RFR: 8210549: Runtime.exec: in closeDescriptors(), use FD_CLOEXEC instead of close()

[Thomas Stuefe]

Hi, please consider the following patch.

This patch replaces the existing close-file-descriptors-logic we follow before exec'ing a target binary: instead of explicitly closing the file descriptors, we mark them as CLOEXEC. That simplifies the logic: it gets rid of the awkward tiptoeing around the fact that we need to keep alive a few file descriptors (the fail pipe fd needs to be kept open right up to the execve(), and we have internal file descriptors in use during iteration of the proc file system to find open file descriptors).

This patch also makes future developments easier: I am working on improving logging during child process spawning (https://bugs.openjdk.org/browse/JDK-8357100), and there we have a similar problem of needing to keep a log file descriptor open right until execve happens).

Note: Using fcntl with FD_CLOEXEC should work on all our POSIX platforms, since we rely on it already, see unconditional use of that flag here: https://github.com/openjdk/jdk/blob/3acfa9e4e7be2f37ac55f97348aad4f74ba802a0/src/java.base/unix/native/libjava/childproc.c#L408-L409

This patch also fixes two subtle bugs:
- we didn't check the return value of the close() inside closeAllFileDescriptors
- the final fcntl for the fail pipe was subtly wrong (should have or'd the FD_CLOEXEC flag with the existing state before setting it)

----

Testing:

We already have the PipelineLeak test, but I also added a new test that checks that we don't accidentally leak file descriptors even if those had been opened outside the JVM and with out without FD_CLOEXEC.

- in the parent JVM, the test opens a file in native code without FD_CLOEXEC
- test then spawns a child program that checks that no file descriptors beyond the expected stdin/out/err are open

I verified that the test correctly detects a broken implementation that leaks file descriptors. 

I verified that with this patch, we close all file descriptors. I also verified the fallback path (where we brute-force-iterate all descriptors up to _SC_OPEN_MAX).

I ran manually all tests from test/jdk/java/base/Process*, and verified that these tests run as part of the GHAs, which are green.

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

Commit messages:
 - close dir fd on fcntl error
 - Mark fds with cloexec, plus test

Changes: https://git.openjdk.org/jdk/pull/25301/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25301&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8210549
  Stats: 204 lines in 5 files changed: 181 ins; 9 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/25301.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25301/head:pull/25301

PR: https://git.openjdk.org/jdk/pull/25301