RFR: 8307990: jspawnhelper must close its writing side of a pipe before reading from it [v4]

[Volker Simonis]

> Since JDK13, executing commands in a sub-process defaults to the so called `POSIX_SPAWN` launching mechanism (i.e. `-Djdk.lang.Process.launchMechanism=POSIX_SPAWN`) on Linux. This works by using `posix_spawn(3)` to firstly start a tiny helper program called `jspawnhelper` in a subprocess. In a second step, `jspawnhelper` reads the command data from the parent Java process over a Unix pipe and finally executes (i.e. `execvp(3)`) the requested command.
> 
> In cases where the parent process terminates abnormally before `jspawnhelper` has read all the expected data from the pipe, `jspawnhelper` will block indefinitely on the reading end of the pipe. This is especially harmful if the parent process had open sockets, because in that case, the forked `jspawnhelper` process will inherit them and keep all the corresponding ports open effectively preventing other processes to bind to them. Notice that this is not an abstract scenario. We've observed this regularly in production with services which couldn't be restarted after a crash after migrating to JDK 17.
> 
> The fix of the issue is rather trivial. `jspawnhelper` has to close its writing end of the pipe which connects it with the parent Java process *before* starting to read from that pipe such that reads from the pipe can immediately return with EOF if the parent process terminates abnormally.
> 
> Also did some cleanup:
>  - in `jspawnhelper.c` correctly chek the result of `readFully()`
>  - in `ProcessImpl_md.c` use `restartableWrite()` instead of `write()` to write the command data from the parent process to `jspawnhelper`
>  - in `childproc.{c,h}` remove remaining traces of `MODE_CLONE` because that's long gone.

Volker Simonis has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - Merge branch 'master' into JDK-8307990
 - Address comments from tstuefe and RogerRiggs
 - REALLY adding the test :)
 - Added test case
 - 8307990: jspawnhelper must close its writing side of a pipe before reading from it

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Changes: https://git.openjdk.org/jdk/pull/13956/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13956&range=03
  Stats: 260 lines in 5 files changed: 239 ins; 1 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/13956.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13956/head:pull/13956

PR: https://git.openjdk.org/jdk/pull/13956