Monkey patch generic-pool to work through errors

 * previously without a minimum the pool would stop handling database
   requests due to worker exhaustion (for any requests started before
   worker exhaustion)
 * This patch ensures that if we have waiting work, we will continue
   processing it
 * a test that fails before this patch and succeeds after this patch:
  'postgresql connector errors'

Author: bobbysmith007

lib/postgresql.js

@@ -50,6 +50,38 @@ exports.initialize = function initializeDataSource(dataSource, callback) {
   }
 };
 
+// there is a bug in the generic pool where it will not recreate
+// destroyed workers (even if there is waiting work to do) unless
+// there is a min specified. Make sure we keep some connections
+// SEE: https://github.com/coopernurse/node-pool/pull/186
+// SEE: https://github.com/brianc/node-pg-pool/issues/48
+// SEE: https://github.com/strongloop/loopback-connector-postgresql/issues/231
+function _ensureMinimum() {
+  var i, diff, waiting;
+  if (this._draining) return;
+  waiting = this._waitingClients.size();
+  if (this._factory.min > 0) { // we have positive specified minimum
+    diff = this._factory.min - this._count;
+  } else if (waiting > 0) { // we have no minimum, but we do have work to do
+    diff = Math.min(waiting, this._factory.max - this._count);
+  }
+  for (i = 0; i < diff; i++) {
+    this._createResource();
+  }
+};
+function makePool(options) {
+  var pg = new postgresql.Pool(options);
+  // Monkey patch to ensure we always finish our work
+  //  - There is a bug where callbacks go uncalled if min is not set
+  //  - We might still not want a connection to *always* exist
+  //  - but we do want to create up to max connections if we have work
+  //  - still waiting
+  // This should be safe till the version of pg-pool is upgraded
+  // SEE: https://github.com/coopernurse/node-pool/pull/186
+  pg.pool._ensureMinimum = _ensureMinimum;
+  return pg;
+}
+
 /**
  * PostgreSQL connector constructor
  *
@@ -78,7 +110,8 @@ function PostgreSQL(postgresql, settings) {
     this.clientConfig.connectionString = settings.url;
   }
   this.clientConfig.Promise = Promise;
-  this.pg = new postgresql.Pool(this.clientConfig);
+  this.pg = makePool(this.clientConfig);
+
   this.settings = settings;
   if (settings.debug) {
     debug('Settings %j', settings);

test/postgresql.initialization.test.js

@@ -5,7 +5,7 @@
 
 'use strict';
 require('./init');
-
+var Promise = require('bluebird');
 var connector = require('..');
 var DataSource = require('loopback-datasource-juggler').DataSource;
 var should = require('should');
@@ -52,3 +52,37 @@ describe('initialization', function() {
     clientConfig.connectionString.should.equal(urlOnly.url);
   });
 });
+
+describe('postgresql connector errors', function() {
+  it('Should complete these 4 queries without dying', function(done) {
+    var dataSource = getDataSource();
+    var db = dataSource.connector;
+    var pool = db.pg.pool;
+    pool._factory.max = 5;
+    pool._factory.min = null;
+    var errors = 0;
+    var shouldGet = 0;
+    function runErrorQuery() {
+      shouldGet++;
+      return new Promise(function(resolve, reject) {
+        db.executeSQL("SELECT 'asd'+1 ", [], {}, function(err, res) {
+          if (err) {
+            errors++;
+            console.log('got err', errors);
+            resolve(err);
+          } else {
+            reject(res); // this should always error
+          }
+        });
+      });
+    };
+    var ps = [];
+    for (var i = 0; i < 12; i++) {
+      ps.push(runErrorQuery());
+    }
+    Promise.all(ps).then(function() {
+      shouldGet.should.equal(errors);
+      done();
+    });
+  });
+});