os: add deadline methods for File type

Add SetDeadline, SetReadDeadline, and SetWriteDeadline methods to os.File,
just as they exist today for the net package.

Fixes #22114

Change-Id: I4d390d739169b991175baba676010897dc8568fa
Reviewed-on: https://go-review.googlesource.com/71770
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/os/error.go

@@ -6,6 +6,7 @@ package os
 
 import (
 	"errors"
+	"internal/poll"
 )
 
 // Portable analogs of some common system call errors.
@@ -15,8 +16,13 @@ var (
 	ErrExist      = errors.New("file already exists")
 	ErrNotExist   = errors.New("file does not exist")
 	ErrClosed     = errors.New("file already closed")
+	ErrNoDeadline = poll.ErrNoDeadline
 )
 
+type timeout interface {
+	Timeout() bool
+}
+
 // PathError records an error and the operation and file path that caused it.
 type PathError struct {
 	Op   string
@@ -26,6 +32,12 @@ type PathError struct {
 
 func (e *PathError) Error() string { return e.Op + " " + e.Path + ": " + e.Err.Error() }
 
+// Timeout reports whether this error represents a timeout.
+func (e *PathError) Timeout() bool {
+	t, ok := e.Err.(timeout)
+	return ok && t.Timeout()
+}
+
 // SyscallError records an error from a specific system call.
 type SyscallError struct {
 	Syscall string
@@ -34,6 +46,12 @@ type SyscallError struct {
 
 func (e *SyscallError) Error() string { return e.Syscall + ": " + e.Err.Error() }
 
+// Timeout reports whether this error represents a timeout.
+func (e *SyscallError) Timeout() bool {
+	t, ok := e.Err.(timeout)
+	return ok && t.Timeout()
+}
+
 // NewSyscallError returns, as an error, a new SyscallError
 // with the given system call name and error details.
 // As a convenience, if err is nil, NewSyscallError returns nil.
@@ -65,6 +83,13 @@ func IsPermission(err error) bool {
 	return isPermission(err)
 }
 
+// IsTimeout returns a boolean indicating whether the error is known
+// to report that a timeout occurred.
+func IsTimeout(err error) bool {
+	terr, ok := underlyingError(err).(timeout)
+	return ok && terr.Timeout()
+}
+
 // underlyingError returns the underlying error for known os error types.
 func underlyingError(err error) error {
 	switch err := err.(type) {