net: export ErrClosed

This permits programs to reliably detect whether they are using a
closed network connection.

Fixes #4373

Change-Id: Ib4ce8cc82bbb134c4689f0ebc8b9b11bb8b32a22
Reviewed-on: https://go-review.googlesource.com/c/go/+/250357
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>

doc/go1.16.html

@@ -99,6 +99,18 @@ Do not send CLs removing the interior tags from such phrases.
   TODO
 </p>
 
+<h3 id="net"><a href="/pkg/net/">net</a></h3>
+
+<p><!-- CL -->
+  The case of I/O on a closed network connection, or I/O on a network
+  connection that is closed before any of the I/O completes, can now
+  be detected using the new <a href="/pkg/net/#ErrClosed">ErrClosed</a> error.
+  A typical use would be <code>errors.Is(err, net.ErrClosed)</code>.
+  In earlier releases the only way to reliably detect this case was to
+  match the string returned by the <code>Error</code> method
+  with <code>"use of closed network connection"</code>.
+</p>
+
 <h3 id="unicode"><a href="/pkg/unicode/">unicode</a></h3>
 
 <p><!-- CL 248765 -->

src/net/error_test.go

@@ -8,6 +8,7 @@ package net
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"internal/poll"
 	"io"
@@ -101,7 +102,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case errCanceled, poll.ErrNetClosing, errMissingAddress, errNoSuitableAddress,
+	case errCanceled, ErrClosed, errMissingAddress, errNoSuitableAddress,
 		context.DeadlineExceeded, context.Canceled:
 		return nil
 	}
@@ -436,7 +437,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrNetClosing, errTimeout, poll.ErrNotPollable, os.ErrDeadlineExceeded:
+	case ErrClosed, errTimeout, poll.ErrNotPollable, os.ErrDeadlineExceeded:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -478,7 +479,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case errCanceled, poll.ErrNetClosing, errMissingAddress, errTimeout, os.ErrDeadlineExceeded, ErrWriteToConnected, io.ErrUnexpectedEOF:
+	case errCanceled, ErrClosed, errMissingAddress, errTimeout, os.ErrDeadlineExceeded, ErrWriteToConnected, io.ErrUnexpectedEOF:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -508,6 +509,10 @@ func parseCloseError(nestedErr error, isShutdown bool) error {
 		return fmt.Errorf("error string %q does not contain expected string %q", nestedErr, want)
 	}
 
+	if !isShutdown && !errors.Is(nestedErr, ErrClosed) {
+		return fmt.Errorf("errors.Is(%v, errClosed) returns false, want true", nestedErr)
+	}
+
 	switch err := nestedErr.(type) {
 	case *OpError:
 		if err := err.isValid(); err != nil {
@@ -531,7 +536,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrNetClosing:
+	case ErrClosed:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -627,7 +632,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrNetClosing, errTimeout, poll.ErrNotPollable, os.ErrDeadlineExceeded:
+	case ErrClosed, errTimeout, poll.ErrNotPollable, os.ErrDeadlineExceeded:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)
@@ -706,7 +711,7 @@ second:
 		goto third
 	}
 	switch nestedErr {
-	case poll.ErrNetClosing:
+	case ErrClosed:
 		return nil
 	}
 	return fmt.Errorf("unexpected type on 2nd nested level: %T", nestedErr)

src/net/net.go

@@ -81,6 +81,7 @@ package net
 import (
 	"context"
 	"errors"
+	"internal/poll"
 	"io"
 	"os"
 	"sync"
@@ -632,6 +633,17 @@ func (e *DNSError) Timeout() bool { return e.IsTimeout }
 // error and return a DNSError for which Temporary returns false.
 func (e *DNSError) Temporary() bool { return e.IsTimeout || e.IsTemporary }
 
+// errClosed exists just so that the docs for ErrClosed don't mention
+// the internal package poll.
+var errClosed = poll.ErrNetClosing
+
+// ErrClosed is the error returned by an I/O call on a network
+// connection that has already been closed, or that is closed by
+// another goroutine before the I/O is completed. This may be wrapped
+// in another error, and should normally be tested using
+// errors.Is(err, net.ErrClosed).
+var ErrClosed = errClosed
+
 type writerOnly struct {
 	io.Writer
 }