net: support error.Is of network errors and context errors

Change timeouts to be Is(context.DeadlineExceeded) and cancelation to
be Is(context.Canceled).

Fixes #51428

Change-Id: Ic580bd9da0f338e993fb79138875a78d99cc1a1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/396877
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

doc/go1.19.html

@@ -89,6 +89,18 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       Please report any such problems on <a href="/issue/new">the
       issue tracker</a>.
     </p>
+
+    <p><!-- CL 396877 -->
+      When a net package function or method returns an "I/O timeout"
+      error, the error will now satisfy <code>errors.Is(err,
+      context.Canceled)</code>.  When a net package function returns
+      an "operation was canceled" error, the error will now satisfy
+      <code>errors.Is(err, context.DeadlineExceeded)</code>.
+      These changes are intended to make it easier for code to test
+      for cases in which a context cancelation or timeout causes a net
+      package function or method to return an error, while preserving
+      backward compatibility for error messages.
+    </p>
   </dd>
 </dl><!-- net -->
 

src/net/error_test.go

@@ -795,3 +795,12 @@ func parseLookupPortError(nestedErr error) error {
 	}
 	return fmt.Errorf("unexpected type on 1st nested level: %T", nestedErr)
 }
+
+func TestContextError(t *testing.T) {
+	if !errors.Is(errCanceled, context.Canceled) {
+		t.Error("errCanceled is not context.Canceled")
+	}
+	if !errors.Is(errTimeout, context.DeadlineExceeded) {
+		t.Error("errTimeout is not context.DeadlineExceeded")
+	}
+}

src/net/net.go

@@ -413,15 +413,20 @@ var (
 	errMissingAddress = errors.New("missing address")
 
 	// For both read and write operations.
-	errCanceled         = errors.New("operation was canceled")
+	errCanceled         = canceledError{}
 	ErrWriteToConnected = errors.New("use of WriteTo with pre-connected connection")
 )
 
+// canceledError lets us return the same error string we have always
+// returned, while still being Is context.Canceled.
+type canceledError struct{}
+
+func (canceledError) Error() string { return "operation was canceled" }
+
+func (canceledError) Is(err error) bool { return err == context.Canceled }
+
 // mapErr maps from the context errors to the historical internal net
 // error values.
-//
-// TODO(bradfitz): get rid of this after adjusting tests and making
-// context.DeadlineExceeded implement net.Error?
 func mapErr(err error) error {
 	switch err {
 	case context.Canceled:
@@ -580,10 +585,12 @@ func (e InvalidAddrError) Temporary() bool { return false }
 // errTimeout exists to return the historical "i/o timeout" string
 // for context.DeadlineExceeded. See mapErr.
 // It is also used when Dialer.Deadline is exceeded.
+// error.Is(errTimeout, context.DeadlineExceeded) returns true.
 //
 // TODO(iant): We could consider changing this to os.ErrDeadlineExceeded
-// in the future, but note that that would conflict with the TODO
-// at mapErr that suggests changing it to context.DeadlineExceeded.
+// in the future, if we make
+//   errors.Is(os.ErrDeadlineExceeded, context.DeadlineExceeded)
+// return true.
 var errTimeout error = &timeoutError{}
 
 type timeoutError struct{}
@@ -592,6 +599,10 @@ func (e *timeoutError) Error() string   { return "i/o timeout" }
 func (e *timeoutError) Timeout() bool   { return true }
 func (e *timeoutError) Temporary() bool { return true }
 
+func (e *timeoutError) Is(err error) bool {
+	return err == context.DeadlineExceeded
+}
+
 // DNSConfigError represents an error reading the machine's DNS configuration.
 // (No longer used; kept for compatibility.)
 type DNSConfigError struct {