81 lines
1.9 KiB
Go
81 lines
1.9 KiB
Go
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// +build linux
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package overlay2
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import (
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"crypto/rand"
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"encoding/base32"
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"fmt"
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"io"
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"os"
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"syscall"
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"time"
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"github.com/Sirupsen/logrus"
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)
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// generateID creates a new random string identifier with the given length
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func generateID(l int) string {
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const (
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// ensures we backoff for less than 450ms total. Use the following to
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// select new value, in units of 10ms:
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// n*(n+1)/2 = d -> n^2 + n - 2d -> n = (sqrt(8d + 1) - 1)/2
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maxretries = 9
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backoff = time.Millisecond * 10
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)
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var (
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totalBackoff time.Duration
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count int
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retries int
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size = (l*5 + 7) / 8
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u = make([]byte, size)
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)
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// TODO: Include time component, counter component, random component
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for {
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// This should never block but the read may fail. Because of this,
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// we just try to read the random number generator until we get
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// something. This is a very rare condition but may happen.
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b := time.Duration(retries) * backoff
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time.Sleep(b)
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totalBackoff += b
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n, err := io.ReadFull(rand.Reader, u[count:])
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if err != nil {
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if retryOnError(err) && retries < maxretries {
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count += n
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retries++
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logrus.Errorf("error generating version 4 uuid, retrying: %v", err)
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continue
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}
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// Any other errors represent a system problem. What did someone
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// do to /dev/urandom?
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panic(fmt.Errorf("error reading random number generator, retried for %v: %v", totalBackoff.String(), err))
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}
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break
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}
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s := base32.StdEncoding.EncodeToString(u)
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return s[:l]
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}
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// retryOnError tries to detect whether or not retrying would be fruitful.
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func retryOnError(err error) bool {
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switch err := err.(type) {
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case *os.PathError:
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return retryOnError(err.Err) // unpack the target error
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case syscall.Errno:
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if err == syscall.EPERM {
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// EPERM represents an entropy pool exhaustion, a condition under
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// which we backoff and retry.
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return true
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}
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}
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return false
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}
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