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io: make chained multiReader Read more efficient

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before this change, when io.MultiReader was called many times but contain few
underlying readers, calls to Read were unnecessarily expensive.

Fixes #13558

Change-Id: I3ec4e88c7b50c075b148331fb1b7348a5840adbe
Reviewed-on: https://go-review.googlesource.com/17873
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
andrew werner 2015-12-15 14:42:28 -08:00 committed by Brad Fitzpatrick
parent 28c201860e
commit ccdca832c5
2 changed files with 39 additions and 0 deletions
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7
src/io/multi.go
View File

@ -10,6 +10,13 @@ type multiReader struct {
func (mr *multiReader) Read(p []byte) (n int, err error) {
for len(mr.readers) > 0 {
// Optimization to flatten nested multiReaders (Issue 13558)
if len(mr.readers) == 1 {
if r, ok := mr.readers[0].(*multiReader); ok {
mr.readers = r.readers
continue
}
}
n, err = mr.readers[0].Read(p)
if n > 0 || err != EOF {
if err == EOF {

32
src/io/multi_test.go
View File

@ -7,9 +7,11 @@ package io_test
import (
"bytes"
"crypto/sha1"
"errors"
"fmt"
. "io"
"io/ioutil"
"runtime"
"strings"
"testing"
)
@ -164,3 +166,33 @@ func TestMultiWriterCopy(t *testing.T) {
t.Errorf("buf.String() = %q, want %q", buf.String(), "hello world")
}
}
// readerFunc is an io.Reader implemented by the underlying func.
type readerFunc func(p []byte) (int, error)
func (f readerFunc) Read(p []byte) (int, error) {
return f(p)
}
// Test that MultiReader properly flattens chained multiReaders when Read is called
func TestMultiReaderFlatten(t *testing.T) {
pc := make([]uintptr, 1000) // 1000 should fit the full stack
var myDepth = runtime.Callers(0, pc)
var readDepth int // will contain the depth from which fakeReader.Read was called
var r Reader = MultiReader(readerFunc(func(p []byte) (int, error) {
readDepth = runtime.Callers(1, pc)
return 0, errors.New("irrelevant")
}))
// chain a bunch of multiReaders
for i := 0; i < 100; i++ {
r = MultiReader(r)
}
r.Read(nil) // don't care about errors, just want to check the call-depth for Read
if readDepth != myDepth+2 { // 2 should be multiReader.Read and fakeReader.Read
t.Errorf("multiReader did not flatten chained multiReaders: expected readDepth %d, got %d",
myDepth+2, readDepth)
}
}