--- a/src/jdk.attach/linux/classes/sun/tools/attach/VirtualMachineImpl.java	Thu Oct 17 20:27:44 2019 +0100
+++ b/src/jdk.attach/linux/classes/sun/tools/attach/VirtualMachineImpl.java	Thu Oct 17 20:53:35 2019 +0100
@@ -60,8 +60,11 @@
         int pid;
         try {
             pid = Integer.parseInt(vmid);
+            if (pid < 1) {
+                throw new NumberFormatException();
+            }
         } catch (NumberFormatException x) {
-            throw new AttachNotSupportedException("Invalid process identifier");
+            throw new AttachNotSupportedException("Invalid process identifier: " + vmid);
         }
 
         // Try to resolve to the "inner most" pid namespace
@@ -233,7 +236,7 @@
      * InputStream for the socket connection to get target VM
      */
     private class SocketInputStream extends InputStream {
-        int s;
+        int s = -1;
 
         public SocketInputStream(int s) {
             this.s = s;
@@ -260,8 +263,12 @@
             return VirtualMachineImpl.read(s, bs, off, len);
         }
 
-        public void close() throws IOException {
-            VirtualMachineImpl.close(s);
+        public synchronized void close() throws IOException {
+            if (s != -1) {
+                int toClose = s;
+                s = -1;
+                VirtualMachineImpl.close(toClose);
+            }
         }
     }