os: implement CreateTemp

Until tinygo implements fastrand(), use a placeholder RNG here.

src/os/tempfile.go

@@ -1,5 +1,113 @@
+// Copyright 2010 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 os
 
-func CreateTemp(dir, pattern string) (*File, error) {
-	return nil, &PathError{"createtemp", pattern, ErrNotImplemented}
+import (
+	"errors"
+	"internal/itoa"
+	"sync"
+	"time"
+)
+
+var minrandPreviousValue uint32
+var minrandMutex sync.Mutex
+
+func init() {
+	// Avoid getting same results on every run
+	now := time.Now()
+	seed := uint32(Getpid()) ^ uint32(now.Nanosecond()) ^ uint32(now.Unix())
+	// initial state must be odd
+	minrandPreviousValue = seed | 1
+}
+
+// minrand() is a simple and rather poor placeholder for fastrand()
+// TODO: provide fastrand in runtime, as go does.  It is hard to implement properly elsewhere.
+func minrand() uint32 {
+	// c++11's minstd_rand
+	// https://en.wikipedia.org/wiki/Linear_congruential_generator
+	// m=2^32, c=0, a=48271
+	minrandMutex.Lock()
+	minrandPreviousValue *= 48271
+	val := minrandPreviousValue
+	minrandMutex.Unlock()
+	return val
+}
+
+// We generate random temporary file names so that there's a good
+// chance the file doesn't exist yet - keeps the number of tries in
+// TempFile to a minimum.
+func nextRandom() string {
+	// Discard lower four bits of minrand.
+	// They're not very random, and we don't need the full range here.
+	return itoa.Uitoa(uint(minrand() >> 4))
+}
+
+// CreateTemp creates a new temporary file in the directory dir,
+// opens the file for reading and writing, and returns the resulting file.
+// The filename is generated by taking pattern and adding a random string to the end.
+// If pattern includes a "*", the random string replaces the last "*".
+// If dir is the empty string, CreateTemp uses the default directory for temporary files, as returned by TempDir.
+// Multiple programs or goroutines calling CreateTemp simultaneously will not choose the same file.
+// The caller can use the file's Name method to find the pathname of the file.
+// It is the caller's responsibility to remove the file when it is no longer needed.
+func CreateTemp(dir, pattern string) (*File, error) {
+	if dir == "" {
+		dir = TempDir()
+	}
+
+	prefix, suffix, err := prefixAndSuffix(pattern)
+	if err != nil {
+		return nil, &PathError{Op: "createtemp", Path: pattern, Err: err}
+	}
+	prefix = joinPath(dir, prefix)
+
+	try := 0
+	for {
+		name := prefix + nextRandom() + suffix
+		f, err := OpenFile(name, O_RDWR|O_CREATE|O_EXCL, 0600)
+		if IsExist(err) {
+			if try++; try < 10000 {
+				continue
+			}
+			return nil, &PathError{Op: "createtemp", Path: dir + string(PathSeparator) + prefix + "*" + suffix, Err: ErrExist}
+		}
+		return f, err
+	}
+}
+
+var errPatternHasSeparator = errors.New("pattern contains path separator")
+
+// prefixAndSuffix splits pattern by the last wildcard "*", if applicable,
+// returning prefix as the part before "*" and suffix as the part after "*".
+func prefixAndSuffix(pattern string) (prefix, suffix string, err error) {
+	for i := 0; i < len(pattern); i++ {
+		if IsPathSeparator(pattern[i]) {
+			return "", "", errPatternHasSeparator
+		}
+	}
+	if pos := lastIndex(pattern, '*'); pos != -1 {
+		prefix, suffix = pattern[:pos], pattern[pos+1:]
+	} else {
+		prefix = pattern
+	}
+	return prefix, suffix, nil
+}
+
+func joinPath(dir, name string) string {
+	if len(dir) > 0 && IsPathSeparator(dir[len(dir)-1]) {
+		return dir + name
+	}
+	return dir + string(PathSeparator) + name
+}
+
+// LastIndexByte from the strings package.
+func lastIndex(s string, sep byte) int {
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == sep {
+			return i
+		}
+	}
+	return -1
 }