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