Adjust GC threshold if num_roots higher than threshold after collection

This fixes an edge causing the GC to be triggered repeatedly.

Destructors might add potential garbage to the buffer, so it may happen that
num_root it higher than threshold after collection, thus triggering a GC run
almost immediately. This can happen by touching enough objects in a destructor,
e.g. by iterating over an array. If the happens again in the new run, and the
threshold is not updated, the GC may be triggered repeatedly.

Also prevent GC runs during zend_gc_check_root_tmpvars(), as the threshold is
not adjusted yet a this point.

Author: arnaud-lb

Zend/tests/gh13670_001.phpt

@@ -0,0 +1,43 @@
+--TEST--
+GH-13670 001
+--FILE--
+<?php
+
+register_shutdown_function(function () {
+    global $shutdown;
+    $shutdown = true;
+});
+
+class Cycle {
+    public $self;
+    public function __construct() {
+        $this->self = $this;
+    }
+    public function __destruct() {
+        global $shutdown;
+        if (!$shutdown) {
+            new Cycle();
+        }
+    }
+}
+
+$defaultThreshold = gc_status()['threshold'];
+for ($i = 0; $i < $defaultThreshold+1; $i++) {
+    new Cycle();
+}
+
+$objs = [];
+for ($i = 0; $i < 100; $i++) {
+    $obj = new stdClass;
+    $objs[] = $obj;
+}
+
+$st = gc_status();
+
+if ($st['runs'] > 10) {
+    var_dump($st);
+}
+?>
+==DONE==
+--EXPECT--
+==DONE==

Zend/tests/gh13670_002.phpt

@@ -0,0 +1,66 @@
+--TEST--
+GH-13670 002
+--FILE--
+<?php
+
+register_shutdown_function(function () {
+    global $shutdown;
+    $shutdown = true;
+});
+
+class Cycle {
+    public $self;
+    public function __construct() {
+        $this->self = $this;
+    }
+}
+
+class Canary {
+    public $self;
+    public function __construct() {
+        $this->self = $this;
+    }
+    public function __destruct() {
+        global $shutdown;
+        if (!$shutdown) {
+            work();
+        }
+    }
+}
+
+function work() {
+    global $objs, $defaultThreshold;
+    new Canary();
+    // Create some collectable garbage so the next run will not adjust
+    // threshold
+    for ($i = 0; $i < 100; $i++) {
+        new Cycle();
+    }
+    // Add potential garbage to buffer
+    foreach (array_slice($objs, 0, $defaultThreshold) as $obj) {
+        $o = $obj;
+    }
+}
+
+$defaultThreshold = gc_status()['threshold'];
+$objs = [];
+for ($i = 0; $i < $defaultThreshold*2; $i++) {
+    $obj = new stdClass;
+    $objs[] = $obj;
+}
+
+work();
+
+foreach ($objs as $obj) {
+    $o = $obj;
+}
+
+$st = gc_status();
+
+if ($st['runs'] > 10) {
+    var_dump($st);
+}
+?>
+==DONE==
+--EXPECT--
+==DONE==

Zend/tests/gh13670_003.phpt

@@ -0,0 +1,67 @@
+--TEST--
+GH-13670 003
+--FILE--
+<?php
+
+register_shutdown_function(function () {
+    global $shutdown;
+    $shutdown = true;
+});
+
+class Cycle {
+    public $self;
+    public function __construct() {
+        $this->self = $this;
+    }
+}
+
+class Canary {
+    public $self;
+    public function __construct() {
+        $this->self = $this;
+    }
+    public function __destruct() {
+        global $shutdown;
+        if (!$shutdown) {
+            work();
+        }
+    }
+}
+
+function work() {
+    global $objs, $defaultThreshold;
+    new Canary();
+    // Create some collectable garbage so the next run will not adjust
+    // threshold
+    for ($i = 0; $i < 100; $i++) {
+        new Cycle();
+    }
+    // Add potential garbage to buffer
+    foreach (array_slice($objs, 0, $defaultThreshold) as $obj) {
+        $o = $obj;
+    }
+}
+
+$defaultThreshold = gc_status()['threshold'];
+$objs = [];
+for ($i = 0; $i < $defaultThreshold*2; $i++) {
+    $obj = new stdClass;
+    $objs[] = $obj;
+}
+
+work();
+
+// Iterate on a tmpvar
+foreach (array_slice($objs, -10) as $obj) {
+    $o = $obj;
+}
+
+$st = gc_status();
+
+if ($st['runs'] > 10) {
+    var_dump($st);
+}
+?>
+==DONE==
+--EXPECT--
+==DONE==

Zend/zend_gc.c

@@ -557,7 +557,7 @@ static void gc_adjust_threshold(int count)
 	/* TODO Very simple heuristic for dynamic GC buffer resizing:
 	 * If there are "too few" collections, increase the collection threshold
 	 * by a fixed step */
-	if (count < GC_THRESHOLD_TRIGGER) {
+	if (count < GC_THRESHOLD_TRIGGER || GC_G(num_roots) >= GC_G(gc_threshold)) {
 		/* increase */
 		if (GC_G(gc_threshold) < GC_THRESHOLD_MAX) {
 			new_threshold = GC_G(gc_threshold) + GC_THRESHOLD_STEP;
@@ -1674,7 +1674,11 @@ ZEND_API int zend_gc_collect_cycles(void)
 
 finish:
 	zend_get_gc_buffer_release();
+
+	GC_G(gc_active) = 1;
 	zend_gc_check_root_tmpvars();
+	GC_G(gc_active) = 0;
+
 	return total_count;
 }