patchkernel: fix partitionig of empty patches

Even if we are partitiniong an empty patch, we still need to go
through the whole preparation to make sure all internal data
structures are properly initialized.

Author: andrea-iob

src/patchkernel/patch_kernel_parallel.cpp

@@ -1833,25 +1833,25 @@ double PatchKernel::evalPartitioningUnbalance(const std::unordered_map<long, dou
 */
 std::vector<adaption::Info> PatchKernel::_partitioningPrepare(const std::unordered_map<long, double> &cellWeights, double defaultWeight, bool trackPartitioning)
 {
-	// Early return if the mesh is empty
-	if (empty(true)) {
-		return std::vector<adaption::Info>();
-	}
-
-	// Early return if there is a single partition
-	if (getProcessorCount() == 1) {
-		return std::vector<adaption::Info>();
-	}
+	// Check if the patch needs a non-trivial partitioning
+	//
+	// A non-trivial partitioning is needed only if the patch is not empty
+	// and if there are multiple processes associated with the patch.
+	bool isCellRankEvaluationNeeded = (!empty(true) && (getProcessorCount() > 1));
 
 	// Default partitioning can only be used when the patch is all on a single process
-	if (isDistributed()) {
-		throw std::runtime_error("Default partitioning can only be used when the patch is all on a single process");
+	if (isCellRankEvaluationNeeded) {
+		if (isDistributed()) {
+			throw std::runtime_error("Default partitioning can only be used when the patch is all on a single process");
+		}
 	}
 
 	// Evaluate partitioning
+	//
+	// Only the owner of the patch need to evaluate the partitioning.
 #if BITPIT_ENABLE_METIS==1
 	std::unordered_map<long, int> cellRanks;
-	if (getRank() == getOwner()) {
+	if (isCellRankEvaluationNeeded && getRank() == getOwner()) {
 		// Initialize map between patch vertices and METIS nodes
 		//
 		// METIS node numbering needs to be contiguous and needs to start from zero.