More work on how to configure the sqlite3 JS API bootstrapping process from higher-level code. Initial version of sqlite3-worker1-promiser, a Promise-based proxy for the Worker API #1.

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+/*
+  2022-08-24
+
+  The author disclaims copyright to this source code.  In place of a
+  legal notice, here is a blessing:
+
+  *   May you do good and not evil.
+  *   May you find forgiveness for yourself and forgive others.
+  *   May you share freely, never taking more than you give.
+
+  ***********************************************************************
+
+  This file implements a Promise-based proxy for the sqlite3 Worker
+  API #1. It is intended to be included either from the main thread or
+  a Worker, but only if (A) the environment supports nested Workers
+  and (B) it's _not_ a Worker which loads the sqlite3 WASM/JS
+  module. This file's features will load that module and provide a
+  slightly simpler client-side interface than the slightly-lower-level
+  Worker API does.
+
+  This script necessarily exposes on global symbol, but clients may
+  freely `delete` that symbol after calling it.
+*/
+'use strict';
+/**
+   Configures an sqlite3 Worker API #1 Worker such that it can be
+   manipulated via a Promise-based interface and returns a factory
+   function which returns Promises for communicating with the worker.
+   This proxy has an _almost_ identical interface to the normal
+   worker API, with any exceptions noted below.
+
+   It requires a configuration object with the following properties:
+
+   - `worker` (required): a Worker instance which loads
+   `sqlite3-worker1.js` or a functional equivalent. Note that this
+   function replaces the worker.onmessage property. This property
+   may alternately be a function, in which case this function
+   re-assigns this property with the result of calling that
+   function, enabling delayed instantiation of a Worker.
+
+   - `onready` (optional, but...): this callback is called with no
+   arguments when the worker fires its initial
+   'sqlite3-api'/'worker1-ready' message, which it does when
+   sqlite3.initWorker1API() completes its initialization. This is
+   the simplest way to tell the worker to kick of work at the
+   earliest opportunity.
+
+   - `onerror` (optional): a callback to pass error-type events from
+   the worker. The object passed to it will be the error message
+   payload from the worker. This is _not_ the same as the
+   worker.onerror property!
+
+   - `onunhandled` (optional): a callback which gets passed the
+   message event object for any worker.onmessage() events which
+   are not handled by this proxy. Ideally that "should" never
+   happen, as this proxy aims to handle all known message types.
+
+   - `generateMessageId` (optional): a function which, when passed
+   an about-to-be-posted message object, generates a _unique_
+   message ID for the message, which this API then assigns as the
+   messageId property of the message. It _must_ generate unique
+   IDs so that dispatching can work. If not defined, a default
+   generator is used.
+
+   - `dbId` (optional): is the database ID to be used by the
+   worker. This must initially be unset or a falsy value. The
+   first `open` message sent to the worker will cause this config
+   entry to be assigned to the ID of the opened database. That ID
+   "should" be set as the `dbId` property of the message sent in
+   future requests, so that the worker uses that database.
+   However, if the worker is not given an explicit dbId, it will
+   use the first-opened database by default. If client code needs
+   to work with multiple database IDs, the client-level code will
+   need to juggle those themselves. A `close` message will clear
+   this property if it matches the ID of the closed db. Potential
+   TODO: add a config callback specifically for reporting `open`
+   and `close` message results, so that clients may track those
+   values.
+
+   - `debug` (optional): a console.debug()-style function for logging
+   information about messages.
+
+
+   This function returns a stateful factory function with the following
+   interfaces:
+
+   - Promise function(messageType, messageArgs)
+   - Promise function({message object})
+
+   The first form expects the "type" and "args" values for a Worker
+   message. The second expects an object in the form {type:...,
+   args:...}  plus any other properties the client cares to set. This
+   function will always set the messageId property on the object,
+   even if it's already set, and will set the dbId property to
+   config.dbId if it is _not_ set in the message object.
+
+   The function throws on error.
+
+   The function installs a temporarily message listener, posts a
+   message to the configured Worker, and handles the message's
+   response via the temporary message listener. The then() callback
+   of the returned Promise is passed the `message.data` property from
+   the resulting message, i.e. the payload from the worker, stripped
+   of the lower-level event state which the onmessage() handler
+   receives.
+
+   Example usage:
+
+   ```
+   const config = {...};
+   const eventPromiser = sqlite3Worker1Promiser(config);
+   eventPromiser('open', {filename:"/foo.db"}).then(function(msg){
+     console.log("open response",msg); // => {type:'open', result: {filename:'/foo.db'}, ...}
+     // Recall that config.dbId will be set for the first 'open'
+     // call and cleared for a matching 'close' call.
