[pluginDiscovery] dedupe packs by absolute path (#16829)

* [pluginDiscovery] dedupe packs by absolute path

* [pluginDiscovery] extract distinct keySelector, add more docs

src/plugin_discovery/__tests__/find_plugin_specs.js

@@ -70,5 +70,27 @@ describe('plugin discovery', () => {
       expect(specs.map(s => s.getId()).sort())
         .to.eql(['bar:two', 'foo']);
     });
+
+    it('dedupes duplicate packs', async () => {
+      const { spec$ } = findPluginSpecs({
+        plugins: {
+          scanDirs: [PLUGIN_FIXTURES],
+          paths: [
+            resolve(PLUGIN_FIXTURES, 'foo'),
+            resolve(PLUGIN_FIXTURES, 'foo'),
+            resolve(PLUGIN_FIXTURES, 'bar'),
+            resolve(PLUGIN_FIXTURES, 'bar'),
+          ],
+        }
+      });
+
+      const specs = await spec$.toArray().toPromise();
+      expect(specs).to.have.length(3);
+      specs.forEach(spec => {
+        expect(spec).to.be.a(PluginSpec);
+      });
+      expect(specs.map(s => s.getId()).sort())
+        .to.eql(['bar:one', 'bar:two', 'foo']);
+    });
   });
 });

src/plugin_discovery/find_plugin_specs.js

@@ -1,4 +1,5 @@
 import { Observable } from 'rxjs';
+import { realpathSync } from 'fs';
 
 import { transformDeprecations, Config } from '../server/config';
 
@@ -31,6 +32,27 @@ function bufferAllResults(observable) {
     .mergeMap(array => array);
 }
 
+/**
+ * Determine a distinct value for each result from find$
+ * so they can be deduplicated
+ * @param  {{error?,pack?}} result
+ * @return {Any}
+ */
+function getDistinctKeyForFindResult(result) {
+  // errors are distinct by their message
+  if (result.error) {
+    return result.error.message;
+  }
+
+  // packs are distinct by their absolute and real path
+  if (result.pack) {
+    return realpathSync(result.pack.getPath());
+  }
+
+  // non error/pack results shouldn't exist, but if they do they are all unique
+  return result;
+}
+
 /**
  *  Creates a collection of observables for discovering pluginSpecs
  *  using Kibana's defaults, settings, and config service
@@ -46,6 +68,7 @@ export function findPluginSpecs(settings, config = defaultConfig(settings)) {
     ...config.get('plugins.paths').map(createPackAtPath$),
     ...config.get('plugins.scanDirs').map(createPacksInDirectory$)
   )
+    .distinct(getDistinctKeyForFindResult)
     .share();
 
   const extendConfig$ = find$