vfs_block: warn only once if sync is not supported

Issue #4825
Issue #4820

repos/libports/recipes/pkg/test-libc_vfs_block/runtime

@@ -68,6 +68,16 @@
 					<write  at="15" content="123"/>
 					<expect content="abcdefghIJKLMNO123stuvwxYZABCDEF"/>
 				</sequence>
+
+				<!-- Test write-sync-write-sync-read -->
+					 write operation by reading back the content. -->
+				<sequence>
+					<write  content="................................"/>
+					<sync/>
+					<write  content="abcdefghIJKLMNOPqrstuvwxYZABCDEF"/>
+					<sync/>
+					<expect content="abcdefghIJKLMNOPqrstuvwxYZABCDEF"/>
+				</sequence>
 			</config>
 		</start>
 	</config>

repos/libports/src/test/libc_vfs_block/main.cc

@@ -194,6 +194,8 @@ class Test::Block_device
 
 			return result;
 		}
+
+		void sync() { fsync(_fd.value); }
 };
 
 
@@ -253,6 +255,12 @@ void Test::Main::_exec_step(Genode::Xml_node step, Block_device &block_device)
 		Genode::error("step '", step, "' failed");
 		throw Step_failed();
 	}
+
+	if (step.has_type("sync")) {
+		Genode::log("sync");
+		block_device.sync();
+		return;
+	}
 }
 
 

repos/os/src/lib/vfs/block_file_system.h

@@ -332,7 +332,12 @@ class Vfs::Block_file_system::Data_file_system : public Single_file_system
 					_tx_source->release_packet(p);
 
 					if (!p.succeeded()) {
-						Genode::error("vfs_block: syncing blocks failed");
+						/* only warn once if sync is not supported */
+						static bool print_sync_failed = true;
+						if (print_sync_failed) {
+							Genode::warning("vfs_block: syncing blocks failed");
+							print_sync_failed = false;
+						}
 						return SYNC_ERR_INVALID;
 					}