[External] : Re: VirtualMachineImpl.checkCatchesAndSendQuitTo leaks file handles

[Laurence Cable]

https://github.com/openjdk/jdk/pull/25526

On 5/29/25 11:05 AM, Kevin Walls wrote:
> OK thanks Philippe, and Larry -
>
> I can't provoke a problem as it stands but yes it would be good to 
> take the recommendation of the API Note, that should keep us 
> efficient. 8-)
>
> I created a JBS issue: https://bugs.openjdk.org/browse/JDK-8358088
>
> Thanks!
> Kevin
> ------------------------------------------------------------------------
> *From:* Philippe Marschall <kustos at gmx.net>
> *Sent:* Thursday, May 29, 2025 5:47 PM
> *To:* Kevin Walls <kevin.walls at oracle.com>; 
> serviceability-dev at openjdk.java.net <serviceability-dev at openjdk.java.net>
> *Subject:* [External] : Re: 
> VirtualMachineImpl.checkCatchesAndSendQuitTo leaks file handles
>
>
>
> On 29.05.25 12:21, Kevin Walls wrote:
> > Hi --
> >
> > Just to be clear, is this an actual persistent leak that we can 
> observe, or is it that we could close earlier with try-with-resources?
> > I'm not seeing a leak when calling a line like this over and over in 
> a tight loop:
> >    final var cmdline = Files.lines(path).findFirst();
>
> I believe it is persistent until the Cleaner of FileChannel closes it.
> The #line Javadoc [1] mentions the need to close
>
>  > This method must be used within a try-with-resources statement or
>  > similar control structure to ensure that the stream's open file is
>  > closed promptly after the stream's operations have completed.
>
> The Stream class Javadoc [2] explicitly mentions #lines as well
>
>  > Generally, only streams whose source is an IO channel, such as those
>  > returned by Files.lines(Path), will require closing. If a stream does
>  > require closing, it must be opened as a resource within a
>  > try-with-resources statement or similar control structure to ensure
>  > that it is closed promptly after its operations have completed.
>
> If I run something like the code below I get
> java.nio.file.FileSystemException: marker: Too many open files. You can
> also lower the number, set a breakpoint at System.out.println and check
> /proc/<pid>/fd
>
>
> Path marker = Path.of("marker");
> if (!Files.exists(marker)) {
>    Files.writeString(marker, "line1");
> }
> // usually over the max fd limit
> int fileCount = 100_000;
> // prevent streams from being garbage collected and cleaner to run and
> close fd
> List<Stream<?>> streams = new ArrayList<>(fileCount);
> for (int i = 0; i < fileCount; i++) {
>    Stream<String> stream = Files.lines(marker);
>    streams.add(stream);
>    Optional<String> firstLine = stream.findFirst();
>    if (firstLine.isPresent()) {
>      if (firstLine.get().hashCode() == 42) {
>        System.out.println('x');
>      }
>    }
> }
> System.out.println(streams.hashCode());
>
>   [1]
> https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/nio/file/Files.html#lines(java.nio.file.Path) 
>
>   [2]
> https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/util/stream/Stream.html 
>
>
> Regards
> Philippe
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