import asyncio
import collections
import hashlib
import itertools
import json
import os
import re
import sys
import threading
import traceback
import urllib.parse
from concurrent import futures
from pathlib import Path

import click
from markupsafe import Markup
import jinja2
from jinja2 import ChoiceLoader, Environment, FileSystemLoader, PrefixLoader, escape
from jinja2.environment import Template
from jinja2.exceptions import TemplateNotFound
import uvicorn

from .views.base import DatasetteError, ureg, AsgiRouter
from .views.database import DatabaseDownload, DatabaseView
from .views.index import IndexView
from .views.special import JsonDataView, PatternPortfolioView
from .views.table import RowView, TableView
from .renderer import json_renderer
from .database import Database, QueryInterrupted

from .utils import (
    escape_css_string,
    escape_sqlite,
    format_bytes,
    module_from_path,
    parse_metadata,
    sqlite3,
    to_css_class,
)
from .utils.asgi import (
    AsgiLifespan,
    NotFound,
    Request,
    Response,
    asgi_static,
    asgi_send,
    asgi_send_html,
    asgi_send_json,
    asgi_send_redirect,
)
from .tracer import AsgiTracer
from .plugins import pm, DEFAULT_PLUGINS, get_plugins
from .version import __version__

app_root = Path(__file__).parent.parent

MEMORY = object()

ConfigOption = collections.namedtuple("ConfigOption", ("name", "default", "help"))
CONFIG_OPTIONS = (
    ConfigOption("default_page_size", 100, "Default page size for the table view"),
    ConfigOption(
        "max_returned_rows",
        1000,
        "Maximum rows that can be returned from a table or custom query",
    ),
    ConfigOption(
        "num_sql_threads",
        3,
        "Number of threads in the thread pool for executing SQLite queries",
    ),
    ConfigOption(
        "sql_time_limit_ms", 1000, "Time limit for a SQL query in milliseconds"
    ),
    ConfigOption(
        "default_facet_size", 30, "Number of values to return for requested facets"
    ),
    ConfigOption(
        "facet_time_limit_ms", 200, "Time limit for calculating a requested facet"
    ),
    ConfigOption(
        "facet_suggest_time_limit_ms",
        50,
        "Time limit for calculating a suggested facet",
    ),
    ConfigOption(
        "hash_urls",
        False,
        "Include DB file contents hash in URLs, for far-future caching",
    ),
    ConfigOption(
        "allow_facet",
        True,
        "Allow users to specify columns to facet using ?_facet= parameter",
    ),
    ConfigOption(
        "allow_download",
        True,
        "Allow users to download the original SQLite database files",
    ),
    ConfigOption("suggest_facets", True, "Calculate and display suggested facets"),
    ConfigOption("allow_sql", True, "Allow arbitrary SQL queries via ?sql= parameter"),
    ConfigOption(
        "default_cache_ttl",
        5,
        "Default HTTP cache TTL (used in Cache-Control: max-age= header)",
    ),
    ConfigOption(
        "default_cache_ttl_hashed",
        365 * 24 * 60 * 60,
        "Default HTTP cache TTL for hashed URL pages",
    ),
    ConfigOption(
        "cache_size_kb", 0, "SQLite cache size in KB (0 == use SQLite default)"
    ),
    ConfigOption(
        "allow_csv_stream",
        True,
        "Allow .csv?_stream=1 to download all rows (ignoring max_returned_rows)",
    ),
    ConfigOption(
        "max_csv_mb",
        100,
        "Maximum size allowed for CSV export in MB - set 0 to disable this limit",
    ),
    ConfigOption(
        "truncate_cells_html",
        2048,
        "Truncate cells longer than this in HTML table view - set 0 to disable",
    ),
    ConfigOption(
        "force_https_urls",
        False,
        "Force URLs in API output to always use https:// protocol",
    ),
    ConfigOption(
        "template_debug",
        False,
        "Allow display of template debug information with ?_context=1",
    ),
    ConfigOption("base_url", "/", "Datasette URLs should use this base"),
)

