Add disk based edge caching support. (#5182)

This PR adds disk based edge caching support for minio server.

Cache settings can be configured in config.json to take list of disk drives,
cache expiry in days and file patterns to exclude from cache or via environment
variables MINIO_CACHE_DRIVES, MINIO_CACHE_EXCLUDE and MINIO_CACHE_EXPIRY

Design assumes that Atime support is enabled and the list of cache drives is
fixed.
 - Objects are cached on both GET and PUT/POST operations.
 - Expiry is used as hint to evict older entries from cache, or if 80% of cache
   capacity is filled.
 - When object storage backend is down, GET, LIST and HEAD operations fetch
   object seamlessly from cache.

Current Limitations
 - Bucket policies are not cached, so anonymous operations are not supported in
   offline mode.
 - Objects are distributed using deterministic hashing among list of cache
   drives specified.If one or more drives go offline, or cache drive
   configuration is altered - performance could degrade to linear lookup.

Fixes #4026

cmd/utils.go

@@ -334,3 +334,34 @@ func newContext(r *http.Request, api string) context.Context {
 
 	return logger.SetContext(context.Background(), &logger.ReqInfo{r.RemoteAddr, r.Header.Get("user-agent"), "", api, bucket, object, nil})
 }
+
+// isNetworkOrHostDown - if there was a network error or if the host is down.
+func isNetworkOrHostDown(err error) bool {
+	if err == nil {
+		return false
+	}
+	switch err.(type) {
+	case *net.DNSError, *net.OpError, net.UnknownNetworkError:
+		return true
+	case *url.Error:
+		// For a URL error, where it replies back "connection closed"
+		if strings.Contains(err.Error(), "Connection closed by foreign host") {
+			return true
+		}
+		return true
+	default:
+		if strings.Contains(err.Error(), "net/http: TLS handshake timeout") {
+			// If error is - tlsHandshakeTimeoutError,.
+			return true
+		} else if strings.Contains(err.Error(), "i/o timeout") {
+			// If error is - tcp timeoutError.
+			return true
+		} else if strings.Contains(err.Error(), "connection timed out") {
+			// If err is a net.Dial timeout.
+			return true
+		} else if strings.Contains(err.Error(), "net/http: HTTP/1.x transport connection broken") {
+			return true
+		}
+	}
+	return false
+}