Don't call flush() when emitting responses

Summary: Fixes T7620. I don't fully understand exactly what's going on here, but we don't actually need to call `flush()`.

Test Plan:
  - Put timing code around the `echo`.
  - Made a fake page that emitted a lot of data.
  - Saw the `echo` block proportionate to data size under `curl --limit-rate ...`.
  - See T7620.
  - Downloaded a large file, got a reasonable progress bar and no obvious memory use issues.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: jlarouche, rbalik, epriestley

Maniphest Tasks: T7620

Differential Revision: https://secure.phabricator.com/D12127

src/aphront/sink/AphrontPHPHTTPSink.php

@@ -22,10 +22,11 @@ final class AphrontPHPHTTPSink extends AphrontHTTPSink {
   protected function emitData($data) {
     echo $data;
 
-    // Try to push the data to the browser. This has a lot of caveats around
-    // browser buffering and display behavior, but approximately works most
-    // of the time.
-    flush();
+    // NOTE: We don't call flush() here because it breaks HTTPS under Apache.
+    // See T7620 for discussion. Even without an explicit flush, PHP appears to
+    // have reasonable behavior here: the echo will block if internal buffers
+    // are full, and data will be sent to the client once enough of it has
+    // been buffered.
   }
 
   protected function isWritable() {