From 2f6eceda913c36fa95f8c0e36f0f3b93c54935d2 Mon Sep 17 00:00:00 2001
From: Mostyn Bramley-Moore <mostyn@antipode.se>
Date: Thu, 18 Jun 2020 20:36:04 +0200
Subject: [PATCH] add some customized tempfile code

We are discussing allowing concurrent uploads for the same blob in #267.

Part of the proposed new design involves using pseudo-randomly named
temp files. Unfortunately, ioutil.TempFile creates files with more
restrictive permissions than we want- 0600 before umask, whereas we
want 0666 before umask.

This code is an adapted version of the algorithm in ioutil.TempFile,
refactored for our more narrow use case- in particular we skip the
use of the pid in the seed, since our use case is single-process,
and we don't bother re-seeding if we encounter lots of collisions
(which is unlikely).

(We do not use this code, yet.)
---
 utils/tempfile/tempfile.go | 55 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)
 create mode 100644 utils/tempfile/tempfile.go

diff --git a/utils/tempfile/tempfile.go b/utils/tempfile/tempfile.go
new file mode 100644
index 000000000..ea2289214
--- /dev/null
+++ b/utils/tempfile/tempfile.go
@@ -0,0 +1,55 @@
+package tempfile
+
+import (
+	"os"
+	"strconv"
+	"sync"
+	"time"
+)
+
+type Creator struct {
+	mu   sync.Mutex
+	idum uint32 // Psuedo-random number generator state.
+}
+
+// NewCreator returns a new Creator, for creating temp files.
+func NewCreator() *Creator {
+	return &Creator{idum: uint32(time.Now().UnixNano())}
+}
+
+// Fast "quick and dirty" linear congruential (pseudo-random) number
+// generator from Numerical Recipes. Excerpt here:
+// https://www.unf.edu/~cwinton/html/cop4300/s09/class.notes/LCGinfo.pdf
+// This is the same algorithm as used in the ioutil.TempFile go standard
+// library function.
+func (c *Creator) ranqd1() string {
+	c.mu.Lock()
+	c.idum = c.idum*1664525 + 1013904223
+	r := c.idum
+	c.mu.Unlock()
+	return strconv.Itoa(int(1e9 + r%1e9))[1:]
+}
+
+const flags = os.O_RDWR | os.O_CREATE | os.O_EXCL
+const mode = 0666 // Before the umask is applied.
+
+// Create attempts to create a temp file for eventualFile, and returns
+// the corresponding *os.File and an error if any occurred. The temp
+// file is created in the same directory as eventualFile, with permissions
+// 0666 before the umask is applied.
+func (c *Creator) Create(eventualFile string) (*os.File, error) {
+	var err error
+	var f *os.File
+
+	base := eventualFile + ".tmp"
+
+	for i := 0; i < 10000; i++ {
+		name := base + c.ranqd1()
+		f, err = os.OpenFile(name, flags, mode)
+		if os.IsExist(err) {
+			continue
+		}
+		return f, err
+	}
+	return nil, err
+}