Make sure the values returned by newLastWriterID() are unique

... by including the time, PID, and a per-process counter.

Pad the rest with rando values for continuity.

This feels a bit like overkill, OTOH adding the PID might be
useful for debugging, so there's that.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

pkg/lockfile/lockfile_unix.go

@@ -5,13 +5,15 @@ package lockfile
 
 import (
 	"bytes"
+	cryptorand "crypto/rand"
+	"encoding/binary"
 	"fmt"
 	"os"
 	"path/filepath"
 	"sync"
+	"sync/atomic"
 	"time"
 
-	"github.com/containers/storage/pkg/stringid"
 	"github.com/containers/storage/pkg/system"
 	"github.com/pkg/errors"
 	"golang.org/x/sys/unix"
@@ -32,11 +34,35 @@ type lockfile struct {
 	recursive  bool
 }
 
+const lastWriterIDSize = 64    // This must be the same as len(stringid.GenerateRandomID)
+var lastWriterIDCounter uint64 // Private state for newLastWriterID
+
 // newLastWriterID returns a new "last writer" ID.
 // The value must be different on every call, and also differ from values
 // generated by other processes.
 func newLastWriterID() []byte {
-	return []byte(stringid.GenerateRandomID())
+	// The ID is (PID, time, per-process counter, random)
+	// PID + time represents both a unique process across reboots,
+	// and a specific time within the process; the per-process counter
+	// is an extra safeguard for in-process concurrency.
+	// The random part disambiguates across process namespaces
+	// (where PID values might collide), serves as a general-purpose
+	// extra safety, _and_ is used to pad the output to lastWriterIDSize,
+	// because other versions of this code exist and they don't work
+	// efficiently if the size of the value changes.
+	pid := os.Getpid()
+	tm := time.Now().UnixNano()
+	counter := atomic.AddUint64(&lastWriterIDCounter, 1)
+
+	res := make([]byte, lastWriterIDSize)
+	binary.LittleEndian.PutUint64(res[0:8], uint64(tm))
+	binary.LittleEndian.PutUint64(res[8:16], counter)
+	binary.LittleEndian.PutUint32(res[16:20], uint32(pid))
+	if n, err := cryptorand.Read(res[20:lastWriterIDSize]); err != nil || n != lastWriterIDSize-20 {
+		panic(err) // This shouldn't happen
+	}
+
+	return res
 }
 
 // openLock opens the file at path and returns the corresponding file