FilePathMem needs to implement read/write(Buffer,position)

otherwise client code will end up calling the version in FileBase, which
has unnecessary synchronisation

h2/src/main/org/h2/store/fs/FilePathMem.java

@@ -302,6 +302,19 @@ class FileMem extends FileBase {
         return this;
     }
 
+    @Override
+    public int write(ByteBuffer src, long position) throws IOException {
+        int len = src.remaining();
+        if (len == 0) {
+            return 0;
+        }
+        data.touch(readOnly);
+        data.readWrite(position, src.array(),
+                src.arrayOffset() + src.position(), len, true);
+        src.position(src.position() + len);
+        return len;
+    }
+
     @Override
     public int write(ByteBuffer src) throws IOException {
         int len = src.remaining();
@@ -315,6 +328,22 @@ class FileMem extends FileBase {
         return len;
     }
 
+    @Override
+    public int read(ByteBuffer dst, long position) throws IOException {
+        int len = dst.remaining();
+        if (len == 0) {
+            return 0;
+        }
+        long newPos = data.readWrite(position, dst.array(),
+                dst.arrayOffset() + dst.position(), len, false);
+        len = (int) (newPos - position);
+        if (len <= 0) {
+            return -1;
+        }
+        dst.position(dst.position() + len);
+        return len;
+    }
+
     @Override
     public int read(ByteBuffer dst) throws IOException {
         int len = dst.remaining();