Stream confusion

[Mark Raynsford]

On Wed, 2023-11-22 at 10:24 -0800, Nathan Reynolds wrote:
> 
> I haven't played with Loom threads yet.  From what I have read, it
> doesn't
> sound like Loom threads change the functional behavior of the
> program.
> Loom threads only change the performance behavior.

They actually do. I was surprised by (and bitten by this):

  private final Map<String, String> data;

  void unsafe()
  {
    this.data.put("x", "y");
    System.in.read();
    this.data.put("x", "z");
  } 

The above unsafe() method is safe when called on a single platform
thread. This is just normal "thread confinement" of the kind we've
been using for years. The unsafe() method isn't safe when called by
a single virtual thread. Why? Because the call to System.in.read()
will cause the virtual thread to yield, and it may be resumed on
a different _carrier_ thread after the read() call finishes. Nothing
ensures those calls to Map.put() will result in safe publication.

It just means that code that was safe to call on a single platform
thread that assumed thread confinement (and so didn't synchronize its
own private data) will break in perhaps surprising ways when called on
a single virtual thread.

-- 
Mark Raynsford | https://www.io7m.com