DEFAULT_CONFIG = {option.name: option.default for option in CONFIG_OPTIONS}


async def favicon(scope, receive, send):
    await asgi_send(send, "", 200)


class Datasette:
    def __init__(
        self,
        files,
        immutables=None,
        cache_headers=True,
        cors=False,
        inspect_data=None,
        metadata=None,
        sqlite_extensions=None,
        template_dir=None,
        plugins_dir=None,
        static_mounts=None,
        memory=False,
        config=None,
        version_note=None,
        config_dir=None,
    ):
        assert config_dir is None or isinstance(
            config_dir, Path
        ), "config_dir= should be a pathlib.Path"
        self.files = tuple(files) + tuple(immutables or [])
        if config_dir:
            self.files += tuple([str(p) for p in config_dir.glob("*.db")])
        if (
            config_dir
            and (config_dir / "inspect-data.json").exists()
            and not inspect_data
        ):
            inspect_data = json.load((config_dir / "inspect-data.json").open())
            if immutables is None:
                immutable_filenames = [i["file"] for i in inspect_data.values()]
                immutables = [
                    f for f in self.files if Path(f).name in immutable_filenames
                ]
        self.inspect_data = inspect_data
        self.immutables = set(immutables or [])
        if not self.files:
            self.files = [MEMORY]
        elif memory:
            self.files = (MEMORY,) + self.files
        self.databases = collections.OrderedDict()
        for file in self.files:
            path = file
            is_memory = False
            if file is MEMORY:
                path = None
                is_memory = True
            is_mutable = path not in self.immutables
            db = Database(self, path, is_mutable=is_mutable, is_memory=is_memory)
            if db.name in self.databases:
                raise Exception("Multiple files with same stem: {}".format(db.name))
            self.add_database(db.name, db)
        self.cache_headers = cache_headers
        self.cors = cors
        metadata_files = []
        if config_dir:
            metadata_files = [
                config_dir / filename
                for filename in ("metadata.json", "metadata.yaml", "metadata.yml")
                if (config_dir / filename).exists()
            ]
        if config_dir and metadata_files and not metadata:
            with metadata_files[0].open() as fp:
                metadata = parse_metadata(fp.read())
        self._metadata = metadata or {}
        self.sqlite_functions = []
        self.sqlite_extensions = sqlite_extensions or []
        if config_dir and (config_dir / "templates").is_dir() and not template_dir:
            template_dir = str((config_dir / "templates").resolve())
        self.template_dir = template_dir
        if config_dir and (config_dir / "plugins").is_dir() and not plugins_dir:
            plugins_dir = str((config_dir / "plugins").resolve())
        self.plugins_dir = plugins_dir
        if config_dir and (config_dir / "static").is_dir() and not static_mounts:
            static_mounts = [("static", str((config_dir / "static").resolve()))]
        self.static_mounts = static_mounts or []
        if config_dir and (config_dir / "config.json").exists() and not config:
            config = json.load((config_dir / "config.json").open())
        self._config = dict(DEFAULT_CONFIG, **(config or {}))
        self.renderers = {}  # File extension -> renderer function
        self.version_note = version_note
        self.executor = futures.ThreadPoolExecutor(
            max_workers=self.config("num_sql_threads")
        )
        self.max_returned_rows = self.config("max_returned_rows")
        self.sql_time_limit_ms = self.config("sql_time_limit_ms")
        self.page_size = self.config("default_page_size")
        # Execute plugins in constructor, to ensure they are available
        # when the rest of `datasette inspect` executes
        if self.plugins_dir:
            for filename in os.listdir(self.plugins_dir):
                filepath = os.path.join(self.plugins_dir, filename)
                mod = module_from_path(filepath, name=filename)
                try:
                    pm.register(mod)
                except ValueError:
                    # Plugin already registered
                    pass

