Implement a server-side config service interface.

server/config/config.go

Index: server/config/config.go
diff --git a/server/config/config.go b/server/config/config.go
new file mode 100644
index 0000000000000000000000000000000000000000..66e66fc4b7656883597387c31c7dfb0ac14ac4cb
--- /dev/null
+++ b/server/config/config.go
@@ -0,0 +1,188 @@
+// 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 (
+	"encoding/json"
+	"net/url"
+
+	"github.com/luci/luci-go/common/config"
+	"github.com/luci/luci-go/common/errors"
+
+	"golang.org/x/net/context"
+)
+
+// ErrNoConfig is a sentinel error returned by Get when the requested
+// configuration is not found.

[iannucci, 2017/01/07 20:12:16]

I would mention that it's an alias of config.ErrNoConfig too (unless that shows
up in godoc. I forget)

[dnj, 2017/01/10 03:25:57]

Done.

+var ErrNoConfig = config.ErrNoConfig
+
+// GetAllType is the type of configuration to retrieve with GetAll.
+//
+// GetAllType marshals/unmarshals to/from a compact JSON representation. This is
+// used by the caching layer.
+type GetAllType string
+
+const (
+	// Project indicates that project configus should be retrieved.
+	Project = GetAllType("Project")
+	// Ref indicates that ref configs should be retrieved.
+	Ref = GetAllType("Ref")
+)
+
+// MarshalJSON implements json.Marshaler.
+func (gat GetAllType) MarshalJSON() ([]byte, error) {
+	switch gat {
+	case Project:
+		return json.Marshal("P")
+	case Ref:
+		return json.Marshal("R")
+	default:
+		return nil, errors.Reason("unknown GetAllType: %(value)v").D("value", gat).Err()
+	}
+}
+
+// UnmarshalJSON implements json.Unmarshaler.
+func (gat *GetAllType) UnmarshalJSON(d []byte) error {

[iannucci, 2017/01/07 20:12:15]

same as other *JSON methods re: constant []byte values

[dnj, 2017/01/10 03:25:57]

Done.

+	var v string
+	if err := json.Unmarshal(d, &v); err != nil {
+		return errors.Annotate(err).Err()
+	}
+
+	switch v {
+	case "P":
+		*gat = Project
+		return nil
+	case "R":
+		*gat = Ref
+		return nil
+	default:
+		return errors.Reason("unknown GetAllType: %(value)q").D("value", v).Err()
+	}
+}
+
+// Meta is metadata about a configuration.
+type Meta struct {
+	// ConfigSet is the item's config set.
+	ConfigSet string
+	// Path is the item's path within its config set.
+	Path string
+
+	// ContentHash is the content hash.
+	ContentHash string
+	// Revision is the revision string.
+	Revision string
+}
+
+// ServiceURL returns the URL of the config service.
+func ServiceURL(c context.Context) url.URL { return getBackend(c).ServiceURL(c) }
+
+// Get retrieves a single configuration.

[iannucci, 2017/01/07 20:12:16]

can we call them 'configuration file's? Since from a program's pov, a single
configuration file may actually contain multiple application-specific
configurations.

[dnj, 2017/01/10 03:25:57]

Done.

+//
+// r, if supplied, is a MultiResolver that will load the configuration data.
+// If nil, the configuration data will be discarded (useful if you only care
+// about metas).
+//
+// meta, if not nil, will have the configuration's Meta loaded into it on
+// success.
+func Get(c context.Context, a Authority, configSet, path string, r Resolver, meta *Meta) error {
+	be := getBackend(c)
+
+	params := Params{
+		Authority: a,
+		Content:   true,
+	}
+
+	if fr, ok := r.(FormattingResolver); ok {
+		params.Format, params.FormatData = fr.Format()
+	}
+
+	item, err := be.Get(c, configSet, path, params)

[iannucci, 2017/01/07 20:12:15]

maybe the Params thing would be clearer if backend were a different package? It
looks like backend is also internal so maybe params should be too? or can users
provide params as well?

I'm just being picky on names because we're really deep into the 'generic names'
bucket in the package. 'config.Params' is very nondescript :/

[dnj, 2017/01/10 03:25:58]

Yeah I was debating doing this. I think I was out of energy with the first path
set, but now that it's been a few I went ahead and split this out into "backend"
and "types".

+	if err != nil {
+		return err
+	}
+
+	if meta != nil {
+		*meta = item.Meta
+	}
+	if r == nil {
+		return nil
+	}
+	return r.Resolve(item)
+}
+
+// GetAll retrieves all configurations of a given type.
+//
+// r, if supplied, is a MultiResolver that will load the configuration data.
+// If nil, the configuration data will be discarded (useful if you only care
+// about metas). If the MultiResolver operates on a slice (which it probably
+// will), each meta and/or error index will correspond to its slice index.
+//
+// If meta is not nil, it will be populated with a slice of *Meta entries
+// for each loaded configuration, in the same order that r receives them. If
+// r resolves to a slice, the indexes for each resolved slice entry and meta
+// entry should align unless r is doing something funky.
+//
+// Two types of failure may happen here. A systemic failure fails to load the
+// set of project configurations. This will be returned directly.
+//
+// A resolver failure happens when the configuratiokns load, but could not be
+// resolved. In this case, any successful resolutions and "meta" will be
+// populated and an errors.MultiError will be returned containing non-nil
+// errors at indices whose configs failed to resolve.
+func GetAll(c context.Context, a Authority, t GetAllType, path string, r MultiResolver, meta *[]*Meta) error {
+	be := getBackend(c)
+
+	params := Params{
+		Authority: a,
+		Content:   true,
+	}
+
+	if fr, ok := r.(FormattingResolver); ok {
+		params.Format, params.FormatData = fr.Format()
+	}
+
+	items, err := be.GetAll(c, t, path, params)
+	if err != nil {
+		return err
+	}
+
+	// Load our metas.
+	if meta != nil {
+		metaSlice := *meta
+		if metaSlice == nil {
+			metaSlice = make([]*Meta, 0, len(items))
+		} else {
+			// Re-use the supplied slice.
+			metaSlice = metaSlice[:0]
+		}
+
+		for _, item := range items {
+			metaSlice = append(metaSlice, &item.Meta)
+		}
+
+		*meta = metaSlice
+	}
+
+	if r == nil {
+		return nil
+	}
+
+	// Resolve our items. If any individual resolution fails, return that failure
+	// as a positional error in a MultiError.
+	lme := errors.NewLazyMultiError(len(items))
+	r.PrepareMulti(len(items))
+	for i, it := range items {
+		if err := r.ResolveItemAt(i, it); err != nil {
+			lme.Assign(i, err)
+		}
+	}
+	return lme.Get()
+}
+
+// GetConfigSetURL returns the URL where the specified ConfigSet is
+// configured. If the config set doesn't exist, ErrNoConfig will be returned.
+func GetConfigSetURL(c context.Context, a Authority, configSet string) (url.URL, error) {
+	return getBackend(c).ConfigSetURL(c, a, configSet)
+}