cipd: Client can resolve tags to instance IDs.

go/src/infra/tools/cipd/apps/cipd/main.go

Index: go/src/infra/tools/cipd/apps/cipd/main.go
diff --git a/go/src/infra/tools/cipd/apps/cipd/main.go b/go/src/infra/tools/cipd/apps/cipd/main.go
index 35c5636ecc5d17a54e9fa40f72c62ebed572c884..b02e814af8f3ec14100323ada32ded7e8c8e74ef 100644
--- a/go/src/infra/tools/cipd/apps/cipd/main.go
+++ b/go/src/infra/tools/cipd/apps/cipd/main.go
@@ -707,7 +707,7 @@ var cmdFetch = &subcommands.Command{
 	CommandRun: func() subcommands.CommandRun {
 		c := &fetchRun{}
 		c.Flags.StringVar(&c.packageName, "name", "<name>", "package name")
-		c.Flags.StringVar(&c.instanceID, "instance-id", "<instance id>", "package instance ID to fetch")
+		c.Flags.StringVar(&c.version, "version", "<version>", "package version to fetch")
 		c.Flags.StringVar(&c.outputPath, "out", "<path>", "path to a file to write fetch to")
 		c.ServiceOptions.registerFlags(&c.Flags)
 		return c
@@ -719,7 +719,7 @@ type fetchRun struct {
 	ServiceOptions
 
 	packageName string
-	instanceID  string
+	version     string
 	outputPath  string
 }
 
@@ -727,8 +727,7 @@ func (c *fetchRun) Run(a subcommands.Application, args []string) int {
 	if !checkCommandLine(args, c.GetFlags(), 0) {
 		return 1
 	}
-	pin := common.Pin{PackageName: c.packageName, InstanceID: c.instanceID}
-	err := fetchInstanceFile(pin, c.outputPath, c.ServiceOptions)
+	err := fetchInstanceFile(c.packageName, c.version, c.outputPath, c.ServiceOptions)
 	if err != nil {
 		reportError("Error while fetching the package: %s", err)
 		return 1
@@ -736,7 +735,16 @@ func (c *fetchRun) Run(a subcommands.Application, args []string) int {
 	return 0
 }
 
-func fetchInstanceFile(pin common.Pin, instanceFile string, serviceOpts ServiceOptions) error {
+func fetchInstanceFile(packageName, version, instanceFile string, serviceOpts ServiceOptions) error {
+	client, err := serviceOpts.makeCipdClient()
+	if err != nil {
+		return err
+	}
+	pin, err := client.ResolveVersion(packageName, version)
+	if err != nil {
+		return err
+	}
+
 	out, err := os.OpenFile(instanceFile, os.O_CREATE|os.O_WRONLY, 0666)
 	if err != nil {
 		return err
@@ -749,10 +757,6 @@ func fetchInstanceFile(pin common.Pin, instanceFile string, serviceOpts ServiceO
 		}
 	}()
 
-	client, err := serviceOpts.makeCipdClient()
-	if err != nil {
-		return err
-	}
 	err = client.FetchInstance(pin, out)
 	if err != nil {
 		return err