feat(plugin): serve Loop as a proxy stub

Composer::getLoop() answers, so a plugin reaches the graph's own downloader
and executor through the route Composer's own docblocks point plugin authors
at, rather than through an instance of its own.

wait() drains the promises it is given and rethrows the first rejection once
the group is done, which is what React\Promise\all() hands PHP. abortJobs()
has nothing to cancel while every request settles before the call that
started it returns. A non-null $progress is an explicit error: a ProgressBar
is a symfony/console object each world runs its own implementation of, so
there is none to hand across.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

2 files changed, 47 insertions, 0 deletions

diff --git a/crates/shirabe/tests/plugin/e2e_http_downloader_test.rs b/crates/shirabe/tests/plugin/e2e_http_downloader_test.rs
index f5fdfa97..f19b992a 100644
--- a/crates/shirabe/tests/plugin/e2e_http_downloader_test.rs
+++ b/crates/shirabe/tests/plugin/e2e_http_downloader_test.rs
@@ -82,6 +82,11 @@ add-missing=ok rejected=\"Composer\\\\Downloader\\\\TransportException\"
 addCopy=ok file=\"{\\\"probe\\\":true,\\\"n\\\":42}\"
 countActiveJobs=0 wait=ok
 collect=ok
+loop class=\"Composer\\\\Util\\\\Loop\" downloader=\"Composer\\\\Util\\\\HttpDownloader\" same=true executor=\"Composer\\\\Util\\\\ProcessExecutor\"
+loop wait=ok bodies=[\"{\\\"probe\\\":true,\\\"n\\\":42}\",\"{\\\"probe\\\":true,\\\"n\\\":42}\"]
+loop wait-rejected=\"Composer\\\\Downloader\\\\TransportException\"
+abortJobs=ok
+own loop=ok
 ",
         upstream.trace
     );
diff --git a/crates/shirabe/tests/plugin/fixtures/e2e-http-downloader/plugin/src/Plugin.php b/crates/shirabe/tests/plugin/fixtures/e2e-http-downloader/plugin/src/Plugin.php
index 6e91ad00..b60c8df0 100644
--- a/crates/shirabe/tests/plugin/fixtures/e2e-http-downloader/plugin/src/Plugin.php
+++ b/crates/shirabe/tests/plugin/fixtures/e2e-http-downloader/plugin/src/Plugin.php
@@ -137,6 +137,48 @@ class Plugin implements PluginInterface, EventSubscriberInterface
             $response->collect();
         });
 
+        // The route Composer's own docblocks point plugin authors at: the loop the run built
+        // hands out the downloader and the executor the rest of the run uses.
+        $loop = $event->getComposer()->getLoop();
+        $shared = $loop->getHttpDownloader();
+        $lines[] = 'loop class=' . json_encode(\get_class($loop))
+            . ' downloader=' . json_encode(\get_class($shared))
+            . ' same=' . json_encode($shared === $loop->getHttpDownloader())
+            . ' executor=' . json_encode($loop->getProcessExecutor() === null ? null : \get_class($loop->getProcessExecutor()));
+
+        // The shared downloader is the one the run enabled async on, so it takes a request group
+        // without the plugin having to enable anything.
+        $waited = [];
+        $lines[] = 'loop wait=' . $this->describe(static function () use ($loop, $shared, $payload, &$waited): void {
+            $loop->wait([
+                $shared->add('file://' . $payload)->then(static function ($result) use (&$waited): void {
+                    $waited[] = $result->getBody();
+                }),
+                $shared->add('file://' . $payload)->then(static function ($result) use (&$waited): void {
+                    $waited[] = $result->getBody();
+                }),
+            ]);
+        }) . ' bodies=' . json_encode($waited);
+
+        // Only the class: the reason a stream failed to open comes from the PHP warning the
+        // reader raised, which this port does not have.
+        $rejected = null;
+        try {
+            $loop->wait([$shared->add('file:///shirabe-probe-missing.json')]);
+        } catch (\Throwable $e) {
+            $rejected = $e;
+        }
+        $lines[] = 'loop wait-rejected=' . json_encode($rejected === null ? null : \get_class($rejected));
+
+        $lines[] = 'abortJobs=' . $this->describe(static function () use ($loop): void {
+            $loop->abortJobs();
+        });
+
+        // A plugin may drive its own loop over the services it already holds.
+        $lines[] = 'own loop=' . $this->describe(static function () use ($downloader): void {
+            (new \Composer\Util\Loop($downloader))->wait([]);
+        });
+
         file_put_contents('http-downloader-trace.txt', implode("\n", $lines) . "\n");
     }