Handle reads from closed sockets and pipes better.

After closing a pipe or socket, a read will return an internal error whose text
contains "use of closed network connection". Unfortunately, that appears to be
the only way to detect that error, and golang/go#4373
suggests it's not going to improve on its own.

Do what ~everyone else seems to have done in this case, check for the string.
That gives us nicer diagnostics when a client or server shuts down in the
normal course of its connections being terminated.

client.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"strconv"
 	"sync"
 
@@ -68,8 +69,12 @@ func (c *Client) accept(ch channel.Receiver) error {
 		c.log("Recoverable decoding error: %v", err)
 		return nil
 	} else if err != nil {
-		c.log("Unrecoverable decoding error: %v", err)
-		c.stop(err)
+		if err == io.EOF || isErrClosing(err) {
+			c.stop(nil) // don't remark on this as a failure
+		} else {
+			c.log("Unrecoverable decoding error: %v", err)
+			c.stop(err)
+		}
 		return err
 	}
 

error.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"strings"
 
 	"bitbucket.org/creachadair/jrpc2/code"
 )
@@ -70,3 +71,11 @@ func DataErrorf(code code.Code, v interface{}, msg string, args ...interface{})
 	}
 	return e
 }
+
+// isErrClosing detects the internal error returned by a read from a pipe or
+// socket that is closed.
+func isErrClosing(err error) bool {
+	// That we must check the string here appears to be working as intended, or at least
+	// there is no intent to make it better.  https://github.com/golang/go/issues/4373
+	return err != nil && strings.Contains(err.Error(), "use of closed network connection")
+}

server.go

@@ -416,7 +416,12 @@ func (s *Server) read(ch channel.Receiver) {
 			} else if isRecoverableJSONError(err) {
 				s.pushError(nil, jerrorf(code.ParseError, "invalid JSON request message"))
 			} else {
-				s.stop(err)
+				// Don't remark on EOF or a closed pipe as a failure.
+				if isErrClosing(err) {
+					s.stop(io.EOF)
+				} else {
+					s.stop(err)
+				}
 				s.mu.Unlock()
 				return
 			}