Experimental fix addressing #2058

datasette/app.py

@@ -424,25 +424,45 @@ class Datasette:
             await init_internal_db(internal_db)
             self.internal_db_created = True
 
-        current_schema_versions = {
-            row["database_name"]: row["schema_version"]
+        current_schema_versions_and_hashes = {
+            row["database_name"]: (row["schema_version"], row["schema_hash"])
             for row in await internal_db.execute(
-                "select database_name, schema_version from databases"
+                "select database_name, schema_version, schema_hash from databases"
             )
         }
         for database_name, db in self.databases.items():
-            schema_version = (await db.execute("PRAGMA schema_version")).first()[0]
-            # Compare schema versions to see if we should skip it
-            if schema_version == current_schema_versions.get(database_name):
-                continue
-            placeholders = "(?, ?, ?, ?)"
-            values = [database_name, str(db.path), db.is_memory, schema_version]
+            schema_version = await db.schema_version()
+            current_version_and_hash = current_schema_versions_and_hashes.get(
+                database_name
+            )
+            if current_version_and_hash:
+                # We might get to skip this database
+                if schema_version is not None and current_version_and_hash:
+                    # Use this to decide if the schema has changed
+                    if schema_version == current_version_and_hash[0]:
+                        continue
+                else:
+                    # Use the schema hash instead
+                    schema_hash = await db.schema_hash()
+                    if schema_hash == current_version_and_hash[1]:
+                        continue
+
+            # Calculate new schema hash
+            schema_hash = await db.schema_hash()
+            placeholders = "(?, ?, ?, ?, ?)"
+            values = [
+                database_name,
+                str(db.path),
+                db.is_memory,
+                schema_version,
+                schema_hash,
+            ]
             if db.path is None:
-                placeholders = "(?, null, ?, ?)"
-                values = [database_name, db.is_memory, schema_version]
+                placeholders = "(?, null, ?, ?, ?)"
+                values = [database_name, db.is_memory, schema_version, schema_hash]
             await internal_db.execute_write(
                 """
-                INSERT OR REPLACE INTO databases (database_name, path, is_memory, schema_version)
+                INSERT OR REPLACE INTO databases (database_name, path, is_memory, schema_version, schema_hash)
                 VALUES {}
             """.format(
                     placeholders

datasette/database.py

@@ -1,6 +1,7 @@
 import asyncio
 from collections import namedtuple
 from pathlib import Path
+import hashlib
 import janus
 import queue
 import sys
@@ -50,6 +51,24 @@ class Database:
         # This is used to track all file connections so they can be closed
         self._all_file_connections = []
 
+    async def schema_version(self):
+        # This can return 'None' if the schema_version cannot be read
+        # See https://github.com/simonw/datasette/issues/2058
+        try:
+            return (await self.execute("PRAGMA schema_version")).first()[0]
+        except sqlite3.OperationalError:
+            return None
+
+    async def schema_hash(self):
+        return hashlib.md5(
+            (
+                (
+                    await self.execute("SELECT group_concat(sql) FROM sqlite_master")
+                ).first()[0]
+                or ""
+            ).encode("utf8")
+        ).hexdigest()
+
     @property
     def cached_table_counts(self):
         if self._cached_table_counts is not None:

datasette/utils/internal_db.py

@@ -9,7 +9,8 @@ async def init_internal_db(db):
         database_name TEXT PRIMARY KEY,
         path TEXT,
         is_memory INTEGER,
-        schema_version INTEGER
+        schema_version INTEGER,
+        schema_hash TEXT
     );
     CREATE TABLE IF NOT EXISTS tables (
         database_name TEXT,

tests/test_internal_db.py

@@ -1,4 +1,7 @@
 import pytest
+from unittest.mock import patch
+from datasette.app import Datasette
+from datasette.database import Database
 
 
 @pytest.mark.asyncio
@@ -83,3 +86,51 @@ async def test_internal_foreign_keys(ds_client):
         "table_name",
         "from",
     }
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("schema_version_returns_none", (True, False))
+async def test_detects_schema_changes(schema_version_returns_none):
+    ds = Datasette()
+    db_name = "test_detects_schema_changes_{}".format(schema_version_returns_none)
+    db = ds.add_memory_database(db_name)
+    # Test if Datasette correctly detects schema changes, whether or not
+    # the schema_version method is working.
+    # https://github.com/simonw/datasette/issues/2058
+
+    _internal = ds.get_database("_internal")
+
+    async def get_tables():
+        return [
+            dict(r)
+            for r in await _internal.execute(
+                "select table_name from tables where database_name = ?", [db_name]
+            )
+        ]
+
+    async def test_it():
+        await ds.refresh_schemas()
+        initial_hash = await db.schema_hash()
+        # _internal should list zero tables
+        tables = await get_tables()
+        assert tables == []
+        # Create a new table
+        await db.execute_write("CREATE TABLE test (id INTEGER PRIMARY KEY)")
+        await ds.refresh_schemas()
+        assert await db.schema_hash() != initial_hash
+        # _internal should list one table
+        tables = await get_tables()
+        assert tables == [
+            {"table_name": "test"},
+        ]
+
+    async def schema_version_none(self):
+        return None
+
+    if schema_version_returns_none:
+        with patch(
+            "datasette.database.Database.schema_version", new=schema_version_none
+        ):
+            await test_it()
+    else:
+        await test_it()