fix(rpn): correct the expression evaluator against rrdtool semantics

.github/workflows/plugin-ci-workflow.yml

@@ -35,21 +35,12 @@ jobs:
   integration-test:
     runs-on: ${{ matrix.os }}
 
-    # A failure against the pinned release is a real failure. The develop entry
-    # is advisory: it is how a core regression becomes visible here, but it must
-    # not turn the plugin's own pull requests red.
-    continue-on-error: ${{ matrix.cacti != 'release/1.2.31' }}
-
     strategy:
       fail-fast: false
       matrix:
         php: ['8.1', '8.2', '8.3', '8.4']
         os: [ubuntu-latest]
         cacti: ['release/1.2.31']
-        include:
-          - php: '8.4'
-            os: ubuntu-latest
-            cacti: 'develop'
 
     services:
       mariadb:
@@ -95,7 +86,24 @@ jobs:
         echo "PHP_BINARY=$(command -v php)" >> "$GITHUB_ENV"
 
     - name: Run apt-get update
-      run: sudo apt-get update
+      run: |
+        for attempt in 1 2 3; do
+          if sudo timeout 3m apt-get \
+            -o Dpkg::Lock::Timeout=60 \
+            -o Acquire::Retries=3 \
+            -o Acquire::http::Timeout=30 \
+            -o Acquire::https::Timeout=30 \
+            update; then
+            exit 0
+          fi
+
+          if [ "$attempt" -lt 3 ]; then
+            sleep 10
+          fi
+        done
+
+        echo 'apt-get update failed after three bounded attempts.' >&2
+        exit 1
 
     - name: Install System Dependencies
       run: sudo apt-get install -y apache2 snmp snmpd rrdtool fping

tests/Unit/TholdCalculateExpressionTest.php

@@ -0,0 +1,134 @@
+<?php
+/*
+ +-------------------------------------------------------------------------+
+ | Copyright (C) 2004-2026 The Cacti Group                                 |
+ |                                                                         |
+ | This program is free software; you can redistribute it and/or           |
+ | modify it under the terms of the GNU General Public License             |
+ | as published by the Free Software Foundation; either version 2          |
+ | of the License, or (at your option) any later version.                  |
+ +-------------------------------------------------------------------------+
+ | Cacti: The Complete RRDtool-based Graphing Solution                     |
+ +-------------------------------------------------------------------------+
+ | http://www.cacti.net/                                                   |
+ +-------------------------------------------------------------------------+
+*/
+
+/**
+ * thold_calculate_expression() evaluates a whole RPN expression and returns
+ * the single value the threshold is compared against.
+ */
+final class TholdCalculateExpressionTest extends TestCase {
+	/**
+	 * @return void
+	 */
+	public static function setUpBeforeClass(): void {
+		self::loadPluginSource('thold_functions.php');
+	}
+
+	/**
+	 * @param string $expression
+	 *
+	 * @return mixed
+	 */
+	private function evaluate($expression) {
+		$thold = [
+			'id'             => 1,
+			'name'           => 'CPU',
+			'local_data_id'  => 4,
+			'local_graph_id' => 7,
+			'expression'     => $expression,
+		];
+
+		$reindexed      = [];
+		$time_reindexed = [];
+
+		return thold_calculate_expression($thold, 0, $reindexed, $time_reindexed);
+	}
+
+	/**
+	 * @return array<string, array{0: string, 1: float|int}>
+	 */
+	public static function expressionProvider() {
+		return [
+			'addition'       => ['2,3,+', 5],
+			'subtraction'    => ['8,2,-', 6],
+			'multiplication' => ['4,3,*', 12],
+			'nested'         => ['2,3,+,4,*', 20],
+			'single value'   => ['7', 7],
+		];
+	}
+
+	/**
+	 * @dataProvider expressionProvider
+	 *
+	 * @param string    $expression
+	 * @param float|int $expected
+	 *
+	 * @return void
+	 */
+	public function testExpressionsReduceToTheTopOfTheStack($expression, $expected): void {
+		$this->assertEqualsWithDelta($expected, $this->evaluate($expression), 1.0e-9);
+	}
+
+	/**
+	 * An expression that leaves more than one value is an authoring error. It
+	 * used to return the first operand pushed, which looks like a plausible
+	 * reading and is compared against the bounds as though it were one.
+	 *
+	 * @return void
+	 */
+	public function testUnbalancedExpressionIsRejectedRatherThanReturningAnOperand(): void {
+		$this->assertSame(0, $this->evaluate('1,2,3,+'));
+		$this->assertTrue($GLOBALS['rpn_error']);
+	}
+
+	/**
+	 * @return void
+	 */
+	public function testUnsupportedTokenFailsTheExpression(): void {
+		$this->assertSame(0, $this->evaluate('2,NOSUCHOP'));
+		$this->assertTrue($GLOBALS['rpn_error']);
+	}
+
+	/**
+	 * @return array<string, array{0: string}>
+	 */
+	public static function specialTokenProvider() {
+		return [
+			'graph maximum' => ['CURRENT_GRAPH_MAXIMUM_VALUE'],
+			'graph minimum' => ['CURRENT_GRAPH_MINIMUM_VALUE'],
+		];
+	}
+
+	/**
+	 * CURRENT_GRAPH_MAXIMUM_VALUE was missing from the dispatch list even
+	 * though the evaluator handles it, so every expression using it failed as
+	 * an unsupported field and returned zero.
+	 *
+	 * Reaching the handler is the whole assertion: what it then reads out of
+	 * the RRD needs a live rrdtool and is not this test's concern.
+	 *
+	 * @dataProvider specialTokenProvider
+	 *
+	 * @param string $token
+	 *
+	 * @return void
+	 */
+	public function testSpecialGraphTokensReachTheirHandler($token): void {
+		// The handler reads the step off the data source before asking rrdtool.
+		CactiStubs::willReturnFor('db_fetch_row_prepared', 'FROM data_template_data', ['rrd_step' => 300]);
+
+		try {
+			$this->evaluate($token);
+		} catch (Throwable $reached_the_rrd_layer) {
+			// Expected: the handler runs and asks rrdtool for a value.
+		}
+
+		$unsupported = array_filter(CactiStubs::$log, static function ($message) {
+			return strpos($message, 'Unsupported Field') !== false;
+		});
+
+		$this->assertSame([], array_values($unsupported));
+	}
+}