Really really only run exit handlers once.

Last time didn't get it apparently, so use a 2-pronged approach to
ensure we only run exit handlers once:

1. Use POSIX::_exit instead of the Perl exit builtin to ensure we skip
running the END { } Perl blocks that control our exit handler.

2. Record the PID when we start and have the exit handler only continue
if it's still the same PID.

This was brought to my attention by Ralf Jung as well (he ended up with
a lot of stray .kdesrc-build-data files from kdesrc-build trying to
record persistent data on shutdown to "current directory").

kdesrc-build still writes to the wrong location if you use a
./kdesrc-buildrc (note the order of . and /), but at least it only
writes the wrong location once now in my testing.

kdesrc-build

@@ -37,7 +37,7 @@ use strict;
 use warnings;
 use Fcntl;    # For sysopen constants
 use Carp;
-use POSIX qw(strftime :sys_wait_h);
+use POSIX qw(strftime :sys_wait_h _exit);
 use File::Find; # For our lndir reimplementation.
 use File::Path qw(make_path remove_tree);
 use File::Glob ':glob';
@@ -3215,10 +3215,8 @@ sub log_command
     }
     else
     {
-        # Child
-
-        # Avoid calling close subroutines in more than one routine.
-        @main::atexit_subs = ();
+        # Child. Note here that we need to avoid running our exit cleanup
+        # handlers in here. For that we need POSIX::_exit.
 
         # Apply altered environment variables.
         while (my ($key, $value) = each %ENV_VARS) {
@@ -3229,7 +3227,7 @@ sub log_command
         if (pretending)
         {
             pretend "\tWould have run g['", join ("' '", @command), "'";
-            exit 0;
+            POSIX::_exit(0);
         }
 
         if (not $logdir or not -e $logdir)
@@ -3274,10 +3272,10 @@ sub log_command
             no strict 'refs'; # Disable restriction on symbolic subroutines.
             if (! &{$cmd}(@command)) # Call sub
             {
-                exit EINVAL;
+                POSIX::_exit (EINVAL);
             }
 
-            exit 0; # Exit child process successfully.
+            POSIX::_exit (0); # Exit child process successfully.
         }
 
         # Don't leave empty output files, give an indication of the particular
@@ -3295,7 +3293,7 @@ Please check your binpath setting (it controls the PATH used by kdesrc-build).
 Currently it is set to g[$ENV{PATH}].
 EOF
             # Don't use return, this is the child still!
-            exit 1;
+            POSIX::_exit (1);
         };
     }
 }
@@ -6642,11 +6640,15 @@ sub finish
     my $ctx = assert_isa(shift, 'ksb::BuildContext');
     my $exitcode = shift // 0;
 
-    $ctx->storePersistentOptions();
-
-    exit $exitcode if pretending; # Abort early when pretending.
+    @main::atexit_subs = ();
+    if (pretending || $main::basePid != $$) {
+        # Abort early if pretending or if we're not the same process
+        # that was started by the user (e.g. async mode, forked pipe-opens
+        exit $exitcode;
+    }
 
     close_lock();
+    $ctx->storePersistentOptions();
 
     my $logdir = $ctx->getLogDir();
     note "Your logs are saved in y[$logdir]";
@@ -7100,8 +7102,7 @@ sub handle_async_build
     { # child
         $ipc->setUpdater();
         # Avoid calling close subroutines in more than one routine.
-        @main::atexit_subs = ();
-        exit handle_updates ($ipc, $ctx);
+        POSIX::_exit (handle_updates ($ipc, $ctx));
     }
 
     # Parent
@@ -7110,8 +7111,7 @@ sub handle_async_build
     { # monitor
         $ipc->setMonitor();
         # Avoid calling close subroutines in more than one routine.
-        @main::atexit_subs = ();
-        exit handle_monitoring ($ipc);
+        POSIX::_exit (handle_monitoring ($ipc));
     }
 
     # Still the parent, let's do the build.
@@ -7225,6 +7225,7 @@ if (defined caller && caller eq 'test')
 
 my $ctx;
 our @atexit_subs;
+our $basePid = $$; # Only run exit handlers from the process with the PID we started with.
 
 END {
     # Basically used to call the finish() handler but only when appropriate.