ulid: add DefaultEntropy() and Make() (#81)

Users of this library have opened many issues regarding the difficulty
of choosing an entropy source that is safe for concurrent use.

This commit introduces a thread safe per process monotonically increase `DefaultEntropy()`
function as well as an easy to use `Make()` function, aimed at users
that want safe defaults chosen for them.

Co-authored-by: Peter Bourgon <peter@bourgon.org>

README.md

@@ -39,21 +39,71 @@ go get github.com/oklog/ulid/v2
 
 ## Usage
 
-An ULID is constructed with a `time.Time` and an `io.Reader` entropy source.
-This design allows for greater flexibility in choosing your trade-offs.
+ULIDs are constructed from two things: a timestamp with millisecond precision,
+and some random data.
+
+Timestamps are modeled as uint64 values representing a Unix time in milliseconds.
+They can be produced by passing a [time.Time](https://pkg.go.dev/time#Time) to
+[ulid.Timestamp](https://pkg.go.dev/github.com/oklog/ulid/v2#Timestamp),
+or by calling  [time.Time.UnixMilli](https://pkg.go.dev/time#Time.UnixMilli)
+and converting the returned value to `uint64`.
+
+Random data is taken from a provided [io.Reader](https://pkg.go.dev/io#Reader).
+This design allows for greater flexibility when choosing trade-offs, but can be
+a bit confusing to newcomers.
+
+If you just want to generate a ULID and don't (yet) care about details like
+performance, cryptographic security, monotonicity, etc., use the
+[ulid.Make](https://pkg.go.dev/github.com/oklog/ulid/v2#Make) helper function.
+This function calls [time.Now](https://pkg.go.dev/time#Now) to get a timestamp,
+and uses a source of entropy which is process-global,
+[pseudo-random](https://pkg.go.dev/math/rand)), and
+[monotonic](https://pkg.go.dev/oklog/ulid/v2#LockedMonotonicReader)).
 
-Please note that `rand.Rand` from the `math` package is *not* safe for concurrent use.
-Instantiate one per long living go-routine or use a `sync.Pool` if you want to avoid the potential contention of a locked `rand.Source` as its been frequently observed in the package level functions.
+```go
+println(ulid.Make())
+// 01G65Z755AFWAKHE12NY0CQ9FH
+```
+
+More advanced use cases should utilize
+[ulid.New](https://pkg.go.dev/github.com/oklog/ulid/v2#New).
 
 ```go
-func ExampleULID() {
-	t := time.Unix(1000000, 0)
-	entropy := ulid.Monotonic(rand.New(rand.NewSource(t.UnixNano())), 0)
-	fmt.Println(ulid.MustNew(ulid.Timestamp(t), entropy))
-	// Output: 0000XSNJG0MQJHBF4QX1EFD6Y3
-}
+entropy := rand.New(rand.NewSource(time.Now().UnixNano()))
+ms := ulid.Timestamp(time.Now())
+println(ulid.New(ms, entropy))
+// 01G65Z755AFWAKHE12NY0CQ9FH
 ```
 
+Care should be taken when providing a source of entropy.
+
+The above example utilizes [math/rand.Rand](https://pkg.go.dev/math/rand#Rand),
+which is not safe for concurrent use by multiple goroutines. Consider
+alternatives such as
+[x/exp/rand](https://pkg.go.dev/golang.org/x/exp/rand#LockedSource).
+Security-sensitive use cases should always use cryptographically secure entropy
+provided by [crypto/rand](https://pkg.go.dev/crypto/rand).
+
+Performance-sensitive use cases should avoid synchronization when generating
+IDs. One option is to use a unique source of entropy for each concurrent
+goroutine, which results in no lock contention, but cannot provide strong
+guarantees about the random data, and does not provide monotonicity within a
+given millisecond. One common performance optimization is to pool sources of
+entropy using a [sync.Pool](https://pkg.go.dev/sync#Pool).
+
+Monotonicity is a property that says each ULID is "bigger than" the previous
+one. ULIDs are automatically monotonic, but only to millisecond precision. ULIDs
+generated within the same millisecond are ordered by their random component,
+which means they are by default un-ordered. You can use
+[ulid.MonotonicEntropy](https://pkg.go.dev/oklog/ulid/v2#MonotonicEntropy) or
+[ulid.LockedMonotonicEntropy](https://pkg.go.dev/oklog/ulid/v2#LockedMonotonicEntropy)
+to create ULIDs that are monotonic within a given millisecond, with caveats. See
+the documentation for details.
+
+If you don't care about time-based ordering of generated IDs, then there's no
+reason to use ULIDs! There are many other kinds of IDs that are easier, faster,
+smaller, etc. Consider UUIDs.
+
 ## Commandline tool
 
