Upgrade File content hashing to SHA256

Summary:
Ref T12464. This defuses any possible SHA1-collision attacks by using SHA256, for which there is no known collision.

(SHA256 hashes are larger -- 256 bits -- so expand the storage column to 64 bytes to hold them.)

Test Plan:
  - Uploaded the same file twice, saw the two files generate the same SHA256 content hash and use the same underlying data.
  - Tried with a fake hash algorihtm ("quackxyz") to make sure the failure mode worked/degraded correctly if we don't have SHA256 for some reason. Got two valid files with two copies of the same data, as expected.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12464

Differential Revision: https://secure.phabricator.com/D17620

src/applications/files/storage/PhabricatorFile.php

@@ -94,7 +94,7 @@ final class PhabricatorFile extends PhabricatorFileDAO
         'storageHandle' => 'text255',
         'authorPHID' => 'phid?',
         'secretKey' => 'bytes20?',
-        'contentHash' => 'bytes40?',
+        'contentHash' => 'bytes64?',
         'ttl' => 'epoch?',
         'isExplicitUpload' => 'bool?',
         'mailKey' => 'bytes20',
@@ -718,9 +718,19 @@ final class PhabricatorFile extends PhabricatorFileDAO
   }
 
   public static function hashFileContent($data) {
+    // NOTE: Hashing can fail if the algorithm isn't available in the current
+    // build of PHP. It's fine if we're unable to generate a content hash:
+    // it just means we'll store extra data when users upload duplicate files
+    // instead of being able to deduplicate it.
+
+    $hash = hash('sha256', $data, $raw_output = false);
+    if ($hash === false) {
       return null;
     }
 
+    return $hash;
+  }
+
   public function loadFileData() {
     $iterator = $this->getFileDataIterator();
     return $this->loadDataFromIterator($iterator);