set the minimum core amount to 2 (1 on single core cpus)

src/com/sheepit/client/Configuration.java

@@ -37,6 +37,7 @@ import com.sheepit.client.os.OS;
 import lombok.Data;
 
 @Data public class Configuration {
+	
 	public enum ComputeType {
 		CPU_GPU, CPU, GPU
 	} // accept job for ...

src/com/sheepit/client/Server.java

@@ -50,6 +50,7 @@ import okhttp3.RequestBody;
 import okhttp3.Response;
 import okhttp3.JavaNetCookieJar;
 
+import com.sheepit.client.hardware.cpu.CPU;
 import com.sheepit.client.Configuration.ComputeType;
 import com.sheepit.client.Error.ServerCode;
 import com.sheepit.client.datamodel.CacheFileMD5;
@@ -195,7 +196,7 @@ public class Server extends Thread {
 				.add("cpu_family", os.getCPU().family())
 				.add("cpu_model", os.getCPU().model())
 				.add("cpu_model_name", os.getCPU().name())
-				.add("cpu_cores", String.valueOf(user_config.getNbCores() == -1 ? os.getCPU().cores() : user_config.getNbCores()))
+				.add("cpu_cores", String.valueOf(user_config.getNbCores() == -1 ? os.getCPU().cores() : Math.max(CPU.MIN_CORES, user_config.getNbCores())))
 				.add("os", os.name())
 				.add("os_version", os.getVersion())
 				.add("ram", String.valueOf(os.getMemory()))
@@ -275,7 +276,8 @@ public class Server extends Thread {
 				.addQueryParameter("computemethod", String.valueOf(user_config.computeMethodToInt()))
 				.addQueryParameter("network_dl", String.valueOf(dlStats.getRawAverageSessionSpeed()))
 				.addQueryParameter("network_up", String.valueOf(ulStats.getRawAverageSessionSpeed()))
-				.addQueryParameter("cpu_cores", String.valueOf(user_config.getNbCores() == -1 ? os.getCPU().cores() : user_config.getNbCores()))
+				.addQueryParameter("cpu_cores", String.valueOf(user_config.getNbCores() == -1 ? os.getCPU().cores() :
+					(Math.max(user_config.getNbCores(), CPU.MIN_CORES))))
 				.addQueryParameter("ram_max", String.valueOf(maxMemory))
 				.addQueryParameter("rendertime_max", String.valueOf(user_config.getMaxRenderTime()));
 			

src/com/sheepit/client/hardware/cpu/CPU.java

@@ -20,7 +20,8 @@
 package com.sheepit.client.hardware.cpu;
 
 public class CPU {
-	final public static int MIN_RENDERBUCKET_SIZE = 32;
+	public static final int MIN_RENDERBUCKET_SIZE = 32;
+	public static final int MIN_CORES = Runtime.getRuntime().availableProcessors() > 1 ? 2 : 1;
 	private String name;
 	private String model;
 	private String family;

src/com/sheepit/client/standalone/Worker.java

@@ -71,7 +71,7 @@ public class Worker {
 	
 	@Option(name = "-compute-method", usage = "CPU: only use cpu, GPU: only use gpu, CPU_GPU: can use cpu and gpu (not at the same time) if -gpu is not use it will not use the gpu", metaVar = "CPU", required = false) private String method = null;
 	
-	@Option(name = "-cores", usage = "Number of cores/threads to use for the render", metaVar = "3", required = false) private int nb_cores = -1;
+	@Option(name = "-cores", usage = "Number of cores/threads to use for the render. The minimum is two cores unless your system only has one", metaVar = "3", required = false) private int nb_cores = -1;
 	
 	@Option(name = "-memory", usage = "Maximum memory allow to be used by renderer, number with unit (800M, 2G, ...)", required = false) private String max_ram = null;
 	

src/com/sheepit/client/standalone/swing/activity/Settings.java

@@ -368,7 +368,7 @@ public class Settings implements Activity {
 				display = (double) cpu.cores() / step;
 			}
 			
-			cpuCores = new JSlider(1, cpu.cores());
+			cpuCores = new JSlider(CPU.MIN_CORES, cpu.cores());
 			cpuCores.setMajorTickSpacing((int) (step));
 			cpuCores.setMinorTickSpacing(1);
 			cpuCores.setPaintTicks(true);
@@ -379,7 +379,7 @@ public class Settings implements Activity {
 				}
 			});
 			cpuCores.setPaintLabels(true);
-			cpuCores.setValue(config.getNbCores() != -1 ? config.getNbCores() : cpuCores.getMaximum());
+			cpuCores.setValue(config.getNbCores() != -1 ? (Math.max(config.getNbCores(), CPU.MIN_CORES)): cpuCores.getMaximum());
 			JLabel coreLabel = new JLabel("CPU cores:");
 			coreLabel.setToolTipText(SwingTooltips.CPU_CORES.getText());