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

[Thomas Stuefe]

On Tue, 28 Nov 2023 06:46:05 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> 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?

Caller needs to take care of it.

In my mind, the caller knows what stream class it uses, so it knows if the stream is one that can truncate. So its his responsibility to handle stream::is_truncated. In this case, the caller that uses a truncatable stream is the dcmd framework, and it handles this.

The printing routines themselves could be completely oblivious to it, I only query truncated in a handful of places to save work - if we know the output stream truncated, there is no reason to continue work printing (and potentially, holding a lock while doing it).

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PR Review Comment: https://git.openjdk.org/jdk/pull/16474#discussion_r1407321302