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// Package uuid provides simple UUID generation. Only version 4 style UUIDs
// can be generated.
//
// Please see http://tools.ietf.org/html/rfc4122 for details on UUIDs.
package uuid
import (
"crypto/rand"
"fmt"
"io"
"os"
"syscall"
"time"
)
const (
// Bits is the number of bits in a UUID
Bits = 128
// Size is the number of bytes in a UUID
Size = Bits / 8
format = "%08x-%04x-%04x-%04x-%012x"
)
var (
// ErrUUIDInvalid indicates a parsed string is not a valid uuid.
ErrUUIDInvalid = fmt.Errorf("invalid uuid")
// Loggerf can be used to override the default logging destination. Such
// log messages in this library should be logged at warning or higher.
Loggerf = func(format string, args ...interface{}) {}
)
// UUID represents a UUID value. UUIDs can be compared and set to other values
// and accessed by byte.
type UUID [Size]byte
// Generate creates a new, version 4 uuid.
func Generate() (u UUID) {
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
)
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++
Loggerf("error 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("error reading random number generator, retried for %v: %v", totalBackoff.String(), err))
}
break
}
u[6] = (u[6] & 0x0f) | 0x40 // set version byte
u[8] = (u[8] & 0x3f) | 0x80 // set high order byte 0b10{8,9,a,b}
return u
}
// Parse attempts to extract a uuid from the string or returns an error.
func Parse(s string) (u UUID, err error) {
if len(s) != 36 {
return UUID{}, ErrUUIDInvalid
}
// create stack addresses for each section of the uuid.
p := make([][]byte, 5)
if _, err := fmt.Sscanf(s, format, &p[0], &p[1], &p[2], &p[3], &p[4]); err != nil {
return u, err
}
copy(u[0:4], p[0])
copy(u[4:6], p[1])
copy(u[6:8], p[2])
copy(u[8:10], p[3])
copy(u[10:16], p[4])
return
}
func (u UUID) String() string {
return fmt.Sprintf(format, u[:4], u[4:6], u[6:8], u[8:10], u[10:])
}
// 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 == syscall.EPERM {
// EPERM represents an entropy pool exhaustion, a condition under
// which we backoff and retry.
return true
}
}
return false
}
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