RFR: JDK-8319307: DCmds should not assert on truncation and should report truncation [v2]

[David Holmes]

On Sat, 4 Nov 2023 06:30:29 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> `bufferedStream` was originally intended to provide intermediate output buffering; the buffer was supposed to drain upon flushing. However, in many cases, `flush()` is a no-op, so the buffer never gets drained.
>> 
>> To prevent infinitely raising buffer sizes for non-flushing `bufferedStream`, [JDK-8220394](https://bugs.openjdk.org/browse/JDK-8220394) introduced the notion of a "maximum reasonable cap". Upon reaching this threshold, we assert in debug VMs since we assumed this to be a condition worth analyzing. In release VMs, we silently truncate.
>> 
>> But DCmds - one of the primary users of `bufferedStream` - can reach the maximum cap under normal conditions; one example would be printing the list of dynamic libraries on Linux (just prints the process memory map) - this can get very large. Similarly, NMT detail reports and VM.info output can get just as large.
>> 
>> Therefore, neither asserting nor silent truncation is optimal. Instead, we should truncate the output, print a visible truncation marker, and - if possible - interrupt the printing.
>> 
>> ---
>> 
>> The patch is minimally invasive to simplify review. Like most stream classes, `bufferedStream` would benefit from an overhaul, but I'd like to leave that to a future RFE.
>> 
>> Testing: Tested manually with a number of commands with artificially increased output size. GHAs (Windows test errors unrelated).
>
> Thomas Stuefe has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Merge branch 'openjdk:master' into JDK-8319307-DCmds-should-not-assert-on-truncation-and-should-report-truncation
>  - fix mac builds
>  - JDK-8319307-DCmds-should-not-assert-on-truncation-and-should-report-truncation

src/hotspot/os/aix/attachListener_aix.cpp line 457:

> 455:       char msg [] = "\n(truncated)\n";
> 456:       AixAttachListener::write_fully(this->socket(), msg, strlen(msg));
> 457:     }

Isn't this happening too late? The st content has already been truncated and we have written the "final" message to the socket for the client to read, and then we try to send a follow-up message that the original was truncated. ???

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PR Review Comment: https://git.openjdk.org/jdk/pull/16474#discussion_r1407278624