 This repo also provides a tool to generate and parse ULIDs at the command line.

ulid.go

@@ -23,6 +23,7 @@ import (
 	"math"
 	"math/bits"
 	"math/rand"
+	"sync"
 	"time"
 )
 
@@ -121,6 +122,32 @@ func MustNew(ms uint64, entropy io.Reader) ULID {
 	return id
 }
 
+var (
+	entropy     io.Reader
+	entropyOnce sync.Once
+)
+
+// DefaultEntropy returns a thread-safe per process monotonically increasing
+// entropy source.
+func DefaultEntropy() io.Reader {
+	entropyOnce.Do(func() {
+		rng := rand.New(rand.NewSource(time.Now().UnixNano()))
+		entropy = &LockedMonotonicReader{
+			MonotonicReader: Monotonic(rng, 0),
+		}
+	})
+	return entropy
+}
+
+// Make returns an ULID with the current time in Unix milliseconds and
+// monotonically increasing entropy for the same millisecond.
+// It is safe for concurrent use, leveraging a sync.Pool underneath for minimal
+// contention.
+func Make() (id ULID) {
+	// NOTE: MustNew can't panic since DefaultEntropy never returns an error.
+	return MustNew(Now(), DefaultEntropy())
+}
+
 // Parse parses an encoded ULID, returning an error in case of failure.
 //
 // ErrDataSize is returned if the len(ulid) is different from an encoded
@@ -531,21 +558,38 @@ func Monotonic(entropy io.Reader, inc uint64) *MonotonicEntropy {
 		m.inc = math.MaxUint32
 	}
 
-	if rng, ok := entropy.(*rand.Rand); ok {
+	if rng, ok := entropy.(rng); ok {
 		m.rng = rng
 	}
 
 	return &m
 }
 
+type rng interface{ Int63n(n int64) int64 }
+
+// LockedMonotonicReader wraps a MonotonicReader with a sync.Mutex for
+// safe concurrent use.
+type LockedMonotonicReader struct {
+	mu sync.Mutex
+	MonotonicReader
+}
+
+// MonotonicRead synchronizes calls to the wrapped MonotonicReader.
+func (r *LockedMonotonicReader) MonotonicRead(ms uint64, p []byte) (err error) {
+	r.mu.Lock()
+	err = r.MonotonicReader.MonotonicRead(ms, p)
+	r.mu.Unlock()
+	return err
+}
+
 // MonotonicEntropy is an opaque type that provides monotonic entropy.
 type MonotonicEntropy struct {
 	io.Reader
 	ms      uint64
 	inc     uint64
 	entropy uint80
 	rand    [8]byte
-	rng     *rand.Rand
+	rng     rng
 }
 
 // MonotonicRead implements the MonotonicReader interface.

ulid_test.go

@@ -21,7 +21,6 @@ import (
 	"math"
 	"math/rand"
 	"strings"
-	"sync"
 	"testing"
 	"testing/iotest"
 	"testing/quick"
@@ -61,6 +60,18 @@ func TestNew(t *testing.T) {
 	})
 }
 
