From fb96ee89f22c4c6f55b5cae67d500f26c7a34067 Mon Sep 17 00:00:00 2001
From: Fiona Ebner <f.ebner@proxmox.com>
Date: Thu, 14 Nov 2024 16:07:33 +0100
Subject: [PATCH] tools: run fork: allow running code in parent after fork

Add an option parameter to the run_fork() run_fork_with_timeout()
functions, where an 'afterfork' subroutine that is run in the parent
process after the fork can be specified. It is made subject to the
timeout too, because the fork already started running at that point
and an error in the 'afterfork' subroutine will take priority over an
error in the child.

In preparation to add a helper to run a Perl subroutine in a user
namespace, which, in turn, will be used for running the container
backup subroutine for external providers inside a user namespace. That
allows them to see the filesystem to back-up from the containers
perspective and also improves security because of isolation.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---
 src/PVE/Tools.pm | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/src/PVE/Tools.pm b/src/PVE/Tools.pm
index d31120f..be34358 100644
--- a/src/PVE/Tools.pm
+++ b/src/PVE/Tools.pm
@@ -1026,7 +1026,7 @@ sub must_stringify {
 # sigkill after $timeout  a $sub running in a fork if it can't write a pipe
 # the $sub has to return a single scalar
 sub run_fork_with_timeout {
-    my ($timeout, $sub) = @_;
+    my ($timeout, $sub, $opts) = @_;
 
     my $res;
     my $error;
@@ -1075,17 +1075,28 @@ sub run_fork_with_timeout {
 	$error = $child_res->{error};
     };
 
+    my $handle_forked = sub {
+	if (my $afterfork = $opts->{afterfork}) {
+	    eval { $afterfork->($child); };
+	    if (my $err = $@) {
+		$error = $err; # propagate error
+		die $err;
+	    }
+	}
+	$readvalues->();
+    };
+
     my $got_timeout = 0;
     my $wantarray = wantarray; # so it can be queried inside eval
     eval {
 	if (defined($timeout)) {
 	    if ($wantarray) {
-		(undef, $got_timeout) = run_with_timeout($timeout, $readvalues);
+		(undef, $got_timeout) = run_with_timeout($timeout, $handle_forked);
 	    } else {
-		run_with_timeout($timeout, $readvalues);
+		run_with_timeout($timeout, $handle_forked);
 	    }
 	} else {
-	    $readvalues->();
+	    $handle_forked->();
 	}
     };
     warn $@ if $@;
@@ -1102,8 +1113,8 @@ sub run_fork_with_timeout {
 }
 
 sub run_fork {
-    my ($code) = @_;
-    return run_fork_with_timeout(undef, $code);
+    my ($code, $opts) = @_;
+    return run_fork_with_timeout(undef, $code, $opts);
 }
 
 # NOTE: NFS syscall can't be interrupted, so alarm does