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

I don't think I agree with this.

If DCmds are regularly asserting, or being truncated, then we should be using a larger buffer shouldn't we? That is what the assert was intended to be used for.

Having clients of the stream actively check for truncation also seems wrong to me. The basic model of our `bufferedStream` is that it is presumed large enough for all usecases (hence the assert when not the case) and users don't have to care. Otherwise the normal API approach here would be for `write()` to return a value indicating truncation so that the caller could consider resizing the buffer.

src/hotspot/share/nmt/memReporter.cpp line 323:

> 321:   const MallocSite* malloc_site;
> 322:   int num_omitted = 0;
> 323:   while (!output()->was_truncated() && (malloc_site = malloc_itr.next()) != nullptr) {

I don't think clients should be aware of, or worrying about truncation, this way. It seems very intrusive.

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PR Review: https://git.openjdk.org/jdk/pull/16474#pullrequestreview-1719114513
PR Review Comment: https://git.openjdk.org/jdk/pull/16474#discussion_r1385815758