Don't assume all database in SQLite are files:

- SQlite allow database to be specified as URL (given that URI filename
  interpretation was turned on on the connection.)

  This commit is necessary for the read_uncommitted transaction
  feature because SQLite doesn't allow to enable the shared-cache mode
  if the database name is `:memory:`. It has to be a URI (`file::memory`)

  Ref https://www.sqlite.org/sharedcache.html#shared_cache_and_in_memory_databases

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -26,7 +26,7 @@ def sqlite3_connection(config)
       # Allow database path relative to Rails.root, but only if the database
       # path is not the special path that tells sqlite to build a database only
       # in memory.
-      if ":memory:" != config[:database]
+      if ":memory:" != config[:database] && !config[:database].to_s.starts_with?("file:")
         config[:database] = File.expand_path(config[:database], Rails.root) if defined?(Rails.root)
         dirname = File.dirname(config[:database])
         Dir.mkdir(dirname) unless File.directory?(dirname)
@@ -116,6 +116,10 @@ def supports_savepoints?
         true
       end
 
+      def supports_transaction_isolation?
+        true
+      end
+
       def supports_partial_index?
         true
       end

activerecord/test/cases/adapters/sqlite3/transaction_test.rb

@@ -107,7 +107,7 @@ def shared_cache_flags
 
     def with_connection(options = {})
       conn_options = options.reverse_merge(
-        database: in_memory_db? ? 'file::memory:' : ActiveRecord::Base.configurations["arunit"][:database]
+        database: in_memory_db? ? "file::memory:" : ActiveRecord::Base.configurations["arunit"][:database]
       )
       conn = ActiveRecord::Base.sqlite3_connection(conn_options)
 

activerecord/test/cases/transaction_isolation_test.rb

@@ -2,7 +2,7 @@
 
 require "cases/helper"
 
-unless ActiveRecord::Base.connection.supports_transaction_isolation?
+unless ActiveRecord::Base.connection.supports_transaction_isolation? && !current_adapter?(:SQLite3Adapter)
   class TransactionIsolationUnsupportedTest < ActiveRecord::TestCase
     self.use_transactional_tests = false
 
@@ -10,6 +10,8 @@ class Tag < ActiveRecord::Base
     end
 
     test "setting the isolation level raises an error" do
+      skip if current_adapter?(:SQLite3Adapter)
+
       assert_raises(ActiveRecord::TransactionIsolationError) do
         Tag.transaction(isolation: :serializable) { Tag.connection.materialize_transactions }
       end

activerecord/test/cases/transactions_test.rb

@@ -1127,7 +1127,7 @@ def test_no_automatic_savepoint_for_inner_transaction
   end
 end if Topic.connection.supports_savepoints?
 
-if ActiveRecord::Base.connection.supports_transaction_isolation?
+if ActiveRecord::Base.connection.supports_transaction_isolation? && !in_memory_db?
   class ConcurrentTransactionTest < TransactionTest
     # This will cause transactions to overlap and fail unless they are performed on
     # separate database connections.