(virConnectPtr conn ATTRIBUTE_UNUSED,
 {
     virCommandPtr cmd = NULL;
     int ret = -1;
+    int volmode_needed = -1;
 
     vol->type = VIR_STORAGE_VOL_BLOCK;
 
@@ -273,6 +315,9 @@ virStorageBackendZFSCreateVol(virConnectPtr conn ATTRIBUTE_UNUSED,
     if (VIR_STRDUP(vol->key, vol->target.path) < 0)
         goto cleanup;
 
+    volmode_needed = virStorageBackendZFSVolModeNeeded();
+    if (volmode_needed < 0)
+        goto cleanup;
     /**
      * $ zfs create -o volmode=dev -V 10240K test/volname
      *
@@ -281,8 +326,10 @@ virStorageBackendZFSCreateVol(virConnectPtr conn ATTRIBUTE_UNUSED,
      *                   will lookup vfs.zfs.vol.mode sysctl value
      * -V -- tells to create a volume with the specified size
      */
-    cmd = virCommandNewArgList(ZFS, "create", "-o", "volmode=dev",
-                               "-V", NULL);
+    cmd = virCommandNewArgList(ZFS, "create", NULL);
+    if (volmode_needed)
+        virCommandAddArgList(cmd, "-o", "volmode=dev", NULL);
+    virCommandAddArg(cmd, "-V");
     virCommandAddArgFormat(cmd, "%lluK",
                            VIR_DIV_UP(vol->target.capacity, 1024));
     virCommandAddArgFormat(cmd, "%s/%s",