[SUREFIRE-1864] Excessive use of discover by the JUnitPlatformProvider

**[Leonard Brünings](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=leonard84)** opened **[SUREFIRE-1864](https://issues.apache.org/jira/browse/SUREFIRE-1864?redirect=false)** and commented

This issue is derived from https://github.com/spockframework/spock/issues/1240

The current implementation of the `JUnitPlatformProvider` together with its helper classes, especially `TestPlanScannerFilter` first make a separate discover call for each class and then another discovery request by calling execute with another `LauncherDiscoveryRequest` instead of reusing the `TestPlan` returned by a previous discovery.

The issue here, is that engines have to compute many things during the discovery that are then simply discarded and need to be recomputed again.

To improve this, surefire should call `org.junit.platform.launcher.core.DefaultLauncher#discover(org.junit.platform.launcher.LauncherDiscoveryRequest)` first with **all** the classes and then use the returned `TestPlan` to execute it instead of recomputing in by calling  `org.junit.platform.launcher.core.DefaultLauncher#execute(org.junit.platform.launcher.TestPlan, org.junit.platform.launcher.TestExecutionListener...)`

instead of `org.junit.platform.launcher.core.DefaultLauncher#execute(org.junit.platform.launcher.LauncherDiscoveryRequest, org.junit.platform.launcher.TestExecutionListener...)`

Alternatively, it could forego a separate discovery and only call `org.junit.platform.launcher.core.DefaultLauncher#execute(org.junit.platform.launcher.LauncherDiscoveryRequest, org.junit.platform.launcher.TestExecutionListener...)`

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**Affects:** 3.0.0-M5

3 votes, 4 watchers

Start with JUnitPlatformProvider and its TestPlanScannerFilter helper, then trace how DefaultLauncher discovery and execution requests are built. Compare the separate per-class discover calls with the later execute call and determine whether the returned TestPlan can be reused or discovery can be avoided. Done means eliminating the redundant engine discovery while preserving execution behavior.

Written by the indexing model from the issue text.