Use utf8_general_ci for "sort" columns in old MySQL

Summary:
Fixes T7287. This trades off 4-byte character support for case insensitivity in these columns, which is a much better trade on the balance.

Also adds more warnings about old MySQL. Note that we already issue a warning when you run "storage adjust" (which I've made stronger) and already "strongly recommend" MySQL 5.5 or newer in the install documentation.

Test Plan:
  - Ran `storage adjust --disable-utf8mb4` to go to old definitions, then ran `storage adjust` to get back to the new ones. Everything seemed OK in both cases.
  - Verified that utf8mb4 data can be migrated out of these colums with `--unsafe` (which will truncate).
  - Verified that manual explains this.
  - Faked my way into the setup warning.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7287

Differential Revision: https://secure.phabricator.com/D11893

src/applications/config/check/PhabricatorMySQLSetupCheck.php

@@ -315,6 +315,26 @@ final class PhabricatorMySQLSetupCheck extends PhabricatorSetupCheck {
         ->addMySQLConfig('innodb_buffer_pool_size');
     }
 
+    $ok = PhabricatorStorageManagementAPI::isCharacterSetAvailableOnConnection(
+      'utf8mb4',
+      id(new PhabricatorUser())->establishConnection('w'));
+    if (!$ok) {
+      $summary = pht(
+        'You are using an old version of MySQL, and should upgrade.');
+
+      $message = pht(
+        'You are using an old version of MySQL which has poor unicode '.
+        'support (it does not support the "utf8mb4" collation set). You will '.
+        'encounter limitations when working with some unicode data.'.
+        "\n\n".
+        'We strongly recommend you upgrade to MySQL 5.5 or newer.');
+
+      $this->newIssue('mysql.utf8mb4')
+        ->setName(pht('Old MySQL Version'))
+        ->setSummary($summary)
+        ->setMessage($message);
+    }
+
   }
 
 }

src/infrastructure/storage/management/PhabricatorStorageManagementAPI.php

@@ -233,7 +233,12 @@ final class PhabricatorStorageManagementAPI {
     }
 
     $conn = $this->getConn(null);
+    return self::isCharacterSetAvailableOnConnection($character_set, $conn);
+  }
 
+  public static function isCharacterSetAvailableOnConnection(
+    $character_set,
+    AphrontDatabaseConnection $conn) {
     $result = queryfx_one(
       $conn,
       'SELECT CHARACTER_SET_NAME FROM INFORMATION_SCHEMA.CHARACTER_SETS
@@ -254,24 +259,32 @@ final class PhabricatorStorageManagementAPI {
       $collate_sort = 'utf8mb4_unicode_ci';
       $collate_full = 'utf8mb4_unicode_ci';
     } else {
-      // If utf8mb4 is not available, we use binary. This allows us to store
-      // 4-byte unicode characters. This has some tradeoffs:
-      //
-      // Unicode characters won't sort correctly. There's nothing we can do
-      // about this while still supporting 4-byte characters.
+      // If utf8mb4 is not available, we use binary for most data. This allows
+      // us to store 4-byte unicode characters.
       //
       // It's possible that strings will be truncated in the middle of a
       // character on insert. We encourage users to set STRICT_ALL_TABLES
       // to prevent this.
       //
-      // There's no valid collation we can use to get a fulltext index on
-      // 4-byte unicode characters: we can't add a fulltext key to a binary
-      // column.
+      // For "fulltext" and "sort" columns, we don't use binary.
+      //
+      // With "fulltext", we can not use binary because MySQL won't let us.
+      // We use 3-byte utf8 instead and accept being unable to index 4-byte
+      // characters.
+      //
+      // With "sort", if we use binary we lose case insensitivity (for
+      // example, "ALincoln@logcabin.com" and "alincoln@logcabin.com" would no
+      // longer be identified as the same email address). This can be very
+      // confusing and is far worse overall than not supporting 4-byte unicode
+      // characters, so we use 3-byte utf8 and accept limited 4-byte support as
+      // a tradeoff to get sensible collation behavior. Many columns where
+      // collation is important rarely contain 4-byte characters anyway, so we
+      // are not giving up too much.
 
       $charset = 'binary';
       $charset_full = 'utf8';
       $collate_text = 'binary';
-      $collate_sort = 'binary';
+      $collate_sort = 'utf8_general_ci';
       $collate_full = 'utf8_general_ci';
     }
 

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

@@ -57,9 +57,11 @@ abstract class PhabricatorStorageManagementWorkflow
     if (!$force && !$api->isCharacterSetAvailable('utf8mb4')) {
       $message = pht(
         "You have an old version of MySQL (older than 5.5) which does not ".
-        "support the utf8mb4 character set. If you apply adjustments now ".
-        "and later update MySQL to 5.5 or newer, you'll need to apply ".
-        "adjustments again (and they will take a long time).\n\n".
+        "support the utf8mb4 character set. We strongly recomend upgrading to ".
+        "5.5 or newer.\n\n".
+        "If you apply adjustments now and later update MySQL to 5.5 or newer, ".
+        "you'll need to apply adjustments again (and they will take a long ".
+        "time).\n\n".
         "You can exit this workflow, update MySQL now, and then run this ".
         "workflow again. This is recommended, but may cause a lot of downtime ".
         "right now.\n\n".