bump min go version to go1.15

The DialTLSConfig option bumped the min version to go1.15 from the
previous go1.13. The recent introduction of net.ErrClosed bumped the min
version to go1.16.

We can still support go1.15 by having a separate build file.

We will drop 1.13: it's very old, go1.17 is out now (so go1.15 is
technically unsupported), and some of the deps use go1.15 (notably,
compress).

Closes twmb#69

Author: twmb

go.mod

@@ -1,6 +1,6 @@
 module github.com/twmb/franz-go
 
-go 1.13
+go 1.15
 
 require (
 	github.com/golang/snappy v0.0.4 // indirect

pkg/kgo/errors.go

@@ -10,6 +10,9 @@ import (
 )
 
 func isRetriableBrokerErr(err error) bool {
+	if err == nil { // sanity
+		return true
+	}
 	// https://github.com/golang/go/issues/45729
 	//
 	// Temporary is relatively useless. We will still check for the
@@ -38,7 +41,7 @@ func isRetriableBrokerErr(err error) bool {
 	}
 	// EOF can be returned if a broker kills a connection unexpectedly, and
 	// we can retry that. Same for ErrClosed.
-	if errors.Is(err, net.ErrClosed) || errors.Is(err, io.EOF) {
+	if isNetClosedErr(err) || errors.Is(err, io.EOF) {
 		return true
 	}
 	// We could have chosen a broker, and then a concurrent metadata update

pkg/kgo/go115.go

@@ -0,0 +1,10 @@
+//go:build go1.15
+// +build go1.15
+
+package kgo
+
+import "strings"
+
+func isNetClosedErr(err error) bool {
+	return strings.Contains(err.Error(), "use of closed network connection")
+}

pkg/kgo/go116.go

@@ -0,0 +1,13 @@
+//go:build go1.16 && !go1.15
+// +build go1.16,!go1.15
+
+package kgo
+
+import (
+	"errors"
+	"net"
+)
+
+func isNetClosedErr(err error) bool {
+	return errors.Is(err, net.ErrClosed)
+}