        # Configure Jinja
        default_templates = str(app_root / "datasette" / "templates")
        template_paths = []
        if self.template_dir:
            template_paths.append(self.template_dir)
        plugin_template_paths = [
            plugin["templates_path"]
            for plugin in get_plugins()
            if plugin["templates_path"]
        ]
        template_paths.extend(plugin_template_paths)
        template_paths.append(default_templates)
        template_loader = ChoiceLoader(
            [
                FileSystemLoader(template_paths),
                # Support {% extends "default:table.html" %}:
                PrefixLoader(
                    {"default": FileSystemLoader(default_templates)}, delimiter=":"
                ),
            ]
        )
        self.jinja_env = Environment(
            loader=template_loader, autoescape=True, enable_async=True
        )
        self.jinja_env.filters["escape_css_string"] = escape_css_string
        self.jinja_env.filters["quote_plus"] = lambda u: urllib.parse.quote_plus(u)
        self.jinja_env.filters["escape_sqlite"] = escape_sqlite
        self.jinja_env.filters["to_css_class"] = to_css_class
        # pylint: disable=no-member
        pm.hook.prepare_jinja2_environment(env=self.jinja_env)

        self.register_renderers()

    def add_database(self, name, db):
        self.databases[name] = db

    def remove_database(self, name):
        self.databases.pop(name)

    def config(self, key):
        return self._config.get(key, None)

    def config_dict(self):
        # Returns a fully resolved config dictionary, useful for templates
        return {option.name: self.config(option.name) for option in CONFIG_OPTIONS}

    def metadata(self, key=None, database=None, table=None, fallback=True):
        """
        Looks up metadata, cascading backwards from specified level.
        Returns None if metadata value is not found.
        """
        assert not (
            database is None and table is not None
        ), "Cannot call metadata() with table= specified but not database="
        databases = self._metadata.get("databases") or {}
        search_list = []
        if database is not None:
            search_list.append(databases.get(database) or {})
        if table is not None:
            table_metadata = ((databases.get(database) or {}).get("tables") or {}).get(
                table
            ) or {}
            search_list.insert(0, table_metadata)
        search_list.append(self._metadata)
        if not fallback:
            # No fallback allowed, so just use the first one in the list
            search_list = search_list[:1]
        if key is not None:
            for item in search_list:
                if key in item:
                    return item[key]
            return None
        else:
            # Return the merged list
            m = {}
            for item in search_list:
                m.update(item)
            return m

    def plugin_config(self, plugin_name, database=None, table=None, fallback=True):
        "Return config for plugin, falling back from specified database/table"
        plugins = self.metadata(
            "plugins", database=database, table=table, fallback=fallback
        )
        if plugins is None:
            return None
        plugin_config = plugins.get(plugin_name)
        # Resolve any $file and $env keys
        if isinstance(plugin_config, dict):
            # Create a copy so we don't mutate the version visible at /-/metadata.json
            plugin_config_copy = dict(plugin_config)
            for key, value in plugin_config_copy.items():
                if isinstance(value, dict):
                    if list(value.keys()) == ["$env"]:
                        plugin_config_copy[key] = os.environ.get(
                            list(value.values())[0]
                        )
                    elif list(value.keys()) == ["$file"]:
                        plugin_config_copy[key] = open(list(value.values())[0]).read()
            return plugin_config_copy
        return plugin_config

    def app_css_hash(self):
        if not hasattr(self, "_app_css_hash"):
            self._app_css_hash = hashlib.sha1(
                open(os.path.join(str(app_root), "datasette/static/app.css"))
                .read()
                .encode("utf8")
            ).hexdigest()[:6]
        return self._app_css_hash

    def get_canned_queries(self, database_name):
        queries = self.metadata("queries", database=database_name, fallback=False) or {}
        names = queries.keys()
        return [self.get_canned_query(database_name, name) for name in names]

    def get_canned_query(self, database_name, query_name):
        queries = self.metadata("queries", database=database_name, fallback=False) or {}
        query = queries.get(query_name)
        if query:
            if not isinstance(query, dict):
                query = {"sql": query}
            query["name"] = query_name
            return query

