Clarify semantic of nparams argument of virDomainGetSchedulerType

Some drivers assumed it can be NULL (e.g. qemu and lxc) and check it
before assigning to it, other drivers assumed it must be non-NULL
(e.g. test and esx) and just assigned to it.

Unify this to nparams being optional and document it.

src/esx/esx_driver.c

@@ -3513,7 +3513,9 @@ esxDomainGetSchedulerType(virDomainPtr domain ATTRIBUTE_UNUSED, int *nparams)
         return NULL;
     }
 
-    *nparams = 3; /* reservation, limit, shares */
+    if (nparams != NULL) {
+        *nparams = 3; /* reservation, limit, shares */
+    }
 
     return type;
 }

src/libvirt.c

@@ -4973,9 +4973,10 @@ error:
 /**
  * virDomainGetSchedulerType:
  * @domain: pointer to domain object
- * @nparams: number of scheduler parameters(return value)
+ * @nparams: pointer to number of scheduler parameters, can be NULL
+ *           (return value)
  *
- * Get the scheduler type.
+ * Get the scheduler type and the number of scheduler parameters.
  *
  * Returns NULL in case of error. The caller must free the returned string.
  */

src/libxl/libxl_driver.c

@@ -2396,14 +2396,16 @@ libxlDomainGetSchedulerType(virDomainPtr dom, int *nparams)
         goto cleanup;
     }
 
-    *nparams = 0;
+    if (nparams)
+        *nparams = 0;
     switch(sched_id) {
     case XEN_SCHEDULER_SEDF:
         ret = strdup("sedf");
         break;
     case XEN_SCHEDULER_CREDIT:
         ret = strdup("credit");
-        *nparams = XEN_SCHED_CREDIT_NPARAM;
+        if (nparams)
+            *nparams = XEN_SCHED_CREDIT_NPARAM;
         break;
     case XEN_SCHEDULER_CREDIT2:
         ret = strdup("credit2");

src/test/test_driver.c

@@ -2624,7 +2624,9 @@ static char *testDomainGetSchedulerType(virDomainPtr domain ATTRIBUTE_UNUSED,
 {
     char *type = NULL;
 
-    *nparams = 1;
+    if (nparams)
+        *nparams = 1;
+
     type = strdup("fair");
     if (!type)
         virReportOOMError();

src/xen/xend_internal.c

@@ -3531,8 +3531,7 @@ xenDaemonGetSchedulerType(virDomainPtr domain, int *nparams)
     const char *ret = NULL;
     char *schedulertype = NULL;
 
-    if ((domain == NULL) || (domain->conn == NULL) || (domain->name == NULL)
-        || (nparams == NULL)) {
+    if (domain->conn == NULL || domain->name == NULL) {
         virXendError(VIR_ERR_INVALID_ARG, __FUNCTION__);
         return NULL;
     }
@@ -3562,14 +3561,16 @@ xenDaemonGetSchedulerType(virDomainPtr domain, int *nparams)
             virReportOOMError();
             goto error;
         }
-        *nparams = XEN_SCHED_CRED_NPARAM;
+        if (nparams)
+            *nparams = XEN_SCHED_CRED_NPARAM;
     } else if (STREQ (ret, "sedf")) {
         schedulertype = strdup("sedf");
         if (schedulertype == NULL){
             virReportOOMError();
             goto error;
         }
-        *nparams = XEN_SCHED_SEDF_NPARAM;
+        if (nparams)
+            *nparams = XEN_SCHED_SEDF_NPARAM;
     } else {
         virXendError(VIR_ERR_INTERNAL_ERROR, "%s", _("Unknown scheduler"));
         goto error;

src/xenapi/xenapi_driver.c

@@ -1767,7 +1767,9 @@ static char *
 xenapiDomainGetSchedulerType (virDomainPtr dom ATTRIBUTE_UNUSED, int *nparams)
 {
     char *result = NULL;
-    *nparams = 0;
+
+    if (nparams)
+        *nparams = 0;
     if (!(result = strdup("credit")))
         virReportOOMError();
     return result;