RFR: 8259070: Add jcmd option to dump CDS

[Thomas Stuefe]

On Fri, 26 Feb 2021 22:01:09 GMT, Calvin Cheung <ccheung at openjdk.org> wrote:

>> Hi, Please review
>> 
>>   Added jcmd option for dumping CDS archive during application runtime. Before this change, user has to dump shared archive in two steps: first run application with
>>   `java -XX:DumpLoadedClassList=<classlist> .... `  
>>  to collect shareable class names and saved in file `<classlist>` , then 
>>   `java -Xshare:dump -XX:SharedClassListFile=<classlist> -XX:SharedArchiveFile=<archivefile> ...`
>>   With this change, user can use jcmd to dump CDS without going through above steps. Also user can choose a moment during the app runtime  to dump an archive.
>>    The bug is associated with the CSR: https://bugs.openjdk.java.net/browse/JDK-8259798 which has been approved.
>>    New added jcmd option:
>>    `jcmd <pid or AppName> VM.cds static_dump <filename>`
>>    or
>>     `jcmd <pid or AppName> VM.cds dynamic_dump <filename>`
>>   To dump dynamic archive, requires start app with newly added flag `-XX:+RecordDynamicDumpInfo`, with this flag, some information related to dynamic dump like loader constraints will be recorded. Note the dumping process changed some object memory locations so for dumping dynamic archive, can only done once for a running app. For static dump, user can dump multiple times against same process. 
>>    The file name is optional, if the file name is not supplied, the file name will take format of `java_pid<number>_static.jsa` or `java_pid<number>_dynamic.jsa` for static and dynamic respectively. The `<number>` is the application process ID.
>> 
>>   Tests: tier1,tier2,tier3,tier4
>> 
>> Thanks
>> Yumin
>
> src/hotspot/share/memory/metaspaceShared.cpp line 783:
> 
>> 781:   char* start = buffer + strlen(buffer);
>> 782:   snprintf(start, buff_len, "%s ", arg);
>> 783: }
> 
> Maybe move the above function to the StringUtils class under share/utilities?
> Use `os::snprintf()` instead of `snprintf()`?

The calculation is also wrong, this would overflow. You need:
  char* start = buffer + strlen(buffer);
  snprintf(start, buff_len - (start - buffer), "%s ", arg);
- and maybe add an assert that strlen(buf) < bufflen. 
- and as Ioi wrote, I'd use either one of os::snprintf or jio_snprintf since both guarantee zero termination on truncation.
- or, just use strncat()

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PR: https://git.openjdk.java.net/jdk/pull/2737