`MongoDriver->getETag()` can return the same etag for two different timestamps if they are within ~500 milliseconds

[kiyanwang]

The following method, computes an ETag for a document based on its _UPDATED_TS:

    public function getETag($resource,$context=null)
    {
        $this->getStat()->increment(MONGO_GET_ETAG);
        $resource = $this->labeller->uri_to_alias($resource);
        $query = array(
            "_id" => array(
                _ID_RESOURCE=>$resource,
                _ID_CONTEXT=>$this->getContextAlias($context)));
        $doc = $this->collection->findOne($query,array('projection' => array(_UPDATED_TS=>true)));
        /* @var $lastUpdatedDate UTCDateTime */
        $lastUpdatedDate = ($doc!=null && array_key_exists(_UPDATED_TS,$doc)) ? $doc[_UPDATED_TS] : null;
        if (isset($lastUpdatedDate) == null) {
            $eTag = '';
        } else {
            // PHP 5.3 used MongoDate::__toString() to generate the etag.
            // This is incompatible with UTCDate::__toString() so we convert it into a microtime representation.
            // This ensures that if it is required to dual run 2 PHP versions, there are no etag compatibility issues.
            // Note that MongoDate doesn't go to 8 decimal place precision but still returns it so we go to 6 and pad
            // with an extra 2
            $seconds = $lastUpdatedDate->__toString() / 1000;
            $eTag = str_pad(number_format(($seconds - floor($seconds)), 6), 10, '0', STR_PAD_RIGHT) . ' ' . floor($seconds);
        }
        return $eTag;

In principle, saving a change to a document, should result in a different value being computed for the Etag because the _UPDATED_TS will have changed. However this is not always the case. Based on the algorithm above, if the document is updated twice within ~500 milliseconds the same ETag is computed (floor rounds a number).

To demonstrate this I have created a branch (https://github.com/talis/tripod-php/tree/spike_etag_bug) with some unit tests that are failing because of this.

	// I've duplicated the core part of the algorithm in this helper method
    // db access is slower on travis ci so hard to get consistent reads
    // have therefore removed it from the equation altogether.
    protected function getETag() {
        $time = microtime(true);
        $lastUpdatedDate = new UTCDateTime($time);
        $seconds = $lastUpdatedDate->__toString() / 1000;
        $eTag = str_pad(number_format(($seconds - floor($seconds)), 6), 10, '0', STR_PAD_RIGHT) . ' ' . floor($seconds);
        return $eTag;
    }

    public function testETagAlgorithm10Millis()
    {
        $originalEtag = $this->getETag();
        usleep(10000);
        $secondEtag = $this->getETag();

        echo "First  ETag: " . $originalEtag . "\n";
        echo "  (10millis)\n";
        echo "Second ETag: " . $secondEtag . "\n\n";
        $this->assertNotEquals($originalEtag, $secondEtag);
    }

    public function testETagAlgorithm100Millis()
    {
        $originalEtag = $this->getETag();
        usleep(100000);
        $secondEtag = $this->getETag();

        echo "First  ETag: " . $originalEtag . "\n";
        echo " (100millis)\n";
        echo "Second ETag: " . $secondEtag . "\n\n";
        $this->assertNotEquals($originalEtag, $secondEtag);
    }

    public function testETagAlgorithm250Millis()
    {
        $originalEtag = $this->getETag();
        usleep(250000);
        $secondEtag = $this->getETag();

        echo "First  ETag: " . $originalEtag . "\n";
        echo " (250millis)\n";
        echo "Second ETag: " . $secondEtag . "\n\n";
        $this->assertNotEquals($originalEtag, $secondEtag);
    }

    public function testETagAlgorithm500Millis()
    {
        $originalEtag = $this->getETag();
        usleep(500000);
        $secondEtag = $this->getETag();

        echo "First  ETag: " . $originalEtag . "\n";
        echo " (500millis)\n";
        echo "Second ETag: " . $secondEtag . "\n\n";
        $this->assertNotEquals($originalEtag, $secondEtag);
    }

All of the above tests fail: https://travis-ci.org/talis/tripod-php/builds/463727103 consistently on PHP 7, 5.6 and 5.5. If I use a sleep duration greater than 500 milliseconds it fails intermittently, and at a second it always seems to work.