storage: Avoid unnecessary ternary operators and refactor the code

Setting of local variables in virStorageBackendCreateQemuImgCmd was
unnecessarily cluttered with ternary operators and repeated testing of
of conditions.

This patch refactors the function to use if statements and improves
error reporting in case inputvol is specified but does not contain
target path. Previously we would complain about "unknown storage vol
type 0" instead of the actual problem.

src/storage/storage_backend.c

@@ -663,53 +663,58 @@ virStorageBackendCreateQemuImgCmd(virConnectPtr conn,
     virCommandPtr cmd = NULL;
     bool do_encryption = (vol->target.encryption != NULL);
     unsigned long long int size_arg;
-    bool preallocate = false;
-
-    /* Treat output block devices as 'raw' format */
-    const char *type =
-        virStorageFileFormatTypeToString(vol->type == VIR_STORAGE_VOL_BLOCK ?
-                                         VIR_STORAGE_FILE_RAW :
-                                         vol->target.format);
-
-    const char *backingType = vol->backingStore.path ?
-        virStorageFileFormatTypeToString(vol->backingStore.format) : NULL;
-
-    const char *inputBackingPath = (inputvol ? inputvol->backingStore.path
-                                             : NULL);
-    const char *inputPath = inputvol ? inputvol->target.path : NULL;
-    /* Treat input block devices as 'raw' format */
-    const char *inputType = inputPath ?
-        virStorageFileFormatTypeToString(inputvol->type == VIR_STORAGE_VOL_BLOCK ?
-                                         VIR_STORAGE_FILE_RAW :
-                                         inputvol->target.format) :
-        NULL;
+    bool preallocate = !!(flags & VIR_STORAGE_VOL_CREATE_PREALLOC_METADATA);
+    const char *type;
+    const char *backingType = NULL;
+    const char *inputPath = NULL;
+    const char *inputType = NULL;
 
     virCheckFlags(VIR_STORAGE_VOL_CREATE_PREALLOC_METADATA, NULL);
 
-    preallocate = !!(flags & VIR_STORAGE_VOL_CREATE_PREALLOC_METADATA);
+    /* Treat output block devices as 'raw' format */
+    type = virStorageFileFormatTypeToString(vol->type == VIR_STORAGE_VOL_BLOCK ?
+                                            VIR_STORAGE_FILE_RAW :
+                                            vol->target.format);
 
-    if (type == NULL) {
+    if (!type) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
                        _("unknown storage vol type %d"),
                        vol->target.format);
         return NULL;
     }
-    if (inputvol && inputType == NULL) {
-        virReportError(VIR_ERR_INTERNAL_ERROR,
-                       _("unknown storage vol type %d"),
-                       inputvol->target.format);
-        return NULL;
-    }
+
     if (preallocate && vol->target.format != VIR_STORAGE_FILE_QCOW2) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                        _("metadata preallocation only available with qcow2"));
         return NULL;
     }
 
+    if (inputvol) {
+        if (!(inputPath = inputvol->target.path)) {
+            virReportError(VIR_ERR_INVALID_ARG, "%s",
+                           _("missing input volume target path"));
+            return NULL;
+        }
+
+        inputType = virStorageFileFormatTypeToString(inputvol->type == VIR_STORAGE_VOL_BLOCK ?
+                                                     VIR_STORAGE_FILE_RAW :
+                                                     inputvol->target.format);
+
+        if (!inputType) {
+            virReportError(VIR_ERR_INTERNAL_ERROR,
+                           _("unknown storage vol type %d"),
+                           inputvol->target.format);
+            return NULL;
+        }
+
+    }
+
     if (vol->backingStore.path) {
         int accessRetCode = -1;
         char *absolutePath = NULL;
 
+        backingType = virStorageFileFormatTypeToString(vol->backingStore.format);
+
         if (preallocate) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                            _("metadata preallocation conflicts with backing"
@@ -722,11 +727,9 @@ virStorageBackendCreateQemuImgCmd(virConnectPtr conn,
          * may cause issues with lvm. Untested essentially.
          */
         if (inputvol &&
-            (!inputBackingPath ||
-             STRNEQ(inputBackingPath, vol->backingStore.path))) {
-            virReportError(VIR_ERR_INTERNAL_ERROR,
-                           "%s", _("a different backing store cannot "
-                                   "be specified."));
+            STRNEQ_NULLABLE(inputvol->backingStore.path, vol->backingStore.path)) {
+            virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                           _("a different backing store cannot be specified."));
             return NULL;
         }