Rotate log file on SIGHUP

Add functionality to rotate the log file when `SIGHUP` signal is received.

Also, a doc is added with few details about logging.

Signed-off-by: Ionut Balutoiu <ibalutoiu@cloudbasesolutions.com>

cmd/garm/main.go

@@ -24,6 +24,7 @@ import (
 	"net/http"
 	"os"
 	"os/signal"
+	"syscall"
 	"time"
 
 	"github.com/cloudbase/garm/apiserver/controllers"
@@ -37,6 +38,7 @@ import (
 	"github.com/cloudbase/garm/util"
 	"github.com/cloudbase/garm/util/appdefaults"
 	"github.com/cloudbase/garm/websocket"
+	lumberjack "gopkg.in/natefinch/lumberjack.v2"
 
 	"github.com/gorilla/handlers"
 	"github.com/gorilla/mux"
@@ -82,6 +84,19 @@ func main() {
 		log.Fatalf("fetching log writer: %+v", err)
 	}
 
+	if cfg.Default.LogFile != "" {
+		// rotate log file on SIGHUP
+		ch := make(chan os.Signal, 1)
+		signal.Notify(ch, syscall.SIGHUP)
+		go func() {
+			for range ch {
+				if err := logWriter.(*lumberjack.Logger).Rotate(); err != nil {
+					log.Printf("failed to rotate log file: %v\n", err)
+				}
+			}
+		}()
+	}
+
 	var writers []io.Writer = []io.Writer{
 		logWriter,
 	}

doc/logging.md

@@ -0,0 +1,17 @@
+# Logging
+
+By default, GARM is logging only on standard output.
+
+If you would like GARM to use a logging file instead, you can use the `log_file` configuration option:
+
+```toml
+[default]
+# Use this if you'd like to log to a file instead of standard output.
+log_file = "/tmp/runner-manager.log"
+```
+
+## Rotating log files
+
+If GARM uses a log file, by default it will rotate it when it reaches 500MB or 28 days, whichever comes first.
+
+However, if you want to manually rotate the log file, you can send a `SIGHUP` signal to the GARM process.