    def update_with_inherited_metadata(self, metadata):
        # Fills in source/license with defaults, if available
        metadata.update(
            {
                "source": metadata.get("source") or self.metadata("source"),
                "source_url": metadata.get("source_url") or self.metadata("source_url"),
                "license": metadata.get("license") or self.metadata("license"),
                "license_url": metadata.get("license_url")
                or self.metadata("license_url"),
                "about": metadata.get("about") or self.metadata("about"),
                "about_url": metadata.get("about_url") or self.metadata("about_url"),
            }
        )

    def prepare_connection(self, conn, database):
        conn.row_factory = sqlite3.Row
        conn.text_factory = lambda x: str(x, "utf-8", "replace")
        for name, num_args, func in self.sqlite_functions:
            conn.create_function(name, num_args, func)
        if self.sqlite_extensions:
            conn.enable_load_extension(True)
            for extension in self.sqlite_extensions:
                conn.execute("SELECT load_extension('{}')".format(extension))
        if self.config("cache_size_kb"):
            conn.execute("PRAGMA cache_size=-{}".format(self.config("cache_size_kb")))
        # pylint: disable=no-member
        pm.hook.prepare_connection(conn=conn, database=database, datasette=self)

    async def execute(
        self,
        db_name,
        sql,
        params=None,
        truncate=False,
        custom_time_limit=None,
        page_size=None,
        log_sql_errors=True,
    ):
        return await self.databases[db_name].execute(
            sql,
            params=params,
            truncate=truncate,
            custom_time_limit=custom_time_limit,
            page_size=page_size,
            log_sql_errors=log_sql_errors,
        )

    async def expand_foreign_keys(self, database, table, column, values):
        "Returns dict mapping (column, value) -> label"
        labeled_fks = {}
        db = self.databases[database]
        foreign_keys = await db.foreign_keys_for_table(table)
        # Find the foreign_key for this column
        try:
            fk = [
                foreign_key
                for foreign_key in foreign_keys
                if foreign_key["column"] == column
            ][0]
        except IndexError:
            return {}
        label_column = await db.label_column_for_table(fk["other_table"])
        if not label_column:
            return {(fk["column"], value): str(value) for value in values}
        labeled_fks = {}
        sql = """
            select {other_column}, {label_column}
            from {other_table}
            where {other_column} in ({placeholders})
        """.format(
            other_column=escape_sqlite(fk["other_column"]),
            label_column=escape_sqlite(label_column),
            other_table=escape_sqlite(fk["other_table"]),
            placeholders=", ".join(["?"] * len(set(values))),
        )
        try:
            results = await self.execute(database, sql, list(set(values)))
        except QueryInterrupted:
            pass
        else:
            for id, value in results:
                labeled_fks[(fk["column"], id)] = value
        return labeled_fks

    def absolute_url(self, request, path):
        url = urllib.parse.urljoin(request.url, path)
        if url.startswith("http://") and self.config("force_https_urls"):
            url = "https://" + url[len("http://") :]
        return url

    def register_custom_units(self):
        "Register any custom units defined in the metadata.json with Pint"
        for unit in self.metadata("custom_units") or []:
            ureg.define(unit)

    def connected_databases(self):
        return [
            {
                "name": d.name,
                "path": d.path,
                "size": d.size,
                "is_mutable": d.is_mutable,
                "is_memory": d.is_memory,
                "hash": d.hash,
            }
            for d in sorted(self.databases.values(), key=lambda d: d.name)
        ]

