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//go:build linux
// +build linux
package overlay
import (
"crypto/rand"
"encoding/base32"
"fmt"
"io"
"os"
"syscall"
"time"
"github.com/sirupsen/logrus"
"golang.org/x/sys/unix"
)
// generateID creates a new random string identifier with the given length
func generateID(l int) string {
const (
// ensures we backoff for less than 450ms total. Use the following to
// select new value, in units of 10ms:
// n*(n+1)/2 = d -> n^2 + n - 2d -> n = (sqrt(8d + 1) - 1)/2
maxretries = 9
backoff = time.Millisecond * 10
)
var (
totalBackoff time.Duration
count int
retries int
size = (l*5 + 7) / 8
u = make([]byte, size)
)
// TODO: Include time component, counter component, random component
for {
// This should never block but the read may fail. Because of this,
// we just try to read the random number generator until we get
// something. This is a very rare condition but may happen.
b := time.Duration(retries) * backoff
time.Sleep(b)
totalBackoff += b
n, err := io.ReadFull(rand.Reader, u[count:])
if err != nil {
if retryOnError(err) && retries < maxretries {
count += n
retries++
logrus.Errorf("Generating version 4 uuid, retrying: %v", err)
continue
}
// Any other errors represent a system problem. What did someone
// do to /dev/urandom?
panic(fmt.Errorf("reading random number generator, retried for %v: %w", totalBackoff.String(), err))
}
break
}
s := base32.StdEncoding.EncodeToString(u)
return s[:l]
}
// retryOnError tries to detect whether or not retrying would be fruitful.
func retryOnError(err error) bool {
switch err := err.(type) {
case *os.PathError:
return retryOnError(err.Err) // unpack the target error
case syscall.Errno:
if err == unix.EPERM {
// EPERM represents an entropy pool exhaustion, a condition under
// which we backoff and retry.
return true
}
}
return false
}
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