os, net, internal/poll: return consistent error for closed socket

In the past we returned "use of closed network connection" when using
a closed network descriptor in some way. In CL 36799 that was changed
to return "use of closed file or network connection". Because programs
have no access to a value of this error type (see issue #4373) they
resort to doing direct string comparisons (see issue #19252). This CL
restores the old error string so that we don't break programs
unnecessarily with the 1.9 release.

This adds a test to the net package for the expected string.

For symmetry check that the os package returns the expected error,
which for os already exists as os.ErrClosed.

Updates #4373.
Fixed #19252.

Change-Id: I5b83fd12cfa03501a077cad9336499b819f4a38b
Reviewed-on: https://go-review.googlesource.com/39997
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 23 insertions, 12 deletions

diff --git a/src/net/error_test.go b/src/net/error_test.go
index 021968b079..9791e6fe4d 100644
--- a/src/net/error_test.go
+++ b/src/net/error_test.go
@@ -13,6 +13,7 @@ import (
 	"net/internal/socktest"
 	"os"
 	"runtime"
+	"strings"
 	"testing"
 	"time"
 )
@@ -98,7 +99,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case errCanceled, poll.ErrClosing, errMissingAddress, errNoSuitableAddress,
+	case errCanceled, poll.ErrNetClosing, errMissingAddress, errNoSuitableAddress,
 		context.DeadlineExceeded, context.Canceled:
 		return nil
 	}
@@ -433,7 +434,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrClosing, poll.ErrTimeout:
+	case poll.ErrNetClosing, poll.ErrTimeout:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -475,7 +476,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case errCanceled, poll.ErrClosing, errMissingAddress, poll.ErrTimeout, ErrWriteToConnected, io.ErrUnexpectedEOF:
+	case errCanceled, poll.ErrNetClosing, errMissingAddress, poll.ErrTimeout, ErrWriteToConnected, io.ErrUnexpectedEOF:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -490,11 +491,21 @@ third:
 // parseCloseError parses nestedErr and reports whether it is a valid
 // error value from Close functions.
 // It returns nil when nestedErr is valid.
-func parseCloseError(nestedErr error) error {
+func parseCloseError(nestedErr error, isShutdown bool) error {
 	if nestedErr == nil {
 		return nil
 	}
 
+	// Because historically we have not exported the error that we
+	// return for an operation on a closed network connection,
+	// there are programs that test for the exact error string.
+	// Verify that string here so that we don't break those
+	// programs unexpectedly. See issues #4373 and #19252.
+	want := "use of closed network connection"
+	if !isShutdown && !strings.Contains(nestedErr.Error(), want) {
+		return fmt.Errorf("error string %q does not contain expected string %q", nestedErr, want)
+	}
+
 	switch err := nestedErr.(type) {
 	case *OpError:
 		if err := err.isValid(); err != nil {
@@ -518,7 +529,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrClosing:
+	case poll.ErrNetClosing:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -548,23 +559,23 @@ func TestCloseError(t *testing.T) {
 
 	for i := 0; i < 3; i++ {
 		err = c.(*TCPConn).CloseRead()
-		if perr := parseCloseError(err); perr != nil {
+		if perr := parseCloseError(err, true); perr != nil {
 			t.Errorf("#%d: %v", i, perr)
 		}
 	}
 	for i := 0; i < 3; i++ {
 		err = c.(*TCPConn).CloseWrite()
-		if perr := parseCloseError(err); perr != nil {
+		if perr := parseCloseError(err, true); perr != nil {
 			t.Errorf("#%d: %v", i, perr)
 		}
 	}
 	for i := 0; i < 3; i++ {
 		err = c.Close()
-		if perr := parseCloseError(err); perr != nil {
+		if perr := parseCloseError(err, false); perr != nil {
 			t.Errorf("#%d: %v", i, perr)
 		}
 		err = ln.Close()
-		if perr := parseCloseError(err); perr != nil {
+		if perr := parseCloseError(err, false); perr != nil {
 			t.Errorf("#%d: %v", i, perr)
 		}
 	}
@@ -577,7 +588,7 @@ func TestCloseError(t *testing.T) {
 
 	for i := 0; i < 3; i++ {
 		err = pc.Close()
-		if perr := parseCloseError(err); perr != nil {
+		if perr := parseCloseError(err, false); perr != nil {
 			t.Errorf("#%d: %v", i, perr)
 		}
 	}
@@ -614,7 +625,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrClosing, poll.ErrTimeout:
+	case poll.ErrNetClosing, poll.ErrTimeout:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -693,7 +704,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrClosing:
+	case poll.ErrNetClosing:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)