--nolock mode to ignore locked files, closes #1744

datasette/app.py

@@ -213,6 +213,7 @@ class Datasette:
         config_dir=None,
         pdb=False,
         crossdb=False,
+        nolock=False,
     ):
         assert config_dir is None or isinstance(
             config_dir, Path
@@ -238,6 +239,7 @@ class Datasette:
         self.databases = collections.OrderedDict()
         self._refresh_schemas_lock = asyncio.Lock()
         self.crossdb = crossdb
+        self.nolock = nolock
         if memory or crossdb or not self.files:
             self.add_database(Database(self, is_memory=True), name="_memory")
         # memory_name is a random string so that each Datasette instance gets its own

datasette/cli.py

@@ -452,6 +452,11 @@ def uninstall(packages, yes):
     is_flag=True,
     help="Enable cross-database joins using the /_memory database",
 )
+@click.option(
+    "--nolock",
+    is_flag=True,
+    help="Ignore locking, open locked files in read-only mode",
+)
 @click.option(
     "--ssl-keyfile",
     help="SSL key file",
@@ -486,6 +491,7 @@ def serve(
     open_browser,
     create,
     crossdb,
+    nolock,
     ssl_keyfile,
     ssl_certfile,
     return_instance=False,
@@ -545,6 +551,7 @@ def serve(
         version_note=version_note,
         pdb=pdb,
         crossdb=crossdb,
+        nolock=nolock,
     )
 
     # if files is a single directory, use that as config_dir=

datasette/database.py

@@ -89,6 +89,8 @@ class Database:
         # mode=ro or immutable=1?
         if self.is_mutable:
             qs = "?mode=ro"
+            if self.ds.nolock:
+                qs += "&nolock=1"
         else:
             qs = "?immutable=1"
         assert not (write and not self.is_mutable)

docs/cli-reference.rst

@@ -115,6 +115,7 @@ datasette serve --help
       --create                  Create database files if they do not exist
       --crossdb                 Enable cross-database joins using the /_memory
                                 database
+      --nolock                  Ignore locking, open locked files in read-only mode
       --ssl-keyfile TEXT        SSL key file
       --ssl-certfile TEXT       SSL certificate file
       --help                    Show this message and exit.

docs/getting_started.rst

@@ -56,7 +56,9 @@ like so:
 
 ::
 
-     datasette ~/Library/Application\ Support/Google/Chrome/Default/History
+     datasette ~/Library/Application\ Support/Google/Chrome/Default/History --nolock
+
+The `--nolock` option ignores any file locks. This is safe as Datasette will open the file in read-only mode.
 
 Now visiting http://localhost:8001/History/downloads will show you a web
 interface to browse your downloads data: