Delete previous plans on autoplan or atlantis plan

When using non-default workspaces, plans are stored
in pr-and-workspace-specific directories.
If a PR is subsequently updated it might happen that
some of the plans are no longer relevant with regards
to the latest changes.
This change ensures that plans are always deleted
when a generic plan is triggered either by autoplan
or by a "atlantis plan" command.
NB Plans are not cleaned up when specific projects are
planned explicitly with "atlantis plan -p/-d/-w".

server/events/command_runner_test.go

@@ -476,7 +476,8 @@ func TestRunAutoplanCommand_DeletePlans(t *testing.T) {
 		ThenReturn(tmp, nil)
 	fixtures.Pull.BaseRepo = fixtures.GithubRepo
 	ch.RunAutoplanCommand(fixtures.GithubRepo, fixtures.GithubRepo, fixtures.Pull, fixtures.User)
-	pendingPlanFinder.VerifyWasCalledOnce().DeletePlans(tmp)
+	// gets called twice: the first time before the plan starts, the second time after the plan errors
+	pendingPlanFinder.VerifyWasCalled(Times(2)).DeletePlans(tmp)
 }
 
 func TestFailedApprovalCreatesFailedStatusUpdate(t *testing.T) {

server/events/plan_command_runner.go

@@ -105,6 +105,10 @@ func (p *PlanCommandRunner) runAutoplan(ctx *CommandContext) {
 		ctx.Log.Warn("unable to update commit status: %s", err)
 	}
 
+	// discard previous plans that might not be relevant anymore
+	ctx.Log.Debug("deleting previous plans")
+	p.deletePlans(ctx)
+
 	// Only run commands in parallel if enabled
 	var result CommandResult
 	if p.isParallelEnabled(projectCmds) {
@@ -179,6 +183,13 @@ func (p *PlanCommandRunner) run(ctx *CommandContext, cmd *CommentCommand) {
 
 	projectCmds, policyCheckCmds := p.partitionProjectCmds(ctx, projectCmds)
 
+	// if the plan is generic, new plans will be generated based on changes
+	// discard previous plans that might not be relevant anymore
+	if !cmd.IsForSpecificProject() {
+		ctx.Log.Debug("deleting previous plans")
+		p.deletePlans(ctx)
+	}
+
 	// Only run commands in parallel if enabled
 	var result CommandResult
 	if p.isParallelEnabled(projectCmds) {