RFR: 8370715: JFR: Races are possible when dumping recordings [v4]

[Robert Toyonaga]

On Tue, 2 Dec 2025 16:55:03 GMT, Robert Toyonaga <duke at openjdk.org> wrote:

>> #### Summary
>> This PR changes the JFR snapshot dumping code so that multiple JFR recordings (potentially from different processes) racing to write the same dump destination won't mix their data. 
>> 
>> #### Problem
>> The dump destination file is created and/or wiped when a recording is started, but not wiped again before actually copying over the chunks during a dump. So in the window of time between creating/wiping and dumping chunks, another recording could write to the same dump destination and have it's chunks added to the snapshot. This can happen with either a single JVM or multiple JVMs that are racing.
>> 
>> #### Proposed fix
>> This PR ensures that any data previously written to the dump destination is wiped before the new recording's data is written.  File locking is also done while chunks are being written. 
>> 
>> Testing:
>> - new test `jdk/jdk/jfr/api/recording/dump/TestDumpOverwrite.java`
>> - Tier 1
>
> Robert Toyonaga has updated the pull request incrementally with one additional commit since the last revision:
> 
>   change WARN to INFO

Yes, I am also okay with changing WARN to INFO. As long as there is some record the user can find of what happened. 

Thank you for the review feedback @egahlin

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PR Comment: https://git.openjdk.org/jdk/pull/28460#issuecomment-3603061200