tstime: write Parse3339 parse that doesn't use time.Parse

It doesn't allocate and it's half the time of time.Parse (which
allocates), and 2/3rds the time of time.ParseInLocation (which
doesn't).

Go with a UTC time:

BenchmarkGoParse3339/Z-8                 2200995               534 ns/op               0 B/op          0 allocs/op
BenchmarkGoParse3339/Z-8                 2254816               554 ns/op               0 B/op          0 allocs/op
BenchmarkGoParse3339/Z-8                 2159504               522 ns/op               0 B/op          0 allocs/op

Go allocates with a "-08:00" suffix instead of ending in "Z":

BenchmarkGoParse3339/TZ-8                1276491               884 ns/op             144 B/op          3 allocs/op
BenchmarkGoParse3339/TZ-8                1355858               942 ns/op             144 B/op          3 allocs/op
BenchmarkGoParse3339/TZ-8                1385484               911 ns/op             144 B/op          3 allocs/op

Go doesn't allocate if you use time.ParseInLocation, but then you need
to parse the string to find the location anyway, so might as well go
all the way (below).

BenchmarkGoParse3339InLocation-8         1912254               597 ns/op               0 B/op          0 allocs/op
BenchmarkGoParse3339InLocation-8         1980043               612 ns/op               0 B/op          0 allocs/op
BenchmarkGoParse3339InLocation-8         1891366               612 ns/op               0 B/op          0 allocs/op

Parsing RFC3339 ourselves, UTC:

BenchmarkParse3339/Z-8                   3889220               307 ns/op               0 B/op          0 allocs/op
BenchmarkParse3339/Z-8                   3718500               309 ns/op               0 B/op          0 allocs/op
BenchmarkParse3339/Z-8                   3621231               303 ns/op               0 B/op          0 allocs/op

Parsing RFC3339 ourselves, with timezone (w/ *time.Location fetched
from sync.Map)

BenchmarkParse3339/TZ-8                  3019612               418 ns/op               0 B/op          0 allocs/op
BenchmarkParse3339/TZ-8                  2921618               401 ns/op               0 B/op          0 allocs/op
BenchmarkParse3339/TZ-8                  3031671               408 ns/op               0 B/op          0 allocs/op

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstime/tstime.go

@@ -6,16 +6,20 @@
 package tstime
 
 import (
+	"errors"
+	"fmt"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
 )
 
-// zoneOf returns the RFC3339 zone suffix, or the empty string
-// if it's invalid or not something we want to cache.
+// zoneOf returns the RFC3339 zone suffix (either "Z" or like
+// "+08:30"), or the empty string if it's invalid or not something we
+// want to cache.
 func zoneOf(s string) string {
 	if strings.HasSuffix(s, "Z") {
-		return ""
+		return "Z"
 	}
 	if len(s) < len("2020-04-05T15:56:00+08:00") {
 		// Too short, invalid? Let time.Parse fail on it.
@@ -36,23 +40,75 @@ func zoneOf(s string) string {
 // *time.Location (from FixedLocation).
 var locCache sync.Map
 
+func getLocation(zone, timeValue string) (*time.Location, error) {
+	if zone == "Z" {
+		return time.UTC, nil
+	}
+	if loci, ok := locCache.Load(zone); ok {
+		return loci.(*time.Location), nil
+	}
+	// TODO(bradfitz): just parse it and call time.FixedLocation.
+	// For now, just have time.Parse do it once:
+	t, err := time.Parse(time.RFC3339Nano, timeValue)
+	if err != nil {
+		return nil, err
+	}
+	loc := t.Location()
+	locCache.LoadOrStore(zone, loc)
+	return loc, nil
+
+}
+
 // Parse3339 is a wrapper around time.Parse(time.RFC3339Nano, s) that caches
 // timezone Locations for future parses.
 func Parse3339(s string) (time.Time, error) {
 	zone := zoneOf(s)
 	if zone == "" {
+		// Invalid or weird timezone offset. Use slow path,
+		// which'll probably return an error.
 		return time.Parse(time.RFC3339Nano, s)
 	}
-	loci, ok := locCache.Load(zone)
-	if ok {
-		// TODO(bradfitz): just rewrite this do the trivial parsing by hand
-		// which will be faster than Go's format-driven one. RFC3339 is trivial.
-		return time.ParseInLocation(time.RFC3339Nano, s, loci.(*time.Location))
-	}
-	t, err := time.Parse(time.RFC3339Nano, s)
+	loc, err := getLocation(zone, s)
 	if err != nil {
 		return time.Time{}, err
 	}
-	locCache.LoadOrStore(zone, t.Location())
-	return t, nil
+	s = s[:len(s)-len(zone)] // remove zone suffix
+	var year, mon, day, hr, min, sec, nsec int
+	const baseLen = len("2020-04-05T15:56:00")
+	if len(s) < baseLen ||
+		!parseInt(s[:4], &year) ||
+		s[4] != '-' ||
+		!parseInt(s[5:7], &mon) ||
+		s[7] != '-' ||
+		!parseInt(s[8:10], &day) ||
+		s[10] != 'T' ||
+		!parseInt(s[11:13], &hr) ||
+		s[13] != ':' ||
+		!parseInt(s[14:16], &min) ||
+		s[16] != ':' ||
+		!parseInt(s[17:19], &sec) {
+		return time.Time{}, errors.New("invalid time")
+	}
+	nsStr := s[baseLen:]
+	if nsStr != "" {
+		if nsStr[0] != '.' {
+			return time.Time{}, errors.New("invalid optional nanosecond prefix")
+		}
+		if !parseInt(nsStr[1:], &nsec) {
+			return time.Time{}, fmt.Errorf("invalid optional nanosecond number %q", nsStr[1:])
+		}
+		for i := 0; i < len("999999999")-(len(nsStr)-1); i++ {
+			nsec *= 10
+		}
+	}
+	return time.Date(year, time.Month(mon), day, hr, min, sec, nsec, loc), nil
+}
+
+func parseInt(s string, dst *int) bool {
+	n, err := strconv.ParseInt(s, 10, 0)
+	if err != nil || n < 0 {
+		return false
+	}
+	*dst = int(n)
+	return true
 }