    def versions(self):
        conn = sqlite3.connect(":memory:")
        self.prepare_connection(conn, ":memory:")
        sqlite_version = conn.execute("select sqlite_version()").fetchone()[0]
        sqlite_extensions = {}
        for extension, testsql, hasversion in (
            ("json1", "SELECT json('{}')", False),
            ("spatialite", "SELECT spatialite_version()", True),
        ):
            try:
                result = conn.execute(testsql)
                if hasversion:
                    sqlite_extensions[extension] = result.fetchone()[0]
                else:
                    sqlite_extensions[extension] = None
            except Exception:
                pass
        # Figure out supported FTS versions
        fts_versions = []
        for fts in ("FTS5", "FTS4", "FTS3"):
            try:
                conn.execute(
                    "CREATE VIRTUAL TABLE v{fts} USING {fts} (data)".format(fts=fts)
                )
                fts_versions.append(fts)
            except sqlite3.OperationalError:
                continue
        datasette_version = {"version": __version__}
        if self.version_note:
            datasette_version["note"] = self.version_note
        return {
            "python": {
                "version": ".".join(map(str, sys.version_info[:3])),
                "full": sys.version,
            },
            "datasette": datasette_version,
            "asgi": "3.0",
            "uvicorn": uvicorn.__version__,
            "sqlite": {
                "version": sqlite_version,
                "fts_versions": fts_versions,
                "extensions": sqlite_extensions,
                "compile_options": [
                    r[0] for r in conn.execute("pragma compile_options;").fetchall()
                ],
            },
        }

    def plugins(self, show_all=False):
        ps = list(get_plugins())
        if not show_all:
            ps = [p for p in ps if p["name"] not in DEFAULT_PLUGINS]
        return [
            {
                "name": p["name"],
                "static": p["static_path"] is not None,
                "templates": p["templates_path"] is not None,
                "version": p.get("version"),
            }
            for p in ps
        ]

    def threads(self):
        threads = list(threading.enumerate())
        d = {
            "num_threads": len(threads),
            "threads": [
                {"name": t.name, "ident": t.ident, "daemon": t.daemon} for t in threads
            ],
        }
        # Only available in Python 3.7+
        if hasattr(asyncio, "all_tasks"):
            tasks = asyncio.all_tasks()
            d.update(
                {
                    "num_tasks": len(tasks),
                    "tasks": [_cleaner_task_str(t) for t in tasks],
                }
            )
        return d

    def table_metadata(self, database, table):
        "Fetch table-specific metadata."
        return (
            (self.metadata("databases") or {})
            .get(database, {})
            .get("tables", {})
            .get(table, {})
        )

    def register_renderers(self):
        """ Register output renderers which output data in custom formats. """
        # Built-in renderers
        self.renderers["json"] = json_renderer

        # Hooks
        hook_renderers = []
        # pylint: disable=no-member
        for hook in pm.hook.register_output_renderer(datasette=self):
            if type(hook) == list:
                hook_renderers += hook
            else:
                hook_renderers.append(hook)

        for renderer in hook_renderers:
            self.renderers[renderer["extension"]] = renderer["callback"]

    async def render_template(
        self, templates, context=None, request=None, view_name=None
    ):
        context = context or {}
        if isinstance(templates, Template):
            template = templates
        else:
            if isinstance(templates, str):
                templates = [templates]
            template = self.jinja_env.select_template(templates)
        body_scripts = []
        # pylint: disable=no-member
        for script in pm.hook.extra_body_script(
            template=template.name,
            database=context.get("database"),
            table=context.get("table"),
            view_name=view_name,
            datasette=self,
        ):
            body_scripts.append(Markup(script))

        extra_template_vars = {}
        # pylint: disable=no-member
        for extra_vars in pm.hook.extra_template_vars(
            template=template.name,
            database=context.get("database"),
            table=context.get("table"),
            view_name=view_name,
            request=request,
            datasette=self,
        ):
            if callable(extra_vars):
                extra_vars = extra_vars()
            if asyncio.iscoroutine(extra_vars):
                extra_vars = await extra_vars
            assert isinstance(extra_vars, dict), "extra_vars is of type {}".format(
                type(extra_vars)
            )
            extra_template_vars.update(extra_vars)

        template_context = {
            **context,
            **{
                "app_css_hash": self.app_css_hash(),
                "zip": zip,
                "body_scripts": body_scripts,
                "format_bytes": format_bytes,
                "extra_css_urls": self._asset_urls("extra_css_urls", template, context),
                "extra_js_urls": self._asset_urls("extra_js_urls", template, context),
                "base_url": self.config("base_url"),
            },
            **extra_template_vars,
        }
        if request and request.args.get("_context") and self.config("template_debug"):
            return "<pre>{}</pre>".format(
                jinja2.escape(json.dumps(template_context, default=repr, indent=4))
            )

