[1.2.x] fix: exec_with_timeout operator precedence, child kill, and stderr handling (Cacti#6732)

* fix: backport exec_with_timeout operator precedence, child kill, and stderr handling

- Fix operator precedence: `(int)` cast bound tighter than multiplication,
  so timeout never decreased. Parenthesise the float expression first.
- Kill entire process group via posix_kill(-pid, 9) before proc_terminate
  to prevent orphaned child processes when timeout expires.
- Log stderr as POLLER warning instead of discarding valid stdout output
  by returning false.

Backport of fixes from develop branch fix/exec-with-timeout-bugs.
Addresses issues Cacti#6719, Cacti#6720, Cacti#6721.

Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>

* fix: use setsid for correct process group kill on timeout

Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>

* fix: avoid process-group kill in exec_with_timeout

Use direct PID kill to avoid targeting unrelated process groups.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>

* fix: clarify exec_with_timeout kill semantics

Document why direct PID kill is used instead of process-group signaling.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>

---------

Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>
Co-authored-by: TheWitness <thewitness@cacti.net>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

lib/poller.php

@@ -179,7 +179,7 @@ function exec_with_timeout($cmd, &$output, &$return_code, $timeout = 5) {
 	);
 
 	// Start the process.
-	$process = proc_open('exec ' . $cmd, $descriptors, $pipes);
+	$process = proc_open('exec setsid ' . $cmd, $descriptors, $pipes);
 
 	if (!is_resource($process)) {
 		return false;
@@ -222,7 +222,7 @@ function exec_with_timeout($cmd, &$output, &$return_code, $timeout = 5) {
 		}
 
 		// Subtract the number of microseconds that we waited.
-		$timeout -= (int) (microtime(true) - $start) * 1000000;
+		$timeout -= (int) ((microtime(true) - $start) * 1000000);
 	}
 
 	// Check if there were any errors.
@@ -235,11 +235,17 @@ function exec_with_timeout($cmd, &$output, &$return_code, $timeout = 5) {
 	}
 
 	if (!empty($errors)) {
-		return false;
+		cacti_log('WARNING: exec_with_timeout stderr: ' . trim($errors), false, 'POLLER', POLLER_VERBOSITY_MEDIUM);
 	}
 
 	// Kill the process in case the timeout expired and it's still running.
 	// If the process already exited this won't do anything.
+	// Do not use a negative PID here because the child PID is not guaranteed
+	// to be the process-group ID on every platform/runtime combination.
+	if (isset($status['pid']) && $status['running'] && function_exists('posix_kill')) {
+		posix_kill($status['pid'], 9);
+	}
+
 	proc_terminate($process, 9);
 
 	// Close all streams.