feat: allow logging to stdout, stderr

The --log option accepts "stdout", "stderr" as filenames. This prevents
rest-server from opending these files, it simply writs the the STDOUT or
STDERR stream provided by the caller.

**BREAKING** in case use really used "stdout", "stderr" to specify file
names, you'll need to update your rest-server invocation to use
"./stdout", "./stderr".

changelog/unreleased/pull-217

@@ -0,0 +1,13 @@
+Feature: Allos "stdout", "stderr" as filenames for the `--log` option.
+
+When (e.g. for debugging purpose) the rest server is invoked like
+
+	sudo -u restic rest-server … --log /dev/stdout
+
+it tries to open `/dev/stdout` (O_CREATE, O_WRITE, O_APPEND). This
+operation fails, as in the above invocation, `/dev/stdout` is owned by
+the caller (here: root) and only writable for the caller. Subprocesses
+get just the filedescriptor. Using `/proc/self/fd/1` didn't work either,
+for the same reasons.
+
+https://github.com/restic/rest-server/pull/217

cmd/rest-server/main.go

@@ -46,7 +46,7 @@ func init() {
 	flags.StringVar(&cpuProfile, "cpu-profile", cpuProfile, "write CPU profile to file")
 	flags.BoolVar(&server.Debug, "debug", server.Debug, "output debug messages")
 	flags.StringVar(&server.Listen, "listen", server.Listen, "listen address")
-	flags.StringVar(&server.Log, "log", server.Log, "write HTTP requests in the combined log format to the specified `filename`")
+	flags.StringVar(&server.Log, "log", server.Log, "write HTTP requests in the combined log format to the specified `filename` (\"stdout\", \"stderr\" can also be used)")
 	flags.Int64Var(&server.MaxRepoSize, "max-size", server.MaxRepoSize, "the maximum size of the repository in bytes")
 	flags.StringVar(&server.Path, "path", server.Path, "data directory")
 	flags.BoolVar(&server.TLS, "tls", server.TLS, "turn on TLS support")

mux.go

@@ -2,6 +2,7 @@ package restserver
 
 import (
 	"fmt"
+	"io"
 	"log"
 	"net/http"
 	"os"
@@ -21,9 +22,19 @@ func (s *Server) debugHandler(next http.Handler) http.Handler {
 }
 
 func (s *Server) logHandler(next http.Handler) http.Handler {
-	accessLog, err := os.OpenFile(s.Log, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0644)
-	if err != nil {
-		log.Fatalf("error: %v", err)
+	var accessLog io.Writer
+
+	switch s.Log {
+	case "stdout":
+		accessLog = os.Stdout
+	case "stderr":
+		accessLog = os.Stderr
+	default:
+		var err error
+		accessLog, err = os.OpenFile(s.Log, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0644)
+		if err != nil {
+			log.Fatalf("error: %v", err)
+		}
 	}
 
 	return handlers.CombinedLoggingHandler(accessLog, next)