        return await template.render_async(template_context)

    def _asset_urls(self, key, template, context):
        # Flatten list-of-lists from plugins:
        seen_urls = set()
        for url_or_dict in itertools.chain(
            itertools.chain.from_iterable(
                getattr(pm.hook, key)(
                    template=template.name,
                    database=context.get("database"),
                    table=context.get("table"),
                    datasette=self,
                )
            ),
            (self.metadata(key) or []),
        ):
            if isinstance(url_or_dict, dict):
                url = url_or_dict["url"]
                sri = url_or_dict.get("sri")
            else:
                url = url_or_dict
                sri = None
            if url in seen_urls:
                continue
            seen_urls.add(url)
            if sri:
                yield {"url": url, "sri": sri}
            else:
                yield {"url": url}

    def app(self):
        "Returns an ASGI app function that serves the whole of Datasette"
        routes = []

        def add_route(view, regex):
            routes.append((regex, view))

        # Generate a regex snippet to match all registered renderer file extensions
        renderer_regex = "|".join(r"\." + key for key in self.renderers.keys())

        add_route(IndexView.as_asgi(self), r"/(?P<as_format>(\.jsono?)?$)")
        # TODO: /favicon.ico and /-/static/ deserve far-future cache expires
        add_route(favicon, "/favicon.ico")

        add_route(
            asgi_static(app_root / "datasette" / "static"), r"/-/static/(?P<path>.*)$"
        )
        for path, dirname in self.static_mounts:
            add_route(asgi_static(dirname), r"/" + path + "/(?P<path>.*)$")

        # Mount any plugin static/ directories
        for plugin in get_plugins():
            if plugin["static_path"]:
                add_route(
                    asgi_static(plugin["static_path"]),
                    "/-/static-plugins/{}/(?P<path>.*)$".format(plugin["name"]),
                )
                # Support underscores in name in addition to hyphens, see https://github.com/simonw/datasette/issues/611
                add_route(
                    asgi_static(plugin["static_path"]),
                    "/-/static-plugins/{}/(?P<path>.*)$".format(
                        plugin["name"].replace("-", "_")
                    ),
                )
        add_route(
            JsonDataView.as_asgi(self, "metadata.json", lambda: self._metadata),
            r"/-/metadata(?P<as_format>(\.json)?)$",
        )
        add_route(
            JsonDataView.as_asgi(self, "versions.json", self.versions),
            r"/-/versions(?P<as_format>(\.json)?)$",
        )
        add_route(
            JsonDataView.as_asgi(self, "plugins.json", self.plugins),
            r"/-/plugins(?P<as_format>(\.json)?)$",
        )
        add_route(
            JsonDataView.as_asgi(self, "config.json", lambda: self._config),
            r"/-/config(?P<as_format>(\.json)?)$",
        )
        add_route(
            JsonDataView.as_asgi(self, "threads.json", self.threads),
            r"/-/threads(?P<as_format>(\.json)?)$",
        )
        add_route(
            JsonDataView.as_asgi(self, "databases.json", self.connected_databases),
            r"/-/databases(?P<as_format>(\.json)?)$",
        )
        add_route(
            PatternPortfolioView.as_asgi(self), r"/-/patterns$",
        )
        add_route(
            DatabaseDownload.as_asgi(self), r"/(?P<db_name>[^/]+?)(?P<as_db>\.db)$"
        )
        add_route(
            DatabaseView.as_asgi(self),
            r"/(?P<db_name>[^/]+?)(?P<as_format>"
            + renderer_regex
            + r"|.jsono|\.csv)?$",
        )
        add_route(
            TableView.as_asgi(self),
            r"/(?P<db_name>[^/]+)/(?P<table_and_format>[^/]+?$)",
        )
        add_route(
            RowView.as_asgi(self),
            r"/(?P<db_name>[^/]+)/(?P<table>[^/]+?)/(?P<pk_path>[^/]+?)(?P<as_format>"
            + renderer_regex
            + r")?$",
        )
        self.register_custom_units()

