replace os.Exit with panic for logger.CriticalIf (#6065)

This commit prevents complete server failures caused by
`logger.CriticalIf` calls. Instead of calling `os.Exit(1)`
the function now executes a panic with a special value
indicating that a critical error happend. At the top HTTP
handler layer panics are recovered and if its a critical
error the client gets an InternalServerError status code.

Further this allows unit tests to cover critical-error code
paths.

cmd/gateway-main.go

@@ -197,7 +197,7 @@ func StartGateway(ctx *cli.Context, gw Gateway) {
 		getCert = globalTLSCerts.GetCertificate
 	}
 
-	globalHTTPServer = xhttp.NewServer([]string{gatewayAddr}, registerHandlers(router, globalHandlers...), getCert)
+	globalHTTPServer = xhttp.NewServer([]string{gatewayAddr}, criticalErrorHandler{registerHandlers(router, globalHandlers...)}, getCert)
 	globalHTTPServer.UpdateBytesReadFunc = globalConnStats.incInputBytes
 	globalHTTPServer.UpdateBytesWrittenFunc = globalConnStats.incOutputBytes
 	go func() {

cmd/generic-handlers.go

@@ -738,3 +738,20 @@ func (s securityHeaderHandler) ServeHTTP(w http.ResponseWriter, r *http.Request)
 	header.Set("Content-Security-Policy", "block-all-mixed-content") // prevent mixed (HTTP / HTTPS content)
 	s.handler.ServeHTTP(w, r)
 }
+
+// criticalErrorHandler handles critical server failures caused by
+// `panic(logger.ErrCritical)` as done by `logger.CriticalIf`.
+//
+// It should be always the first / highest HTTP handler.
+type criticalErrorHandler struct{ handler http.Handler }
+
+func (h criticalErrorHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	defer func() {
+		if err := recover(); err == logger.ErrCritical { // handle
+			writeErrorResponse(w, ErrInternalError, r.URL)
+		} else if err != nil {
+			panic(err) // forward other panic calls
+		}
+	}()
+	h.handler.ServeHTTP(w, r)
+}

cmd/logger/logger.go

@@ -362,13 +362,15 @@ func LogIf(ctx context.Context, err error) {
 	fmt.Println(output)
 }
 
-// CriticalIf :
-// Like LogIf with exit
-// It'll be called for fatal error conditions during run-time
+// ErrCritical is the value panic'd whenever CriticalIf is called.
+var ErrCritical struct{}
+
+// CriticalIf logs the provided error on the console. It fails the
+// current go-routine by causing a `panic(ErrCritical)`.
 func CriticalIf(ctx context.Context, err error) {
 	if err != nil {
 		LogIf(ctx, err)
-		os.Exit(1)
+		panic(ErrCritical)
 	}
 }
 

cmd/server-main.go

@@ -288,7 +288,7 @@ func serverMain(ctx *cli.Context) {
 		getCert = globalTLSCerts.GetCertificate
 	}
 
-	globalHTTPServer = xhttp.NewServer([]string{globalMinioAddr}, handler, getCert)
+	globalHTTPServer = xhttp.NewServer([]string{globalMinioAddr}, criticalErrorHandler{handler}, getCert)
 	globalHTTPServer.UpdateBytesReadFunc = globalConnStats.incInputBytes
 	globalHTTPServer.UpdateBytesWrittenFunc = globalConnStats.incOutputBytes
 	go func() {