Support connection service file and service names

The whole point of supporting this can best be said by directly quoting
the PostgreSQL manual:

> The connection service file allows libpq connection parameters to be
> associated with a single service name. That service name can then be
> specified by a libpq connection, and the associated settings will be
> used. This allows connection parameters to be modified without
> requiring a recompile of the libpq application. The service name can
> also be specified using the PGSERVICE environment variable.

source: https://www.postgresql.org/docs/current/libpq-pgservice.html

Fixes #538

Author: feikesteenbergen

conn.go

@@ -1863,7 +1863,9 @@ func parseEnviron(env []string) (out map[string]string) {
 			accrue("user")
 		case "PGPASSWORD":
 			accrue("password")
-		case "PGSERVICE", "PGSERVICEFILE", "PGREALM":
+		case "PGSERVICE":
+			accrue("service")
+		case "PGREALM":
 			unsupported()
 		case "PGOPTIONS":
 			accrue("options")
@@ -1901,6 +1903,63 @@ func parseEnviron(env []string) (out map[string]string) {
 	return out
 }
 
+// parseServiceFile parses the options from a service file and adds them to the values.
+//
+// The parsing code is based on parseServiceInfo from libpq's fe-connect.c
+func parseServiceFile(service string, o values) error {
+	filename := os.Getenv("PGSERVICEFILE")
+	if filename == "" {
+		// XXX this code doesn't work on Windows where the default filename is
+		// XXX %APPDATA%\postgresql\pgpass.conf
+		// Prefer $HOME over user.Current due to glibc bug: golang.org/issue/13470
+		userHome := os.Getenv("HOME")
+		if userHome == "" {
+			user, err := user.Current()
+			if err != nil {
+				return err
+			}
+			userHome = user.HomeDir
+		}
+		filename = filepath.Join(userHome, ".pg_service.conf")
+	}
+
+	file, err := os.Open(filename)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+
+		// once we find the header of our section, we can start reading
+		if line == fmt.Sprintf("[%s]", service) {
+			for scanner.Scan() {
+				line = strings.TrimSpace(scanner.Text())
+				// once we find the next section, we're done
+				if strings.HasPrefix(line, "[") {
+					return nil
+				} else if line != "" {
+					if err := parseOpts(line, o); err != nil {
+						return err
+					}
+				}
+			}
+			// EOF means we're done
+			return nil
+		}
+	}
+
+	if err := scanner.Err(); err != nil {
+		return err
+	}
+
+	// if we end up here, we didn't find the service that was explicitly provided
+	// that seems reason enough to give up
+	panic(fmt.Sprintf(`definition of service "%s" not found`, service))
+}
+
 // isUTF8 returns whether name is a fuzzy variation of the string "UTF-8".
 func isUTF8(name string) bool {
 	// Recognize all sorts of silly things as "UTF-8", like Postgres does

connector.go

@@ -47,6 +47,7 @@ func NewConnector(dsn string) (*Connector, error) {
 	//
 	// * Very low precedence defaults applied in every situation
 	// * Environment variables
+	// * Service name variables
 	// * Explicitly passed connection information
 	o["host"] = "localhost"
 	o["port"] = "5432"
@@ -68,6 +69,24 @@ func NewConnector(dsn string) (*Connector, error) {
 		return nil, err
 	}
 
+	// whenever a service is specified, we will need to parse the connection service file
+	// and override the defaults with the parameters specified for that service.
+	// See https://www.postgresql.org/docs/current/libpq-pgservice.html
+	if service, ok := o["service"]; ok {
+		if err := parseServiceFile(service, o); err != nil {
+			return nil, err
+		}
+
+		// By overwriting the options with the service parameters we may have masked some
+		// explicitly passed connection information, e.g. "service=staging user=read_only".
+		// By repeating the parseOpts we overcome this issue.
+		if err := parseOpts(dsn, o); err != nil {
+			return nil, err
+		}
+		// "service" itself should not be passed down as a connection parameter
+		delete(o, "service")
+	}
+
 	// Use the "fallback" application name if necessary
 	if fallback, ok := o["fallback_application_name"]; ok {
 		if _, ok := o["application_name"]; !ok {