RFR: 8267666: Add option to jcmd GC.heap_dump to use existing file [v2]

[Anton Kozlov]

> Please review a small change that adds an option to GC.heap_dump to use an existing file. 
> 
> The option is necessary if the target file is a predefined file-like object, like a named pipe. This opens up a lot of possibilities to process a heap dump without storing it to the file system first.
> 
> Reviews of the CSR linked to the bug would be appreciated as well.

Anton Kozlov has updated the pull request incrementally with one additional commit since the last revision:

  Extend HeapDumpTest

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4183/files
  - new: https://git.openjdk.java.net/jdk/pull/4183/files/c3b4bc58..7faa6900

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4183&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4183&range=00-01

  Stats: 12 lines in 2 files changed: 7 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4183.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4183/head:pull/4183

PR: https://git.openjdk.java.net/jdk/pull/4183