+   });
+   eventPromiser({type:'close'}).then((msg)=>{
+     console.log("open response",msg); // => {type:'open', result: {filename:'/foo.db'}, ...}
+     // Recall that config.dbId will be used by default for the message's dbId if
+     // none is explicitly provided, and a 'close' op will clear config.dbId if it
+     // closes that exact db.
+   });
+   ```
+
+   Differences from Worker API #1:
+
+   - exec's {callback: STRING} option does not work via this
+   interface (it triggers an exception), but {callback: function}
+   does and works exactly like the STRING form does in the Worker:
+   the callback is called one time for each row of the result set
+   and once more, at the end, passed only `null`, to indicate that
+   the end of the result set has been reached. Note that the rows
+   arrive via worker-posted messages, with all the implications
+   of that.
+
+
+   TODO?: a config option which causes it to queue up events to fire
+   one at a time and flush the event queue on the first error. The
+   main use for this is test runs which must fail at the first error.
+*/
+self.sqlite3Worker1Promiser = function callee(config = callee.defaultConfig){
+  // Inspired by: https://stackoverflow.com/a/52439530
+  let idNumber = 0;
+  const handlerMap = Object.create(null);
+  const noop = function(){};
+  const err = config.onerror || noop;
+  const debug = config.debug || noop;
+  const genMsgId = config.generateMessageId || function(msg){
+    return msg.type+'#'+(++idNumber);
+  };
+  const toss = (...args)=>{throw new Error(args.join(' '))};
+  if('function'===typeof config.worker) config.worker = config.worker();
+  config.worker.onmessage = function(ev){
+    ev = ev.data;
+    debug('worker1.onmessage',ev);
+    let msgHandler = handlerMap[ev.messageId];
+    if(!msgHandler){
+      if(ev && 'sqlite3-api'===ev.type && 'worker1-ready'===ev.result) {
+        /*fired one time when the Worker1 API initializes*/
+        if(config.onready) config.onready();
+        return;
+      }
+      msgHandler = handlerMap[ev.type] /* check for exec per-row callback */;
+      if(msgHandler && msgHandler.onrow){
+        msgHandler.onrow(ev.row);
+        return;
+      }        
+      if(config.onunhandled) config.onunhandled(arguments[0]);
+      else err("sqlite3Worker1Promiser() unhandled worker message:",ev);
+      return;
+    }
+    delete handlerMap[ev.messageId];
+    switch(ev.type){
+        case 'error':
+          msgHandler.reject(ev);
+          return;
+        case 'open':
+          if(!config.dbId) config.dbId = ev.dbId;
+          break;
+        case 'close':
+          if(config.dbId === ev.dbId) config.dbId = undefined;
+          break;
+        default:
+          break;
+    }
+    msgHandler.resolve(ev);
+  }/*worker.onmessage()*/;
+  return function(/*(msgType, msgArgs) || (msg)*/){
+    let msg;
+    if(1===arguments.length){
+      msg = arguments[0];
+    }else if(2===arguments.length){
+      msg = {
+        type: arguments[0],
+        args: arguments[1]
+      };
+    }else{
+      toss("Invalid arugments for sqlite3Worker1Promiser()-created factory.");
+    }
+    if(!msg.dbId) msg.dbId = config.dbId;
+    msg.messageId = genMsgId(msg);
+    msg.departureTime = performance.now();
+    const proxy = Object.create(null);
+    proxy.message = msg;
+    let cbId /* message handler ID for exec on-row callback proxy */;
+    if('exec'===msg.type && msg.args){
+      if('function'===typeof msg.args.callback){
+        cbId = genMsgId(msg)+':row';
+        proxy.onrow = msg.args.callback;
+        msg.args.callback = cbId;
+        handlerMap[cbId] = proxy;
+      }else if('string' === typeof msg.args.callback){
+        toss("exec callback may not be a string when using the Promise interface.");
+      }
+    }
+    //debug("requestWork", msg);
+    const p = new Promise(function(resolve, reject){
+      proxy.resolve = resolve;
+      proxy.reject = reject;
+      handlerMap[msg.messageId] = proxy;
+      debug("Posting",msg.type,"message to Worker dbId="+(config.dbId||'default')+':',msg);
+      config.worker.postMessage(msg);
+    });
+    if(cbId) p.finally(()=>delete handlerMap[cbId]);
+    return p;
+  };
+}/*sqlite3Worker1Promiser()*/;
+self.sqlite3Worker1Promiser.defaultConfig = {
+  worker: ()=>new Worker('sqlite3-worker1.js'),
+  onerror: console.error.bind(console),
+  dbId: undefined
+};