        async def setup_db():
            # First time server starts up, calculate table counts for immutable databases
            for dbname, database in self.databases.items():
                if not database.is_mutable:
                    await database.table_counts(limit=60 * 60 * 1000)

        asgi = AsgiLifespan(
            AsgiTracer(DatasetteRouter(self, routes)), on_startup=setup_db
        )
        for wrapper in pm.hook.asgi_wrapper(datasette=self):
            asgi = wrapper(asgi)
        return asgi


class DatasetteRouter(AsgiRouter):
    def __init__(self, datasette, routes):
        self.ds = datasette
        super().__init__(routes)

    async def route_path(self, scope, receive, send, path):
        # Strip off base_url if present before routing
        base_url = self.ds.config("base_url")
        if base_url != "/" and path.startswith(base_url):
            path = "/" + path[len(base_url) :]
        return await super().route_path(scope, receive, send, path)

    async def handle_404(self, scope, receive, send, exception=None):
        # If URL has a trailing slash, redirect to URL without it
        path = scope.get("raw_path", scope["path"].encode("utf8"))
        if path.endswith(b"/"):
            path = path.rstrip(b"/")
            if scope["query_string"]:
                path += b"?" + scope["query_string"]
            await asgi_send_redirect(send, path.decode("latin1"))
        else:
            # Is there a pages/* template matching this path?
            template_path = os.path.join("pages", *scope["path"].split("/")) + ".html"
            try:
                template = self.ds.jinja_env.select_template([template_path])
            except TemplateNotFound:
                template = None
            if template:
                headers = {}
                status = [200]

                def custom_header(name, value):
                    headers[name] = value
                    return ""

                def custom_status(code):
                    status[0] = code
                    return ""

                def custom_redirect(location, code=302):
                    status[0] = code
                    headers["Location"] = location
                    return ""

                body = await self.ds.render_template(
                    template,
                    {
                        "custom_header": custom_header,
                        "custom_status": custom_status,
                        "custom_redirect": custom_redirect,
                    },
                    request=Request(scope, receive),
                    view_name="page",
                )
                # Pull content-type out into separate parameter
                content_type = "text/html; charset=utf-8"
                matches = [k for k in headers if k.lower() == "content-type"]
                if matches:
                    content_type = headers[matches[0]]
                await asgi_send(
                    send,
                    body,
                    status=status[0],
                    headers=headers,
                    content_type=content_type,
                )
            else:
                await self.handle_500(
                    scope, receive, send, exception or NotFound("404")
                )

    async def handle_500(self, scope, receive, send, exception):
        title = None
        if isinstance(exception, NotFound):
            status = 404
            info = {}
            message = exception.args[0]
        elif isinstance(exception, DatasetteError):
            status = exception.status
            info = exception.error_dict
            message = exception.message
            if exception.messagge_is_html:
                message = Markup(message)
            title = exception.title
        else:
            status = 500
            info = {}
            message = str(exception)
            traceback.print_exc()
        templates = ["500.html"]
        if status != 500:
            templates = ["{}.html".format(status)] + templates
        info.update({"ok": False, "error": message, "status": status, "title": title})
        headers = {}
        if self.ds.cors:
            headers["Access-Control-Allow-Origin"] = "*"
        if scope["path"].split("?")[0].endswith(".json"):
            await asgi_send_json(send, info, status=status, headers=headers)
        else:
            template = self.ds.jinja_env.select_template(templates)
            await asgi_send_html(
                send, await template.render_async(info), status=status, headers=headers
            )


_cleaner_task_str_re = re.compile(r"\S*site-packages/")


def _cleaner_task_str(task):
    s = str(task)
    # This has something like the following in it:
    # running at /Users/simonw/Dropbox/Development/datasette/venv-3.7.5/lib/python3.7/site-packages/uvicorn/main.py:361>
    # Clean up everything up to and including site-packages
    return _cleaner_task_str_re.sub("", s)