testing/iotest: correct ErrReader signature and remove exported error

Corrects ErrReader's signature to what was accepted in the approved
proposal, and also removes an exported ErrIO which wasn't part of
the proposal and is unnecessary.

The new signature allows users to customize their own errors.

While here, started examples, with ErrReader leading the way.

Updates #38781

Change-Id: Ia7f84721f11061343cfef8b1adc2b7b69bc3f43c
Reviewed-on: https://go-review.googlesource.com/c/go/+/248898
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

4 files changed, 53 insertions, 17 deletions

diff --git a/src/testing/iotest/example_test.go b/src/testing/iotest/example_test.go
new file mode 100644
index 0000000000..10f6bd38f7
--- /dev/null
+++ b/src/testing/iotest/example_test.go
@@ -0,0 +1,22 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest_test
+
+import (
+	"errors"
+	"fmt"
+	"testing/iotest"
+)
+
+func ExampleErrReader() {
+	// A reader that always returns a custom error.
+	r := iotest.ErrReader(errors.New("custom error"))
+	n, err := r.Read(nil)
+	fmt.Printf("n:   %d\nerr: %q\n", n, err)
+
+	// Output:
+	// n:   0
+	// err: "custom error"
+}
diff --git a/src/testing/iotest/logger_test.go b/src/testing/iotest/logger_test.go
index 575f37e05c..fec4467cc6 100644
--- a/src/testing/iotest/logger_test.go
+++ b/src/testing/iotest/logger_test.go
@@ -138,14 +138,14 @@ func TestReadLogger_errorOnRead(t *testing.T) {
 	data := []byte("Hello, World!")
 	p := make([]byte, len(data))
 
-	lr := ErrReader()
+	lr := ErrReader(errors.New("io failure"))
 	rl := NewReadLogger("read", lr)
 	n, err := rl.Read(p)
 	if err == nil {
 		t.Fatalf("Unexpectedly succeeded to read: %v", err)
 	}
 
-	wantLogWithHex := fmt.Sprintf("lr: read %x: %v\n", p[:n], "io")
+	wantLogWithHex := fmt.Sprintf("lr: read %x: io failure\n", p[:n])
 	if g, w := lOut.String(), wantLogWithHex; g != w {
 		t.Errorf("ReadLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
 	}
diff --git a/src/testing/iotest/reader.go b/src/testing/iotest/reader.go
index b18e912f27..bc2f72a911 100644
--- a/src/testing/iotest/reader.go
+++ b/src/testing/iotest/reader.go
@@ -88,16 +88,15 @@ func (r *timeoutReader) Read(p []byte) (int, error) {
 	return r.r.Read(p)
 }
 
-// ErrIO is a fake IO error.
-var ErrIO = errors.New("io")
-
-// ErrReader returns a fake error every time it is read from.
-func ErrReader() io.Reader {
-	return errReader(0)
+// ErrReader returns an io.Reader that returns 0, err from all Read calls.
+func ErrReader(err error) io.Reader {
+	return &alwaysErrReader{err: err}
 }
 
-type errReader int
+type alwaysErrReader struct {
+	err error
+}
 
-func (r errReader) Read(p []byte) (int, error) {
-	return 0, ErrIO
+func (aer *alwaysErrReader) Read(p []byte) (int, error) {
+	return 0, aer.err
 }
diff --git a/src/testing/iotest/reader_test.go b/src/testing/iotest/reader_test.go
index ccba22ee29..6004e841e5 100644
--- a/src/testing/iotest/reader_test.go
+++ b/src/testing/iotest/reader_test.go
@@ -6,6 +6,7 @@ package iotest
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"testing"
 )
@@ -226,11 +227,25 @@ func TestDataErrReader_emptyReader(t *testing.T) {
 }
 
 func TestErrReader(t *testing.T) {
-	n, err := ErrReader().Read([]byte{})
-	if err != ErrIO {
-		t.Errorf("ErrReader.Read(any) should have returned ErrIO, returned %v", err)
-	}
-	if n != 0 {
-		t.Errorf("ErrReader.Read(any) should have read 0 bytes, read %v", n)
+	cases := []struct {
+		name string
+		err  error
+	}{
+		{"nil error", nil},
+		{"non-nil error", errors.New("io failure")},
+		{"io.EOF", io.EOF},
+	}
+
+	for _, tt := range cases {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			n, err := ErrReader(tt.err).Read(nil)
+			if err != tt.err {
+				t.Fatalf("Error mismatch\nGot:  %v\nWant: %v", err, tt.err)
+			}
+			if n != 0 {
+				t.Fatalf("Byte count mismatch: got %d want 0", n)
+			}
+		})
 	}
 }