Add independent config poller service.

logdog/server/service/config/poller.go

Index: logdog/server/service/config/poller.go
diff --git a/logdog/server/service/config/poller.go b/logdog/server/service/config/poller.go
new file mode 100644
index 0000000000000000000000000000000000000000..d7abb80dc71088a8a03c9396fdf162f23f377206
--- /dev/null
+++ b/logdog/server/service/config/poller.go
@@ -0,0 +1,103 @@
+// Copyright 2016 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package config
+
+import (
+	"time"
+
+	"github.com/luci/luci-go/common/clock"
+	"github.com/luci/luci-go/common/errors"
+	log "github.com/luci/luci-go/common/logging"
+	"github.com/luci/luci-go/luci_config/common/cfgtypes"
+	"github.com/luci/luci-go/luci_config/server/cfgclient"
+
+	"golang.org/x/net/context"
+)
+
+// ChangePoller polls a configuration files for changes. If it changes,
+// the OnChange function will be called and the polling will stop.
+type ChangePoller struct {
+	// ConfigSet is the slice of config paths to watch.
+	ConfigSet cfgtypes.ConfigSet
+	// Path is the path of the config to watch.
+	Path string
+
+	// Period is the amount of time in between checks to see if the configuration
+	// has been updated. If <= 0, the poller will refrain from polling, and Run
+	// will immediately exit.
+	Period time.Duration
+	// OnChange is the function that will be called if a configuration change has
+	// been observed.
+	//
+	// Polling will be blocked until OnChange returns. If the Context supplied to
+	// Run is canceled by OnChange, Run will exit at the beginning of the next
+	// poll round.
+	OnChange func()
+
+	// ContentHash is the config's hash. This should be set to the initial config
+	// value, either directly or via a Refresh call, before Run is called.
+	ContentHash string
+}
+
+// Run starts polling for changes. It will stop when the Context is cancelled.
+func (p *ChangePoller) Run(c context.Context) {
+	if p.Period <= 0 {
+		return
+	}
+
+	for {
+		// If our Context has been canceled, terminate.
+		select {
+		case <-c.Done():
+			log.WithError(c.Err()).Warningf(c, "Terminating poll loop: context has been cancelled.")
+			return
+		default:
+			// Continue
+		}
+
+		log.Fields{
+			"timeout": p.Period,
+		}.Debugf(c, "Entering change check poll loop...")
+		if tr := clock.Sleep(c, p.Period); tr.Incomplete() {
+			log.WithError(tr.Err).Debugf(c, "Context cancelled, shutting down change poller.")
+			return
+		}
+
+		log.Infof(c, "Change check timeout triggered, checking configuration...")
+		lastHash := p.ContentHash
+		switch err := p.Refresh(c); {
+		case err != nil:
+			log.WithError(err).Errorf(c, "Failed to refresh config.")
+
+		case lastHash != p.ContentHash:
+			log.Fields{
+				"originalHash": lastHash,
+				"newHash":      p.ContentHash,
+			}.Warningf(c, "Configuration content hash has changed.")
+			if p.OnChange != nil {
+				p.OnChange()
+			}
+
+		default:
+			log.Fields{
+				"currentHash": lastHash,
+			}.Debugf(c, "Content hash matches.")
+		}
+	}
+}
+
+// Refresh reloads the configuration value, updating ContentHash.
+func (p *ChangePoller) Refresh(c context.Context) error {
+	var meta cfgclient.Meta
+	if err := cfgclient.Get(c, cfgclient.AsService, p.ConfigSet, p.Path, nil, &meta); err != nil {
+		return errors.Annotate(err).Reason("failed to reload config %(configSet)s :: %(path)s").
+			D("configSet", p.ConfigSet).
+			D("path", p.Path).
+			Err()
+	}
+
+	p.ContentHash = meta.ContentHash
+	return nil
+}