Add support for Pull Queues to prod implementation.

service/taskqueue/interface.go

 // Copyright 2015 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 taskqueue
 
 import (
         "github.com/luci/luci-go/common/errors"
 
         "golang.org/x/net/context"
@@ skipping 38 matching lines @@
         })
         if err == nil {
                 err = lme.Get()
                 if len(tasks) == 1 {
                         err = errors.SingleError(err)
                 }
         }
         return err
 }
 
-// NOTE(riannucci): No support for pull taskqueues. We're not planning on
-// making pull-queue clients which RUN in appengine (e.g. they'd all be
-// external REST consumers). If someone needs this, it will need to be added
-// here and in RawInterface. The theory is that a good lease API might look
-// like:
+// NOTE(riannucci): Pull task queues API can be extended to support automatic
+// lease management.
+//
+// The theory is that a good lease API might look like:
 //
 //   func Lease(queueName, tag string, batchSize int, duration time.Time, cb func(*Task, error<-))
 //
 // Which blocks and calls cb for each task obtained. Lease would then do all
 // necessary backoff negotiation with the backend. The callback could execute
 // synchronously (stuffing an error into the chan or panicing if it fails), or
 // asynchronously (dispatching a goroutine which will then populate the error
 // channel if needed). If it operates asynchronously, it has the option of
 // processing multiple work items at a time.
 //
 // Lease would also take care of calling ModifyLease as necessary to ensure
 // that each call to cb would have 'duration' amount of time to work on the
 // task, as well as releasing as many leased tasks as it can on a failure.
 
+// Lease leases tasks from a queue.

[Vadim Sh., 2016/11/18 03:42:12]

all docs are copy-paste from official site

+//
+// leaseTime is in seconds. The number of tasks fetched will be at most

[Vadim Sh., 2016/11/18 03:53:53]

actually int for leaseTime is BS, I'll convert it to time.Duration and clarify
that it has second precision.

+// maxTasks.
+func Lease(c context.Context, maxTasks int, queueName string, leaseTime int) ([]*Task, error) {
+        return Raw(c).Lease(maxTasks, queueName, leaseTime)
+}
+
+// LeaseByTag leases tasks from a queue, grouped by tag.
+//
+// If tag is empty, then the returned tasks are grouped by the tag of the task

[Vadim Sh., 2016/11/18 03:42:12]

:( why they did it, I'll have to reimplement all this in impl/memory

+// with earliest ETA.
+//
+// leaseTime is in seconds. The number of tasks fetched will be at most
+// maxTasks.
+func LeaseByTag(c context.Context, maxTasks int, queueName string, leaseTime int, tag string) ([]*Task, error) {
+        return Raw(c).LeaseByTag(maxTasks, queueName, leaseTime, tag)
+}
+
+// ModifyLease modifies the lease of a task.
+//
+// Used to request more processing time, or to abandon processing. leaseTime is
+// in seconds and must not be negative.
+func ModifyLease(c context.Context, task *Task, queueName string, leaseTime int) error {
+        return Raw(c).ModifyLease(task, queueName, leaseTime)
+}
+
 // Purge purges all tasks form the named queue.
 func Purge(c context.Context, queueName string) error {
         return Raw(c).Purge(queueName)
 }
 
 // Stats returns Statistics instances for each of the named task queues.
 //
 // If only one task is provided its error will be returned directly. If more
 // than one task is provided, an errors.MultiError will be returned in the
 // event of an error, with a given error index corresponding to the error
@@ skipping 15 matching lines @@
                 }
         }
         return ret, err
 }
 
 // GetTestable returns a Testable for the current task queue service in c, or
 // nil if it does not offer one.
 func GetTestable(c context.Context) Testable {
         return Raw(c).GetTestable()
 }