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/linux/os_linux.cpp line 1892:

> 1890:   buf[32] = '\0';
> 1891:   unsigned lines = 0;
> 1892:   while (!st->was_truncated() && (bytes = ::read(fd, buf, sizeof(buf)-1)) > 0) {

Okay so we stop printing if `st` is full but where is the notification that this has happened?

src/hotspot/share/utilities/ostream.hpp line 307:

> 305:   virtual void write(const char* c, size_t len) override;
> 306:   bool was_truncated() const override { return truncated; }
> 307:   // Returns number of characters written, exclusing terminating zero.

typo: exclusing -> excluding the

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PR Review Comment: https://git.openjdk.org/jdk/pull/16474#discussion_r1407295977
PR Review Comment: https://git.openjdk.org/jdk/pull/16474#discussion_r1407297243