blaze_util_posix: handle killpg failures (#18403)

In case Bazel JVM server is stuck, it would be useful to know whether
the SIGKILL was sent successfully or not.

If not, log out the error message from errno.

Closes #18299.

PiperOrigin-RevId: 530615358
Change-Id: I4ddf9996ce80520ff19c306fe95429550e931a8b

Co-authored-by: Son Luong Ngoc <sluongng@gmail.com>

src/main/cpp/blaze_util_posix.cc

@@ -724,7 +724,12 @@ void ReleaseLock(BlazeLock* blaze_lock) {
 
 bool KillServerProcess(int pid, const blaze_util::Path& output_base) {
   // Kill the process and make sure it's dead before proceeding.
-  killpg(pid, SIGKILL);
+  errno = 0;
+  if (killpg(pid, SIGKILL) == -1) {
+    BAZEL_DIE(blaze_exit_code::LOCAL_ENVIRONMENTAL_ERROR)
+        << "Attempted to kill stale server process (pid=" << pid
+        << ") using SIGKILL: " << GetLastErrorString();
+  }
   if (!AwaitServerProcessTermination(pid, output_base,
                                      kPostKillGracePeriodSeconds)) {
     BAZEL_DIE(blaze_exit_code::LOCAL_ENVIRONMENTAL_ERROR)