(stable) Manually set "max_allowed_packet" to 1GB for "mysqldump"

Summary:
We have one production instance with failing database backups since they recently uploaded a 52MB hunk. The production configuration specifies a 64MB "max_allowed_packet" in `[mysqld]`, but this doesn't apply to `mysqldump` (we'd need to specify it in a separate `[mysqldump]` section) and `mysqldump` runs with an effective limit of the default (16MB).

We could change our production config to specify a value in `[mysqldump]`, but just change it unconditionally at execution time since there's no reason for any user to ever want this command to fail because they have too much data.

Test Plan: Dumped locally, will verify production backup goes through cleanly.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18834

src/infrastructure/storage/management/workflow/PhabricatorStorageManagementDumpWorkflow.php

@@ -187,6 +187,22 @@ final class PhabricatorStorageManagementDumpWorkflow
     $argv[] = '-h';
     $argv[] = $host;
 
+    // MySQL's default "max_allowed_packet" setting is fairly conservative
+    // (16MB). If we try to dump a row which is larger than this limit, the
+    // dump will fail.
+
+    // We encourage users to increase this limit during setup, but modifying
+    // the "[mysqld]" section of the configuration file (instead of
+    // "[mysqldump]" section) won't apply to "mysqldump" and we can not easily
+    // detect what the "mysqldump" setting is.
+
+    // Since no user would ever reasonably want a dump to fail because a row
+    // was too large, just manually force this setting to the largest supported
+    // value.
+
+    $argv[] = '--max-allowed-packet';
+    $argv[] = '1G';
+
     if ($port) {
       $argv[] = '--port';
       $argv[] = $port;