Add independent config poller service.

logdog/server/service/config/poller.go

+// 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
+}