JENKINS-15638 Xvfb 'display name offset' defaults to 0, not 1 as

described, if unset

src/main/java/org/jenkinsci/plugins/xvfb/XvfbBuildWrapper.java

@@ -42,13 +42,15 @@
 import hudson.tasks.BuildWrapperDescriptor;
 import hudson.tools.ToolInstallation;
 import hudson.util.ArgumentListBuilder;
+import hudson.util.FormValidation;
 
 import java.io.IOException;
 import java.util.Map;
 
 import net.sf.json.JSONObject;
 
 import org.kohsuke.stapler.DataBoundConstructor;
+import org.kohsuke.stapler.QueryParameter;
 import org.kohsuke.stapler.StaplerRequest;
 
 public class XvfbBuildWrapper extends BuildWrapper {
@@ -64,6 +66,18 @@ public XvfbBuildWrapperDescriptor() {
             load();
         }
 
+        public FormValidation doCheckDisplayName(@QueryParameter final String value) throws IOException {
+            return validateOptionalNonNegativeInteger(value);
+        }
+
+        public FormValidation doCheckDisplayNameOffset(@QueryParameter final String value) throws IOException {
+            return validateOptionalPositiveInteger(value);
+        }
+
+        public FormValidation doCheckTimeout(@QueryParameter final String value) throws IOException {
+            return validateOptionalNonNegativeInteger(value);
+        }
+
         @Override
         public String getDisplayName() {
             return Messages.XvfbBuildWrapper_DisplayName();
@@ -91,6 +105,22 @@ public void setInstallations(final XvfbInstallation... installations) {
             this.installations = installations;
             save();
         }
+
+        private FormValidation validateOptionalNonNegativeInteger(final String value) {
+            if (value == null || value.trim().isEmpty()) {
+                return FormValidation.ok();
+            }
+
+            return FormValidation.validateNonNegativeInteger(value);
+        }
+
+        private FormValidation validateOptionalPositiveInteger(final String value) {
+            if (value == null || value.trim().isEmpty()) {
+                return FormValidation.ok();
+            }
+
+            return FormValidation.validatePositiveInteger(value);
+        }
     }
 
     private static final int    MILLIS_IN_SECOND  = 1000;
@@ -120,7 +150,8 @@ public void setInstallations(final XvfbInstallation... installations) {
     private final String        additionalOptions;
 
     @DataBoundConstructor
-    public XvfbBuildWrapper(final String installationName, final Integer displayName, final String screen, final Boolean debug, final int timeout, int displayNameOffset, final String additionalOptions) {
+    public XvfbBuildWrapper(final String installationName, final Integer displayName, final String screen, final Boolean debug, final int timeout, final int displayNameOffset,
+            final String additionalOptions) {
         this.installationName = installationName;
         this.displayName = displayName;
 
@@ -133,7 +164,12 @@ public XvfbBuildWrapper(final String installationName, final Integer displayName
 
         this.debug = Boolean.TRUE.equals(debug);
         this.timeout = timeout;
-        this.displayNameOffset = displayNameOffset;
+        if (displayNameOffset <= 0) {
+            this.displayNameOffset = 1;
+        }
+        else {
+            this.displayNameOffset = displayNameOffset;
+        }
         this.additionalOptions = additionalOptions;
     }