[errors] Remove Fix(), move MultiErrorFromErrors().

common/sync/parallel/workPool.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 parallel
 
 import (
         "github.com/luci/luci-go/common/errors"
 )
 
 // WorkPool creates a fixed-size pool of worker goroutines. A supplied generator
 // method creates task functions and passes them through to the work pool.
 //
 // WorkPool will use at most workers goroutines to execute the supplied tasks.
 // If workers is <= 0, WorkPool will be unbounded and behave like FanOutIn.
 //
 // WorkPool blocks until all the generator completes and all workers have
 // finished their tasks.
 func WorkPool(workers int, gen func(chan<- func() error)) error {
-        return errors.MultiErrorFromErrors(Run(workers, gen))
+        return multiErrorFromErrors(Run(workers, gen))
 }
 
 // FanOutIn is useful to quickly parallelize a group of tasks.
 //
 // You pass it a function which is expected to push simple `func() error`
 // closures into the provided chan. Each function will be executed in parallel
 // and their error results will be collated.
 //
 // The function blocks until all functions are executed, and an
 // errors.MultiError is returned if one or more of your fan-out tasks failed,
 // otherwise this function returns nil.
 //
 // This function is equivalent to WorkPool(0, gen).
 func FanOutIn(gen func(chan<- func() error)) error {
         return WorkPool(0, gen)
 }
+
+// multiErrorFromErrors takes an error-channel, blocks on it, and returns
+// a MultiError for any errors pushed to it over the channel, or nil if
+// all the errors were nil.
+func multiErrorFromErrors(ch <-chan error) error {
+        if ch == nil {
+                return nil
+        }
+        ret := errors.MultiError(nil)
+        for e := range ch {
+                if e == nil {
+                        continue
+                }
+                ret = append(ret, e)
+        }
+        if len(ret) == 0 {
+                return nil
+        }
+        return ret
+}