db.execute_write_fn(create_tables, block=True) hangs a thread if connection fails

[simonw]

Discovered in simonw/latest-datasette-with-all-plugins#3 (comment)

[simonw]

The bug is here:

datasette/datasette/database.py

Lines 89 to 100 in 13b3b51

	def _execute_writes(self):
	# Infinite looping thread that protects the single write connection
	# to this database
	conn = self.connect(write=True)
	while True:
	task = self._write_queue.get()
	try:
	result = task.fn(conn)
	except Exception as e:
	print(e)
	result = e
	task.reply_queue.sync_q.put(result)

If conn = self.connect(write=True) raises an exception the entire server hangs, like this:

% datasette -i fixtures.db  --get /            
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/threading.py", line 932, in _bootstrap_inner
    self.run()
  File "/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/simon/.local/share/virtualenvs/latest-datasette-with-all-plugins-PJL_Xy9e/lib/python3.8/site-packages/datasette/database.py", line 92, in _execute_writes
    conn = self.connect(write=True)
  File "/Users/simon/.local/share/virtualenvs/latest-datasette-with-all-plugins-PJL_Xy9e/lib/python3.8/site-packages/datasette/database.py", line 55, in connect
    assert not (write and not self.is_mutable)
AssertionError

... server hangs here ...

[simonw]

The easiest way to recreate this is to attempt a write against an immutable database, which triggers this assertion error:

datasette/datasette/database.py

Lines 47 to 55 in 13b3b51

	def connect(self, write=False):
	if self.is_memory:
	return sqlite3.connect(":memory:")
	# mode=ro or immutable=1?
	if self.is_mutable:
	qs = "?mode=ro"
	else:
	qs = "?immutable=1"
	assert not (write and not self.is_mutable)

[simonw]

This implementation seems to fix it, need to work out how to test though.

diff --git a/datasette/database.py b/datasette/database.py
index ffa7a79..7ba1456 100644
--- a/datasette/database.py
+++ b/datasette/database.py
@@ -89,14 +89,22 @@ class Database:
     def _execute_writes(self):
         # Infinite looping thread that protects the single write connection
         # to this database
-        conn = self.connect(write=True)
+        conn_exception = None
+        conn = None
+        try:
+            conn = self.connect(write=True)
+        except Exception as e:
+            conn_exception = e
         while True:
             task = self._write_queue.get()
-            try:
-                result = task.fn(conn)
-            except Exception as e:
-                print(e)
-                result = e
+            if conn_exception is not None:
+                result = conn_exception
+            else:
+                try:
+                    result = task.fn(conn)
+                except Exception as e:
+                    print(e)
+                    result = e
             task.reply_queue.sync_q.put(result)
 
     async def execute_fn(self, fn):