RFR: 8257234 : Add gz option to SA jmap to write a gzipped heap dump [v9]

[Lin Zang]

On Mon, 25 Jan 2021 21:25:54 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Lin Zang has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   fix coding style issue
>
> test/hotspot/jtreg/serviceability/sa/ClhsdbDumpheap.java line 73:
> 
>> 71:             File out = new File(deCompressedFile);
>> 72:             try {
>> 73:                 GZIPInputStream gis = new GZIPInputStream(new FileInputStream(dump));
> 
> `printStackTraces()` uses `Reader.getStack()`, which does not know about gzipped hprof files. However, `Reader.readFile()` was modified to automatically detect gzipped hprof files. I suggest you do the same for `getStack()` rather than making the test do all the work. Probably `getStack()` and `readFile()` can share the code that does this.

Hi Chris, 
Thanks for review and nice catch on this.

I have considered using Reader.readFile() but it can not parse the gziped heap dump successfully, and I investigated the reason is that the underlying GzipAccess class requires the gziped heap dump file to have "HPROF BLOCKSIZE=xxx" bytes in the gziped file header.
I also investigated that the implementation of jmap command in jdk.jcmd added these words as a "comment section" in gz file headers, by passing it to the "ZIP_GZip_Fully()" when it is first called. 
But I cannot find a way to do same thing in GZIPOutputStream(), it's writeHeader() does not support "comment" section. 
Also I have verified the gziped heap dump file generated with GZIPOutputStream() could be parsed by heaphero.io successfully. 

So it seems to me that "HPROF BLOCKSIZE" may not be a must for general gziped heap dump, but I am not sure whether it is required for testing the jmap command, so I didn't touch GzipAccess in test and do decompress manually in ClhsdbDumpheap.java. 

I am considering another way to modify GzipAccess for testing to accept heap dump file without "HPROF BLOCKSIZE" header, I will try to investigate whether it could work. 
What do you think? 

Thanks!
Lin

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PR: https://git.openjdk.java.net/jdk/pull/1712