open-consul/agent/uiserver/uiserver.go

package uiserver

import (
	"bytes"
	"fmt"
	"io/ioutil"
	"net/http"
	"os"
	"regexp"
	"strings"
	"sync/atomic"
	"text/template"

	"github.com/hashicorp/consul/agent/config"
	"github.com/hashicorp/consul/logging"
	"github.com/hashicorp/go-hclog"
)

// Handler is the http.Handler that serves the Consul UI. It may serve from the
// compiled-in AssetFS or from and external dir. It provides a few important
// transformations on the index.html file and includes a proxy for metrics
// backends.
type Handler struct {
	// state is a reloadableState struct accessed through an atomic value to make
	// it safe to reload at run time. Each call to ServeHTTP will see the latest
	// version of the state without internal locking needed.
	state  atomic.Value
	logger hclog.Logger
}

// reloadableState encapsulates all the state that might be modified during
// ReloadConfig.
type reloadableState struct {
	cfg *config.UIConfig
	srv http.Handler
	err error
}

// NewHandler returns a Handler that can be used to serve UI http requests. It
// accepts a full agent config since properties like ACLs being enabled affect
// the UI so we need more than just UIConfig parts.
func NewHandler(agentCfg *config.RuntimeConfig, logger hclog.Logger) *Handler {
	h := &Handler{
		logger: logger.Named(logging.UIServer),
	}
	// Don't return the error since this is likely the result of a
	// misconfiguration and reloading config could fix it. Instead we'll capture
	// it and return an error for all calls to ServeHTTP so the misconfiguration
	// is visible. Sadly we can't log effectively
	if err := h.ReloadConfig(agentCfg); err != nil {
		h.state.Store(reloadableState{
			err: err,
		})
	}
	return h
}

// ServeHTTP implements http.Handler and serves UI HTTP requests
func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	// TODO: special case for compiled metrics assets in later PR
	s := h.getState()
	if s == nil {
		panic("nil state")
	}
	if s.err != nil {
		http.Error(w, "UI server is misconfigured.", http.StatusInternalServerError)
		h.logger.Error("Failed to configure UI server: %s", s.err)
		return
	}
	s.srv.ServeHTTP(w, r)
}

// ReloadConfig is called by the agent when the configuration is reloaded and
// updates the UIConfig values the handler uses to serve requests.
func (h *Handler) ReloadConfig(newCfg *config.RuntimeConfig) error {
	newState := reloadableState{
		cfg: &newCfg.UIConfig,
	}

	var fs http.FileSystem

	if newCfg.UIConfig.Dir == "" {
		// Serve from assetFS
		fs = assetFS()
	} else {
		fs = http.Dir(newCfg.UIConfig.Dir)
	}

	// Render a new index.html with the new config values ready to serve.
	buf, info, err := renderIndex(newCfg, fs)
	if _, ok := err.(*os.PathError); ok && newCfg.UIConfig.Dir != "" {
		// A Path error indicates that there is no index.html. This could happen if
		// the user configured their own UI dir and is serving something that is not
		// our usual UI. This won't work perfectly because our uiserver will still
		// redirect everything to the UI but we shouldn't fail the entire UI server
		// with a 500 in this case. Partly that's just bad UX and partly it's a
		// breaking change although quite an edge case. Instead, continue but just
		// return a 404 response for the index.html and log a warning.
		h.logger.Warn("ui_config.dir does not contain an index.html. Index templating and redirects to index.html are disabled.")
	} else if err != nil {
		return err
	}

	// buf can be nil in the PathError case above. We should skip this part but
	// still serve the rest of the files in that case.
	if buf != nil {
		// Create a new fs that serves the rendered index file or falls back to the
		// underlying FS.
		fs = &bufIndexFS{
			fs:            fs,
			indexRendered: buf,
			indexInfo:     info,
		}

		// Wrap the buffering FS our redirect FS. This needs to happen later so that
		// redirected requests for /index.html get served the rendered version not the
		// original.
		fs = &redirectFS{fs: fs}
	}

	newState.srv = http.FileServer(fs)

	// Store the new state
	h.state.Store(newState)
	return nil
}

// getState is a helper to access the atomic internal state
func (h *Handler) getState() *reloadableState {
	if cfg, ok := h.state.Load().(reloadableState); ok {
		return &cfg
	}
	return nil
}

func renderIndex(cfg *config.RuntimeConfig, fs http.FileSystem) ([]byte, os.FileInfo, error) {
	// Open the original index.html
	f, err := fs.Open("/index.html")
	if err != nil {
		return nil, nil, err
	}
	defer f.Close()

	content, err := ioutil.ReadAll(f)
	if err != nil {
		return nil, nil, fmt.Errorf("failed reading index.html: %s", err)
	}
	info, err := f.Stat()
	if err != nil {
		return nil, nil, fmt.Errorf("failed reading metadata for index.html: %s", err)
	}

	// Create template data from the current config.
	tplData, err := uiTemplateDataFromConfig(cfg)
	if err != nil {
		return nil, nil, fmt.Errorf("failed loading UI config for template: %s", err)
	}

	// Sadly we can't perform all the replacements we need with Go template
	// because some of them end up being rendered into an escaped json encoded
	// meta tag by Ember build which messes up the Go template tags. After a few
	// iterations of grossness, this seemed like the least bad for now. note we
	// have to match the encoded double quotes around the JSON string value that
	// is there as a placeholder so the end result is an actual JSON bool not a
	// string containing "false" etc.
	re := regexp.MustCompile(`%22__RUNTIME_BOOL_[A-Za-z0-9-_]+__%22`)

	content = []byte(re.ReplaceAllStringFunc(string(content), func(str string) string {
		// Trim the prefix and __ suffix
		varName := strings.TrimSuffix(strings.TrimPrefix(str, "%22__RUNTIME_BOOL_"), "__%22")
		if v, ok := tplData[varName].(bool); ok && v {
			return "true"
		}
		return "false"
	}))

	tpl, err := template.New("index").Parse(string(content))
	if err != nil {
		return nil, nil, fmt.Errorf("failed parsing index.html template: %s", err)
	}

	var buf bytes.Buffer

	err = tpl.Execute(&buf, tplData)
	if err != nil {
		return nil, nil, fmt.Errorf("failed to render index.html: %s", err)
	}

	return buf.Bytes(), info, nil
}