RFR: 8289643: File descriptor leak with ProcessBuilder.startPipeline [v3]

[Roger Riggs]

> The `ProcessBuilder.pipelineStart()` implementation does not close all of the file descriptors it uses to create the pipeline of processes.
> 
> The process calling `pipelineStart()` is creating the pipes between the stages.
> As each process is launched, the file descriptor is inherited by the child process and
> the child process `dup`s them to the respective stdin/stdout/stderr fd.  
> These copies of inherited file descriptors are handled correctly.
> 
> Between the launching of each Process, the file descriptor for the read-side of the pipe for the output of the previous process is kept open (in the parent process invoking `pipelineStart`).  The file descriptor is correctly passed to the child and is dup'd to the stdin of the next process.
> 
> However, the open file descriptor in the parent is not closed after it has been used as the input for the next Process. 
> The fix is to close the fd after it has been used as the input of the next process.
> 
> A new test verifies that after `pipelineStart` is complete, the same file descriptors are open for Unix Pipes as before the test.
> The test only runs on Linux using the /proc/<pid>/fd filesystem to identify open file descriptors.
> 
> The bug fix is in `ProcessBuilder.pipelineStart` and is applicable to all platforms.

Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:

  Add TWR for Files.walk of /proc

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9414/files
  - new: https://git.openjdk.org/jdk/pull/9414/files/0d628d66..845695d8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9414&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9414&range=01-02

  Stats: 13 lines in 1 file changed: 1 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/9414.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9414/head:pull/9414

PR: https://git.openjdk.org/jdk/pull/9414