fix(watch): don't built schema twice in watch mode (#558)

Author: benjie

packages/graphile-build-pg/src/plugins/PgIntrospectionPlugin.js

@@ -861,7 +861,7 @@ export default (async function PgIntrospectionPlugin(
       this._start();
     }
 
-    async _start() {
+    async _start(isReconnect = false) {
       if (this.stopped) {
         return;
       }
@@ -921,7 +921,9 @@ export default (async function PgIntrospectionPlugin(
           }
 
           // Trigger re-introspection on server reconnect
-          this._handleChange();
+          if (isReconnect) {
+            this._handleChange();
+          }
         }
       } catch (e) {
         // If something goes wrong, disconnect and try again after a short delay
@@ -949,7 +951,7 @@ export default (async function PgIntrospectionPlugin(
       setTimeout(() => {
         if (!this.stopped) {
           // Listen for further changes
-          this._start();
+          this._start(true);
         }
       }, 2000);
     }

packages/graphile-build/src/SchemaBuilder.js

@@ -510,7 +510,15 @@ class SchemaBuilder extends EventEmitter {
     try {
       this._busy = true;
       this._explicitSchemaListener = listener;
+
+      // We want to ignore `triggerChange` calls that occur whilst we're setting
+      // up the listeners to prevent an unnecessary double schema build.
+      let ignoreChangeTriggers = true;
+
       this.triggerChange = () => {
+        if (ignoreChangeTriggers) {
+          return;
+        }
         this._generatedSchema = null;
         // XXX: optionally debounce
         try {
@@ -530,10 +538,16 @@ class SchemaBuilder extends EventEmitter {
       for (const fn of this.watchers) {
         await fn(this.triggerChange);
       }
+
+      // Now we're about to build the first schema, any further `triggerChange`
+      // calls should be honoured.
+      ignoreChangeTriggers = false;
+
       if (listener) {
         this.on("schema", listener);
       }
       this.emit("schema", this.buildSchema());
+
       this._watching = true;
     } finally {
       this._busy = false;