Database(memory_name=) for shared in-memory databases, closes #1151

datasette/database.py

@@ -24,11 +24,18 @@
 
 
 class Database:
-    def __init__(self, ds, path=None, is_mutable=False, is_memory=False):
+    def __init__(
+        self, ds, path=None, is_mutable=False, is_memory=False, memory_name=None
+    ):
         self.ds = ds
         self.path = path
         self.is_mutable = is_mutable
         self.is_memory = is_memory
+        self.memory_name = memory_name
+        if memory_name is not None:
+            self.path = memory_name
+            self.is_memory = True
+            self.is_mutable = True
         self.hash = None
         self.cached_size = None
         self.cached_table_counts = None
@@ -46,6 +53,16 @@ def __init__(self, ds, path=None, is_mutable=False, is_memory=False):
                 }
 
     def connect(self, write=False):
+        if self.memory_name:
+            uri = "file:{}?mode=memory&cache=shared".format(self.memory_name)
+            conn = sqlite3.connect(
+                uri,
+                uri=True,
+                check_same_thread=False,
+            )
+            if not write:
+                conn.execute("PRAGMA query_only=1")
+            return conn
         if self.is_memory:
             return sqlite3.connect(":memory:")
         # mode=ro or immutable=1?
@@ -215,7 +232,10 @@ def mtime_ns(self):
     @property
     def name(self):
         if self.is_memory:
-            return ":memory:"
+            if self.memory_name:
+                return ":memory:{}".format(self.memory_name)
+            else:
+                return ":memory:"
         else:
             return Path(self.path).stem
 

docs/internals.rst

@@ -270,11 +270,16 @@ The ``db`` parameter should be an instance of the ``datasette.database.Database`
 
 This will add a mutable database from the provided file path.
 
-The ``Database()`` constructor takes four arguments: the first is the ``datasette`` instance you are attaching to, the second is a ``path=``, then ``is_mutable`` and ``is_memory`` are both optional arguments.
+To create a shared in-memory database named ``statistics``, use the following:
 
-Use ``is_mutable`` if it is possible that updates will be made to that database - otherwise Datasette will open it in immutable mode and any changes could cause undesired behavior.
+.. code-block:: python
+
+    from datasette.database import Database
 
-Use ``is_memory`` if the connection is to an in-memory SQLite database.
+    datasette.add_database("statistics", Database(
+        datasette,
+        memory_name="statistics"
+    ))
 
 .. _datasette_remove_database:
 
@@ -480,6 +485,32 @@ Database class
 
 Instances of the ``Database`` class can be used to execute queries against attached SQLite databases, and to run introspection against their schemas.
 
+.. _database_constructor:
+
+Database(ds, path=None, is_mutable=False, is_memory=False, memory_name=None)
+----------------------------------------------------------------------------
+
+The ``Database()`` constructor can be used by plugins, in conjunction with :ref:`datasette_add_database`, to create and register new databases.
+
+The arguments are as follows:
+
+``ds`` - :ref:`internals_datasette` (required)
+    The Datasette instance you are attaching this database to.
+
+``path`` - string
+    Path to a SQLite database file on disk.
+
+``is_mutable`` - boolean
+    Set this to ``True`` if it is possible that updates will be made to that database - otherwise Datasette will open it in immutable mode and any changes could cause undesired behavior.
+
+``is_memory`` - boolean
+    Use this to create non-shared memory connections.
+
+``memory_name`` - string or ``None``
+    Use this to create a named in-memory database. Unlike regular memory databases these can be accessed by multiple threads and will persist an changes made to them for the lifetime of the Datasette server process.
+
+The first argument is the ``datasette`` instance you are attaching to, the second is a ``path=``, then ``is_mutable`` and ``is_memory`` are both optional arguments.
+
 .. _database_execute:
 
 await db.execute(sql, ...)

tests/test_internals_database.py

@@ -464,3 +464,33 @@ def test_mtime_ns_is_none_for_memory(app_client):
 def test_is_mutable(app_client):
     assert Database(app_client.ds, is_memory=True, is_mutable=True).is_mutable is True
     assert Database(app_client.ds, is_memory=True, is_mutable=False).is_mutable is False
+
+
+@pytest.mark.asyncio
+async def test_database_memory_name(app_client):
+    ds = app_client.ds
+    foo1 = Database(ds, memory_name="foo")
+    foo2 = Database(ds, memory_name="foo")
+    bar1 = Database(ds, memory_name="bar")
+    bar2 = Database(ds, memory_name="bar")
+    for db in (foo1, foo2, bar1, bar2):
+        table_names = await db.table_names()
+        assert table_names == []
+    # Now create a table in foo
+    await foo1.execute_write("create table foo (t text)", block=True)
+    assert await foo1.table_names() == ["foo"]
+    assert await foo2.table_names() == ["foo"]
+    assert await bar1.table_names() == []
+    assert await bar2.table_names() == []
+
+
+@pytest.mark.asyncio
+async def test_in_memory_databases_forbid_writes(app_client):
+    ds = app_client.ds
+    db = Database(ds, memory_name="test")
+    with pytest.raises(sqlite3.OperationalError):
+        await db.execute("create table foo (t text)")
+    assert await db.table_names() == []
+    # Using db.execute_write() should work:
+    await db.execute_write("create table foo (t text)", block=True)
+    assert await db.table_names() == ["foo"]