+func TestMake(t *testing.T) {
+	t.Parallel()
+	id := ulid.Make()
+	rt, err := ulid.Parse(id.String())
+	if err != nil {
+		t.Fatalf("parse %q: %v", id.String(), err)
+	}
+	if id != rt {
+		t.Fatalf("%q != %q", id.String(), rt.String())
+	}
+}
+
 func TestMustNew(t *testing.T) {
 	t.Parallel()
 
@@ -142,7 +153,7 @@ func TestRoundTrips(t *testing.T) {
 			id == ulid.MustParseStrict(id.String())
 	}
 
-	err := quick.Check(prop, &quick.Config{MaxCount: 1E5})
+	err := quick.Check(prop, &quick.Config{MaxCount: 1e5})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -229,7 +240,7 @@ func TestEncoding(t *testing.T) {
 		return true
 	}
 
-	if err := quick.Check(prop, &quick.Config{MaxCount: 1E5}); err != nil {
+	if err := quick.Check(prop, &quick.Config{MaxCount: 1e5}); err != nil {
 		t.Fatal(err)
 	}
 }
@@ -258,7 +269,7 @@ func TestLexicographicalOrder(t *testing.T) {
 		top = next
 	}
 
-	if err := quick.Check(prop, &quick.Config{MaxCount: 1E6}); err != nil {
+	if err := quick.Check(prop, &quick.Config{MaxCount: 1e6}); err != nil {
 		t.Fatal(err)
 	}
 }
@@ -316,7 +327,7 @@ func TestParseRobustness(t *testing.T) {
 		return err == nil
 	}
 
-	err := quick.Check(prop, &quick.Config{MaxCount: 1E4})
+	err := quick.Check(prop, &quick.Config{MaxCount: 1e4})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -368,7 +379,7 @@ func TestTimestampRoundTrips(t *testing.T) {
 		return ts == ulid.Timestamp(ulid.Time(ts))
 	}
 
-	err := quick.Check(prop, &quick.Config{MaxCount: 1E5})
+	err := quick.Check(prop, &quick.Config{MaxCount: 1e5})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -447,7 +458,7 @@ func TestEntropyRead(t *testing.T) {
 		return eq
 	}
 
-	if err := quick.Check(prop, &quick.Config{MaxCount: 1E4}); err != nil {
+	if err := quick.Check(prop, &quick.Config{MaxCount: 1e4}); err != nil {
 		t.Fatal(err)
 	}
 }
@@ -463,7 +474,7 @@ func TestCompare(t *testing.T) {
 		return a.Compare(b)
 	}
 
-	err := quick.CheckEqual(a, b, &quick.Config{MaxCount: 1E5})
+	err := quick.CheckEqual(a, b, &quick.Config{MaxCount: 1e5})
 	if err != nil {
 		t.Error(err)
 	}
@@ -586,11 +597,8 @@ func TestMonotonicSafe(t *testing.T) {
 	t.Parallel()
 
 	var (
-		src       = rand.NewSource(time.Now().UnixNano())
-		entropy   = rand.New(src)
-		monotonic = ulid.Monotonic(entropy, 0)
-		safe      = &safeMonotonicReader{MonotonicReader: monotonic}
-		t0        = ulid.Timestamp(time.Now())
+		safe = ulid.DefaultEntropy()
+		t0   = ulid.Timestamp(time.Now())
 	)
 
 	errs := make(chan error, 100)
@@ -630,18 +638,6 @@ func TestULID_Bytes(t *testing.T) {
 	}
 }
 
-type safeMonotonicReader struct {
-	mtx sync.Mutex
-	ulid.MonotonicReader
-}
-
-func (r *safeMonotonicReader) MonotonicRead(ms uint64, p []byte) (err error) {
-	r.mtx.Lock()
-	err = r.MonotonicReader.MonotonicRead(ms, p)
-	r.mtx.Unlock()
-	return err
-}
-
 func BenchmarkNew(b *testing.B) {
 	benchmarkMakeULID(b, func(timestamp uint64, entropy io.Reader) {
 		_, _ = ulid.New(timestamp, entropy)