Plugin proxy – handle artifacts hosted on windows.net (#889)

Fixes an issue reported by @gziolo:

> WordPress/gutenberg#57256 - I can’t preview
this PR using Playground
https://playground.wordpress.net/gutenberg.html. What do I miss? All CI
checks are green.

The culprit was that GitHub hosts that artifact on another domain:
`productionresultssa0.blob.core.windows.net`. The current
`file_get_contents` call follows the redirection URL and resends the
same HTTP headers. That used to be fine, but now the windows.net host
rejects those headers.

This PR ensures we don't implicitly follow the 302 redirect, but parse
the target URL and send an explicit request with a fresh set of HTTP
headers. As a bonus, this makes the download much faster as we now use
the `streamHttpResponse` function which immediately passes the response
bytes back to the browser instead of buffering the entire response
first.

This PR also switches from HEAD-request-based PR validation to the same
Query params–based one as the wordpress.html previewer uses.

## Testing instructions

Try to preview the above PR in the Gutenberg PR previewer on
https://playground.wordpress.net/gutenberg.html and confirm that it
works now (this fix is already deployed).

Author: adamziel

packages/playground/website/public/gutenberg.html

@@ -117,7 +117,7 @@
 			// Verify that the PR exists and that GitHub CI finished building it
 			const zipArtifactUrl = `/plugin-proxy.php?org=WordPress&repo=gutenberg&workflow=Build%20Gutenberg%20Plugin%20Zip&artifact=gutenberg-plugin&pr=${prNumber}`;
 			// Send the HEAD request to zipArtifactUrl to confirm the PR and the artifact both exist
-			const response = await fetch(zipArtifactUrl, { method: 'HEAD' });
+			const response = await fetch(zipArtifactUrl + '&verify_only=true');
 			if (response.status !== 200) {
 				errorDiv.innerText = `The PR ${prNumber} does not exist or GitHub CI did not finish building it yet.`;
 				submitting = false;

packages/playground/website/public/plugin-proxy.php

@@ -108,17 +108,23 @@ public function streamFromGithubPR($organization, $repo, $pr, $workflow_name, $a
                 'vary',
                 'cache-Control'
             );
-            $artifact = $this->gitHubRequest($zip_download_api_endpoint, false);
-            foreach ($artifact['headers'] as $header_line) {
+            $artifact_res = $this->gitHubRequest($zip_download_api_endpoint, false, false);
+            ob_end_flush();
+            flush();
+
+            // The API endpoint returns the actual artifact URL as a 302 Location header.
+            foreach ($artifact_res['headers'] as $header_line) {
                 $header_name = strtolower(substr($header_line, 0, strpos($header_line, ':')));
-                if (in_array($header_name, $allowed_headers)) {
-                    header($header_line);
+                $header_value = trim(substr($header_line, 1 + strpos($header_line, ':')));
+                if ($header_name === 'location') {
+                    streamHttpResponse($header_value, 'GET', [], NULL, $allowed_headers, [
+                        'Content-Type: application/zip',
+                    ]);
+                    die();
                 }
             }
-            echo $artifact['body'];
-            ob_flush();
-            flush();
-            return;
+
+            throw new ApiException('artifact_redirect_not_present');
         }
         if (!$artifacts) {
             throw new ApiException('artifact_not_available');
@@ -156,14 +162,15 @@ public function streamFromGithubReleases($repo, $name)
         }
     }
 
-    protected function gitHubRequest($url, $decode = true)
+    protected function gitHubRequest($url, $decode = true, $follow_location = true)
     {
         $headers[] = 'User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.132 Safari/537.36';
         $headers[] = 'Authorization: Bearer ' . $this->githubToken;
         $context = stream_context_create([
             'http' => [
                 'method' => 'GET',
                 'header' => implode("\r\n", $headers),
+                'follow_location' => $follow_location
             ]
         ]);
         $response = file_get_contents($url, false, $context);