Add basic test for TargetManager.targetSet

Verify that if the configs change, target groups are cleaned on
TargetManager.reload (rather than having old ones linger around, even if
they are no longer present in the configs).

This covers the bug fixed in #1907 -- I verified that by checking out
source from before that commit.

This is a start on #1906

retrieval/targetmanager_test.go

@@ -12,3 +12,63 @@
 // limitations under the License.
 
 package retrieval
+
+import (
+	"testing"
+
+	"golang.org/x/net/context"
+	"gopkg.in/yaml.v2"
+
+	"github.com/prometheus/prometheus/config"
+	"github.com/prometheus/prometheus/storage/local"
+)
+
+func TestTargetSetRecreatesTargetGroupsEveryRun(t *testing.T) {
+	scrapeConfig := &config.ScrapeConfig{}
+
+	sOne := `
+job_name: "foo"
+dns_sd_configs:
+- names:
+  - "srv.name.one.example.org"
+`
+	if err := yaml.Unmarshal([]byte(sOne), scrapeConfig); err != nil {
+		t.Fatalf("Unable to load YAML config sOne: %s", err)
+	}
+
+	// Not properly setting it up, but that seems okay
+	mss := &local.MemorySeriesStorage{}
+
+	ts := newTargetSet(scrapeConfig, mss)
+
+	ts.runProviders(context.Background(), providersFromConfig(scrapeConfig))
+
+	verifyPresence(t, ts.tgroups, "dns/0/srv.name.one.example.org", true)
+
+	sTwo := `
+job_name: "foo"
+dns_sd_configs:
+- names:
+  - "srv.name.two.example.org"
+`
+	if err := yaml.Unmarshal([]byte(sTwo), scrapeConfig); err != nil {
+		t.Fatalf("Unable to load YAML config sTwo: %s", err)
+	}
+
+	ts.runProviders(context.Background(), providersFromConfig(scrapeConfig))
+
+	verifyPresence(t, ts.tgroups, "dns/0/srv.name.one.example.org", false)
+	verifyPresence(t, ts.tgroups, "dns/0/srv.name.two.example.org", true)
+
+}
+
+func verifyPresence(t *testing.T, tgroups map[string][]*Target, name string, present bool) {
+	if _, ok := tgroups[name]; ok != present {
+		msg := ""
+		if !present {
+			msg = "not "
+		}
+		t.Fatalf("'%s' should %sbe present in TargetSet.tgroups: %s", name, msg, tgroups)
+	}
+
+}