Add a test case to detect regression on issue fixed by PR #39.

Related issue is #38. Time to start expanding functional test coverage since
the integration tests missed this.

Makefile

@@ -21,7 +21,7 @@ vet:
 	go vet .
 
 test:
-	go test -v .
+	go test -v -cover .
 
 test-integration: postgres_exporter postgres_exporter_integration_test
 	tests/test-smoke ./postgres_exporter ./postgres_exporter_integration_test

postgres_exporter.go

@@ -508,6 +508,9 @@ func makeDescMap(pgVersion semver.Version, metricMaps map[string]map[string]Colu
 			// Force to discard if not compatible.
 			if columnMapping.supportedVersions != nil {
 				if !columnMapping.supportedVersions(pgVersion) {
+					// It's very useful to be able to see what columns are being
+					// rejected.
+					log.Debugln(columnName, "is being forced to discard due to version incompatibility.")
 					thisMap[columnName] = MetricMap{
 						discard: true,
 						conversion: func(in interface{}) (float64, bool) {

postgres_exporter_test.go

@@ -0,0 +1,87 @@
+// +build !integration
+
+package main
+
+import (
+	"testing"
+	. "gopkg.in/check.v1"
+
+	"github.com/blang/semver"
+)
+
+// Hook up gocheck into the "go test" runner.
+func Test(t *testing.T) { TestingT(t) }
+
+type FunctionalSuite struct{
+	e *Exporter
+}
+
+var _ = Suite(&FunctionalSuite{})
+
+func (s *FunctionalSuite) SetUpSuite(c *C) {
+
+}
+
+func (s *FunctionalSuite) TestSemanticVersionColumnDiscard(c *C) {
+	testMetricMap := map[string]map[string]ColumnMapping{
+		"test_namespace" : map[string]ColumnMapping{
+			"metric_which_stays" : {COUNTER, "This metric should not be eliminated", nil, nil},
+			"metric_which_discards" : {COUNTER, "This metric should be forced to DISCARD", nil, nil},
+		},
+	}
+
+	{
+		// No metrics should be eliminated
+		resultMap := makeDescMap(semver.MustParse("0.0.1"), testMetricMap)
+		c.Check(
+			resultMap["test_namespace"].columnMappings["metric_which_stays"].discard,
+			Equals,
+			false,
+		)
+		c.Check(
+			resultMap["test_namespace"].columnMappings["metric_which_discards"].discard,
+			Equals,
+			false,
+		)
+	}
+
+	{
+		// Update the map so the discard metric should be eliminated
+		discardable_metric := testMetricMap["test_namespace"]["metric_which_discards"]
+		discardable_metric.supportedVersions = semver.MustParseRange(">0.0.1")
+		testMetricMap["test_namespace"]["metric_which_discards"] = discardable_metric
+
+		// Discard metric should be discarded
+		resultMap := makeDescMap(semver.MustParse("0.0.1"), testMetricMap)
+		c.Check(
+			resultMap["test_namespace"].columnMappings["metric_which_stays"].discard,
+			Equals,
+			false,
+		)
+		c.Check(
+			resultMap["test_namespace"].columnMappings["metric_which_discards"].discard,
+			Equals,
+			true,
+		)
+	}
+
+	{
+		// Update the map so the discard metric should be kept but has a version
+		discardable_metric := testMetricMap["test_namespace"]["metric_which_discards"]
+		discardable_metric.supportedVersions = semver.MustParseRange(">0.0.1")
+		testMetricMap["test_namespace"]["metric_which_discards"] = discardable_metric
+
+		// Discard metric should be discarded
+		resultMap := makeDescMap(semver.MustParse("0.0.2"), testMetricMap)
+		c.Check(
+			resultMap["test_namespace"].columnMappings["metric_which_stays"].discard,
+			Equals,
+			false,
+		)
+		c.Check(
+			resultMap["test_namespace"].columnMappings["metric_which_discards"].discard,
+			Equals,
+			false,
+		)
+	}
+}