REST API Pagination Does not work as expected

[lbajsarowicz]

Preconditions (*)

1. Magento 2.3.4
2. REST API Call

Steps to reproduce (*)

1. Fixture with 3 orders
2. Request with page_size = 1 and current_page = 99

Expected result (*)

1. total_count returns 3
2. items returns empty array OR OutOfRangeException

Actual result (*)

1. total_count is 3
2. items returns repeated last page of results

Kudos for

@ihor-sviziev , @Vinai , @nuzil for discussion at #appdesign channel about this issue.

[m2-assistant]

Hi @lbajsarowicz. Thank you for your report.
To help us process this issue please make sure that you provided the following information:

• Summary of the issue
• Information on your environment
• Steps to reproduce
• Expected and actual results

Please make sure that the issue is reproducible on the vanilla Magento instance following Steps to reproduce. To deploy vanilla Magento instance on our environment, please, add a comment to the issue:

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For more details, please, review the Magento Contributor Assistant documentation.

@lbajsarowicz do you confirm that you were able to reproduce the issue on vanilla Magento instance following steps to reproduce?

• yes
• no

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[lbajsarowicz]

I'm going to address the issue with data described with dataProvider:

magento2/dev/tests/api-functional/testsuite/Magento/Catalog/Api/ProductRepositoryInterfaceTest.php

Lines 1147 to 1176 in 6771122

	/**
	* Keep in mind: Fixture contains 5 products
	*
	* @return array
	*/
	public function productPaginationDataProvider()
	{
	return [
	'expect-all-items' => [
	'pageSize' => 10,
	'currentPage' => 1,
	'expectedCount' => 5
	],
	'expect-page=size-items' => [
	'pageSize' => 2,
	'currentPage' => 1,
	'expectedCount' => 2
	],
	'expect-less-than-pagesize-elements' => [
	'pageSize' => 3,
	'currentPage' => 2,
	'expectedCount' => 2
	],
	'expect-no-items' => [
	'pageSize' => 100,
	'currentPage' => 99,
	'expectedCount' => 0
	]
	];
	}

[lbajsarowicz]

The source of the issue is located in:

magento2/lib/internal/Magento/Framework/Data/Collection.php

Lines 240 to 249 in 09de30c

	public function getCurPage($displacement = 0)
	{
	if ($this->_curPage + $displacement < 1) {
	return 1;
	} elseif ($this->_curPage + $displacement > $this->getLastPageNumber()) {
	return $this->getLastPageNumber();
	} else {
	return $this->_curPage + $displacement;
	}
	}

Although $this->_curPage is overwritten by the

    if ($this->_curPage + $displacement > $this->getLastPageNumber()) {
        return $this->getLastPageNumber();
    }

That is not passed back to all APIs. There are two options, actually:

1. Remove above-mentioned mechanism to return no values when we reached the last page.
2. Pass the information about overwritten currentPage to API

I choose (1) as for me it's more obvious.