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.
-------------
Commit messages:
- 8307990: jspawnhelper must close its writing side of a pipe before reading
from it
Changes: https://git.openjdk.org/jdk/pull/13956/files
Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13956&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8307990
Stats: 29 lines in 4 files changed: 9 ins; 1 del; 19 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
