Move multierror logic into luci-go.

common/errors/multierror.go

Index: common/errors/multierror.go
diff --git a/common/errors/multierror.go b/common/errors/multierror.go
index 30473250dc7ca7bda89d529523eb4472a6c86a6f..afa31333f508543125f3e12dee64e6016dbfda39 100644
--- a/common/errors/multierror.go
+++ b/common/errors/multierror.go
@@ -6,12 +6,53 @@ package errors
 
 import (
 	"fmt"
+	"reflect"
+	"sync"
+)
+
+var (
+	multiErrorType = reflect.TypeOf(MultiError(nil))
 )
 
 // MultiError is a simple `error` implementation which represents multiple
 // `error` objects in one.
 type MultiError []error
 
+func (m MultiError) Error() string {
+	s, n := "", 0
+	for _, e := range m {
+		if e != nil {
+			if n == 0 {
+				s = e.Error()
+			}
+			n++
+		}
+	}
+	switch n {
+	case 0:
+		return "(0 errors)"
+	case 1:
+		return s
+	case 2:
+		return s + " (and 1 other error)"
+	}
+	return fmt.Sprintf("%s (and %d other errors)", s, n-1)
+}
+
+// SingleError provides a simple way to uwrap a MultiError if you know that it
+// could only ever contain one element.
+//
+// If err is a MultiError, return its first element. Otherwise, return err.
+func SingleError(err error) error {
+	if me, ok := err.(MultiError); ok {
+		if len(me) == 0 {
+			return nil
+		}
+		return me[0]
+	}
+	return err
+}
+
 // 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.
@@ -32,6 +73,54 @@ func MultiErrorFromErrors(ch <-chan error) error {
 	return ret
 }
 
-func (m MultiError) Error() string {
-	return fmt.Sprintf("%+q", []error(m))
+// LazyMultiError is a lazily-constructed MultiError. You specify the target
+// MultiError size up front (as Size), and then you call Assign for each error
+// encountered, and it's potential index. The MultiError will only be allocated
+// if one of the Assign'd errors is non-nil. Similarly, Get will retrieve either
+// the allocated MultiError, or nil if no error was encountered.
+type LazyMultiError struct {
+	sync.Mutex
+
+	Size int
+	me   MultiError
+}
+
+// Assign semantically assigns the error to the given index in the MultiError.
+// If the error is nil, no action is taken. Otherwise the MultiError is
+// allocated to its full size (if not already), and the error assigned into it.
+func (e *LazyMultiError) Assign(i int, err error) {
+	if err == nil {
+		return
+	}
+	e.Lock()
+	defer e.Unlock()
+	if e.me == nil {
+		e.me = make(MultiError, e.Size)
+	}
+	e.me[i] = err
+}
+
+// Get returns the MultiError, or nil, if no non-nil error was Assign'd.
+func (e *LazyMultiError) Get() error {
+	e.Lock()
+	defer e.Unlock()
+	if e.me == nil {
+		return nil
+	}
+	return e.me
+}
+
+// Fix will convert a MultiError compatible type (e.g. []error) to this
+// version of MultiError.
+func Fix(err error) error {
+	if err != nil {
+		// we know that err already conforms to the error interface (or the caller's
+		// method wouldn't compile), so check to see if the error's underlying type
+		// looks like one of the special error types we implement.
+		v := reflect.ValueOf(err)
+		if v.Type().ConvertibleTo(multiErrorType) {
+			err = v.Convert(multiErrorType).Interface().(error)
+		}
+	}
+	return err
 }