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

[Lin Zang]

On Tue, 26 Jan 2021 03:53:17 GMT, Lin Zang <lzang at openjdk.org> wrote:

>> 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

Sorry for typo. the GzipAccess should be GzipRandomAccess.

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