Compress file downloads if the client sends "Accept-Encoding: gzip" and we guess the file might compress alright

Summary:
Ref T13507. We currently compress normal responses, but do not compress file data responses because most files we serve are images and already compressed.

However, there are some cases where large files may be highly compressible (e.g., huge XML files stored in LFS) and we can benefit from compressing responses.

Make a reasonable guess about whether compression is beneficial and enable compression if we guess it is.

Test Plan:
  - Used `curl ...` to download an image with `Accept-Encoding: gzip`. Got raw image data in the response (as expected, because we don't expect images to be worthwhile to recompress).
  - Used `curl ...` to download a text file with `Accept-Encoding: gzip`. Got a compressed response. Decompressed the response into the original file.

Maniphest Tasks: T13507

Differential Revision: https://secure.phabricator.com/D21125

src/applications/files/controller/PhabricatorFileDataController.php

@@ -142,6 +142,10 @@ final class PhabricatorFileDataController extends PhabricatorFileController {
         (string)$request_uri);
     }
 
+    if ($this->shouldCompressFileDataResponse($file)) {
+      $response->setCompressResponse(true);
+    }
+
     return $response;
   }
 
@@ -224,4 +228,51 @@ final class PhabricatorFileDataController extends PhabricatorFileController {
     return $this->file;
   }
 
+  private function shouldCompressFileDataResponse(PhabricatorFile $file) {
+    // If the client sends "Accept-Encoding: gzip", we have the option of
+    // compressing the response.
+
+    // We generally expect this to be a good idea if the file compresses well,
+    // but maybe not such a great idea if the file is already compressed (like
+    // an image or video) or compresses poorly: the CPU cost of compressing and
+    // decompressing the stream may exceed the bandwidth savings during
+    // transfer.
+
+    // Ideally, we'd probably make this decision by compressing files when
+    // they are uploaded, storing the compressed size, and then doing a test
+    // here using the compression savings and estimated transfer speed.
+
+    // For now, just guess that we shouldn't compress images or videos or
+    // files that look like they are already compressed, and should compress
+    // everything else.
+
+    if ($file->isViewableImage()) {
+      return false;
+    }
+
+    if ($file->isAudio()) {
+      return false;
+    }
+
+    if ($file->isVideo()) {
+      return false;
+    }
+
+    $compressed_types = array(
+      'application/x-gzip',
+      'application/x-compress',
+      'application/x-compressed',
+      'application/x-zip-compressed',
+      'application/zip',
+    );
+    $compressed_types = array_fuse($compressed_types);
+
+    $mime_type = $file->getMimeType();
+    if (isset($compressed_types[$mime_type])) {
+      return false;
+    }
+
+    return true;
+  }
+
 }