Support connstring syntax when discovering databases (#473)

* Support connstring syntax when discovering databases

Support connstring DSNs (`host=... user=... password=... dbname=...`) in
addition to URIs (`postgresql://user:pass@host/dbname`) for purposes of
database discovery.

Connstring syntax is needed to support accessing PostgreSQL via Unix
domain sockets (`host=/run/postgres`), which is not really possible with
URI syntax.

* Appease gometalinter, don't shadow namespace

cmd/postgres_exporter/postgres_exporter.go

@@ -601,6 +601,8 @@ func makeDescMap(pgVersion semver.Version, serverLabels prometheus.Labels, metri
 	for namespace, intermediateMappings := range metricMaps {
 		thisMap := make(map[string]MetricMap)
 
+		namespace = strings.Replace(namespace, "pg", *metricPrefix, 1)
+
 		// Get the constant labels
 		var variableLabels []string
 		for columnName, columnMapping := range intermediateMappings.columnMappings {
@@ -627,8 +629,6 @@ func makeDescMap(pgVersion semver.Version, serverLabels prometheus.Labels, metri
 				}
 			}
 
-			namespace := strings.Replace(namespace, "pg", *metricPrefix, 1)
-
 			// Determine how to convert the column based on its usage.
 			// nolint: dupl
 			switch columnMapping.usage {
@@ -1616,11 +1616,27 @@ func (e *Exporter) scrape(ch chan<- prometheus.Metric) {
 }
 
 func (e *Exporter) discoverDatabaseDSNs() []string {
+	// connstring syntax is complex (and not sure if even regular).
+	// we don't need to parse it, so just superficially validate that it starts
+	// with a valid-ish keyword pair
+	connstringRe := regexp.MustCompile(`^ *[a-zA-Z0-9]+ *= *[^= ]+`)
+
 	dsns := make(map[string]struct{})
 	for _, dsn := range e.dsn {
-		parsedDSN, err := url.Parse(dsn)
-		if err != nil {
-			log.Errorf("Unable to parse DSN (%s): %v", loggableDSN(dsn), err)
+		var dsnURI *url.URL
+		var dsnConnstring string
+
+		if strings.HasPrefix(dsn, "postgresql://") {
+			var err error
+			dsnURI, err = url.Parse(dsn)
+			if err != nil {
+				log.Errorf("Unable to parse DSN as URI (%s): %v", loggableDSN(dsn), err)
+				continue
+			}
+		} else if connstringRe.MatchString(dsn) {
+			dsnConnstring = dsn
+		} else {
+			log.Errorf("Unable to parse DSN as either URI or connstring (%s)", loggableDSN(dsn))
 			continue
 		}
 
@@ -1643,8 +1659,16 @@ func (e *Exporter) discoverDatabaseDSNs() []string {
 			if contains(e.excludeDatabases, databaseName) {
 				continue
 			}
-			parsedDSN.Path = databaseName
-			dsns[parsedDSN.String()] = struct{}{}
+
+			if dsnURI != nil {
+				dsnURI.Path = databaseName
+				dsn = dsnURI.String()
+			} else {
+				// replacing one dbname with another is complicated.
+				// just append new dbname to override.
+				dsn = fmt.Sprintf("%s dbname=%s", dsnConnstring, databaseName)
+			}
+			dsns[dsn] = struct{}{}
 		}
 	}