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.

[iannucci, 2017/01/21 00:09:12]

what's the reason for having the loop stop on the first change?

[dnj, 2017/01/21 01:17:54]

The application will quit after this, so there's no point in continuing. I
suppose I might as well keep it going, though - the change function can just
cancel the Context.

+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
+
+        // if the configuration has changed. If it has, KillFunc will be invoked.

[iannucci, 2017/01/21 00:09:12]

something went wrong with this comment I think. Also KillFunc?

[dnj, 2017/01/21 01:17:54]

Fixed.

+        CheckInterval time.Duration
+        // OnChange is the function that will be called if a configuration hash change
+        // has been observed.
+        OnChange func()
+
+        // ContentHash is the config's hash. If empty, it will be populated when first
+        // polled.
+        ContentHash string
+}
+
+// Run starts polling for changes. It will stop when the Context is cancelled.
+func (p *ChangePoller) Run(c context.Context) {
+        if p.CheckInterval <= 0 {
+                return
+        }
+
+        for {
+                if err := c.Err(); err != nil {

[iannucci, 2017/01/21 00:09:12]

so remember that thing we talked about last feburary where we weren't sure if it
was safe to call Err without blocking on Done? Turns out that you're right:
https://github.com/golang/go/issues/14292

However, since clock.Sleep synchronizes with Done, this is actually ok. In fact,
I don't think you need this if statement at all (just let the clock.Sleep do it)

[dnj, 2017/01/21 01:17:54]

Done.

+                        log.WithError(err).Warningf(c, "Terminating poll loop: context has been cancelled.")
+                        return
+                }
+
+                log.Fields{
+                        "timeout": p.CheckInterval,
+                }.Debugf(c, "Entering change check poll loop...")
+                if tr := clock.Sleep(c, p.CheckInterval); 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()
+                        }
+                        return
+
+                default:
+                        log.Fields{
+                                "currentHash": lastHash,
+                        }.Debugf(c, "Content hash matches.")
+                }
+        }
+}
+
+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()

[iannucci, 2017/01/21 00:09:12]

how would

  return errors.Annotate(err).
    Reason(...).
    D(...).
    D(...).
    Err()

Look?

[dnj, 2017/01/21 01:17:54]

Taller, but sure.

+        }
+
+        p.ContentHash = meta.ContentHash
+        return nil
+}