RFR: 8305457: Implement java.io.IO [v7]

[Jaikiran Pai]

On Fri, 10 May 2024 16:41:28 GMT, Naoto Sato <naoto at openjdk.org> wrote:

>> When implementing, I asked myself if I must use any of those monitors and decided that I don't have to. My reasoning is below.
>> 
>> `readLock`:
>> 
>> - is used inside the object that `Reader reader` is initialised with, and
>> 
>> - it also guards fields such as `char[] rcb`, `boolean restoreEcho` and `boolean shutdownHookInstalled`.
>> 
>> Since `println` and `print` don't call methods on `reader` or access the above fields, they don't require `readLock`.
>> 
>> `writeLock`:
>> 
>> - is used inside objects that `Writer out` and `PrintWriter pw` are initialised with, and
>> - also in compound operations that first write and then immediately read. (I assume, it's so that no concurrent write could sneak in between writing and reading parts of such a compound.)
>> 
>> `println` or `print` don't call methods on `out` and certainly don't do any write-read compounds. That said, they call methods on `pw`. But `pw` uses `writeLock` internally. So in that sense we're covered. 
>> 
>> One potential concern is a write-write compound in `print`:
>> 
>> 
>>     @Override
>>     public JdkConsole print(Object obj) {
>>         pw.print(obj);
>>         pw.flush(); // automatic flushing does not cover print
>>         return this;
>>     }
>> 
>> 
>> I'm saying write-_write_, not write-_flush_, because as far as synchronisation is concerned,  `pw.flush()` should behave the same as `pw.print("")`.
>> 
>> While not using `writeLock` is not strictly correct, I believe the potential execution interleavings with other writes are benign. What's the worst that could happen? You flush more than you expected? Big deal!
>> 
>> Since we exhausted all the reasons to use `writeLock`, I don't think we need one.
>> 
>> 
>> 
>> Naoto has already reviewed this PR with only minor comments. While that increases my confidence in that the implementation is correct, it doesn't hurt to request re-review of this specific part: @naotoj, do you think I should use any of those monitors?
>
> I think your investigation is correct. As to the write-write case, there already is the same pattern in (`formatter` basically utilizes `pw` underneath) 
> 
>     public JdkConsole format(String fmt, Object ... args) {
>         formatter.format(fmt, args).flush();
>         return this;
>     }
> 
> So I think it is acceptable.

Thank you for that explanation, Pavel. I think the crucial detail happens to be:

> But pw uses writeLock internally. So in that sense we're covered.

As you note, the same instance of `writeLock` will get used internally by the `PrintWriter`, so I think the current version of this code is good and won't require additionally locking in the outer code.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19112#discussion_r1598268588