Ensure usernames are unique regardless of casing.

app/src/main/java/org/thoughtcrime/securesms/database/RecipientTable.kt

@@ -275,7 +275,8 @@ open class RecipientTable(context: Context, databaseHelper: SignalDatabase) : Da
 
     val CREATE_INDEXS = arrayOf(
       "CREATE INDEX IF NOT EXISTS recipient_type_index ON $TABLE_NAME ($TYPE);",
-      "CREATE INDEX IF NOT EXISTS recipient_aci_profile_key_index ON $TABLE_NAME ($ACI_COLUMN, $PROFILE_KEY) WHERE $ACI_COLUMN NOT NULL AND $PROFILE_KEY NOT NULL"
+      "CREATE INDEX IF NOT EXISTS recipient_aci_profile_key_index ON $TABLE_NAME ($ACI_COLUMN, $PROFILE_KEY) WHERE $ACI_COLUMN NOT NULL AND $PROFILE_KEY NOT NULL",
+      "CREATE UNIQUE INDEX recipient_username_unique_nocase ON recipient(username COLLATE NOCASE)"
     )
 
     private val RECIPIENT_PROJECTION: Array<String> = arrayOf(
@@ -446,7 +447,14 @@ open class RecipientTable(context: Context, databaseHelper: SignalDatabase) : Da
   }
 
   fun getByUsername(username: String): Optional<RecipientId> {
-    return getByColumn(USERNAME, username)
+    return readableDatabase
+      .select(ID)
+      .from(TABLE_NAME)
+      .where("$USERNAME = ? COLLATE NOCASE", username)
+      .run()
+      .readToSingleObject { cursor ->
+        Optional.of(RecipientId.from(cursor.requireLong(ID)))
+      } ?: Optional.empty()
   }
 
   fun getByCallLinkRoomId(callLinkRoomId: CallLinkRoomId): Optional<RecipientId> {
@@ -974,7 +982,7 @@ open class RecipientTable(context: Context, databaseHelper: SignalDatabase) : Da
       writableDatabase
         .update(TABLE_NAME)
         .values(USERNAME to null)
-        .where("$USERNAME = ? AND $ID != ?", username, recipientId.serialize())
+        .where("$USERNAME = ? COLLATE NOCASE AND $ID != ?", username, recipientId.serialize())
         .run()
     }
   }

app/src/main/java/org/thoughtcrime/securesms/database/helpers/SignalDatabaseMigrations.kt

@@ -156,6 +156,7 @@ import org.thoughtcrime.securesms.database.helpers.migration.V299_AddAttachmentM
 import org.thoughtcrime.securesms.database.helpers.migration.V300_AddKeyTransparencyColumn
 import org.thoughtcrime.securesms.database.helpers.migration.V301_RemoveCallLinkEpoch
 import org.thoughtcrime.securesms.database.helpers.migration.V302_AddDeletedByColumn
+import org.thoughtcrime.securesms.database.helpers.migration.V303_CaseInsensitiveUsernames
 import org.thoughtcrime.securesms.database.SQLiteDatabase as SignalSqliteDatabase
 
 /**
@@ -318,10 +319,11 @@ object SignalDatabaseMigrations {
     299 to V299_AddAttachmentMetadataTable,
     300 to V300_AddKeyTransparencyColumn,
     301 to V301_RemoveCallLinkEpoch,
-    302 to V302_AddDeletedByColumn
+    302 to V302_AddDeletedByColumn,
+    303 to V303_CaseInsensitiveUsernames
   )
 
-  const val DATABASE_VERSION = 302
+  const val DATABASE_VERSION = 303
 
   @JvmStatic
   fun migrate(context: Application, db: SignalSqliteDatabase, oldVersion: Int, newVersion: Int) {

app/src/main/java/org/thoughtcrime/securesms/database/helpers/migration/V303_CaseInsensitiveUsernames.kt

@@ -0,0 +1,41 @@
+package org.thoughtcrime.securesms.database.helpers.migration
+
+import android.app.Application
+import org.signal.core.util.Stopwatch
+import org.signal.core.util.logging.Log
+import org.thoughtcrime.securesms.database.SQLiteDatabase
+
+/**
+ * Enforces case-insensitive uniqueness on the username column in the recipient table.
+ * Cleans up any existing case-insensitive duplicates before creating the index.
+ */
+@Suppress("ClassName")
+object V303_CaseInsensitiveUsernames : SignalDatabaseMigration {
+
+  private val TAG = Log.tag(V303_CaseInsensitiveUsernames::class.java)
+
+  override fun migrate(context: Application, db: SQLiteDatabase, oldVersion: Int, newVersion: Int) {
+    val stopwatch = Stopwatch("migration", decimalPlaces = 2)
+
+    // Clear the username if it doesn't have the highest _id of recipient rows with the same case-insensitive username
+    db.execSQL(
+      """
+      UPDATE recipient
+      SET username = NULL
+      WHERE username IS NOT NULL
+        AND _id NOT IN (
+          SELECT MAX(_id)
+          FROM recipient
+          WHERE username IS NOT NULL
+          GROUP BY LOWER(username)
+        )
+      """.trimIndent()
+    )
+    stopwatch.split("dedupe")
+
+    db.execSQL("CREATE UNIQUE INDEX recipient_username_unique_nocase ON recipient(username COLLATE NOCASE)")
+    stopwatch.split("create-index")
+
+    stopwatch.stop(TAG)
+  }
+}