os: add ReadFile, WriteFile, CreateTemp (was TempFile), MkdirTemp (was TempDir) from io/ioutil

io/ioutil was a poorly defined collection of helpers.
Proposal #40025 moved out the generic I/O helpers to io.
This CL for proposal #42026 moves the OS-specific helpers to os,
making the entire io/ioutil package deprecated.

For #42026.

Change-Id: I018bcb2115ef2ff1bc7ca36a9247eda429af21ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/266364
Trust: Russ Cox <rsc@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>

1 files changed, 19 insertions, 51 deletions

diff --git a/src/io/ioutil/ioutil.go b/src/io/ioutil/ioutil.go
index a001c86b2f..45682b89c9 100644
--- a/src/io/ioutil/ioutil.go
+++ b/src/io/ioutil/ioutil.go
@@ -3,6 +3,11 @@
 // license that can be found in the LICENSE file.
 
 // Package ioutil implements some I/O utility functions.
+//
+// As of Go 1.16, the same functionality is now provided
+// by package io or package os, and those implementations
+// should be preferred in new code.
+// See the specific function documentation for details.
 package ioutil
 
 import (
@@ -26,67 +31,30 @@ func ReadAll(r io.Reader) ([]byte, error) {
 // A successful call returns err == nil, not err == EOF. Because ReadFile
 // reads the whole file, it does not treat an EOF from Read as an error
 // to be reported.
+//
+// As of Go 1.16, this function simply calls os.ReadFile.
 func ReadFile(filename string) ([]byte, error) {
-	f, err := os.Open(filename)
-	if err != nil {
-		return nil, err
-	}
-	defer f.Close()
-	// It's a good but not certain bet that FileInfo will tell us exactly how much to
-	// read, so let's try it but be prepared for the answer to be wrong.
-	const minRead = 512
-	var n int64 = minRead
-
-	if fi, err := f.Stat(); err == nil {
-		// As initial capacity for readAll, use Size + a little extra in case Size
-		// is zero, and to avoid another allocation after Read has filled the
-		// buffer. The readAll call will read into its allocated internal buffer
-		// cheaply. If the size was wrong, we'll either waste some space off the end
-		// or reallocate as needed, but in the overwhelmingly common case we'll get
-		// it just right.
-		if size := fi.Size() + minRead; size > n {
-			n = size
-		}
-	}
-
-	if int64(int(n)) != n {
-		n = minRead
-	}
-
-	b := make([]byte, 0, n)
-	for {
-		if len(b) == cap(b) {
-			// Add more capacity (let append pick how much).
-			b = append(b, 0)[:len(b)]
-		}
-		n, err := f.Read(b[len(b):cap(b)])
-		b = b[:len(b)+n]
-		if err != nil {
-			if err == io.EOF {
-				err = nil
-			}
-			return b, err
-		}
-	}
+	return os.ReadFile(filename)
 }
 
 // WriteFile writes data to a file named by filename.
 // If the file does not exist, WriteFile creates it with permissions perm
 // (before umask); otherwise WriteFile truncates it before writing, without changing permissions.
+//
+// As of Go 1.16, this function simply calls os.WriteFile.
 func WriteFile(filename string, data []byte, perm fs.FileMode) error {
-	f, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
-	if err != nil {
-		return err
-	}
-	_, err = f.Write(data)
-	if err1 := f.Close(); err == nil {
-		err = err1
-	}
-	return err
+	return os.WriteFile(filename, data, perm)
 }
 
 // ReadDir reads the directory named by dirname and returns
-// a list of directory entries sorted by filename.
+// a list of fs.FileInfo for the directory's contents,
+// sorted by filename. If an error occurs reading the directory,
+// ReadDir returns no directory entries along with the error.
+//
+// As of Go 1.16, os.ReadDir is a more efficient and correct choice:
+// it returns a list of fs.DirEntry instead of fs.FileInfo,
+// and it returns partial results in the case of an error
+// midway through reading a directory.
 func ReadDir(dirname string) ([]fs.FileInfo, error) {
 	f, err := os.Open(dirname)
 	if err != nil {