all: add Unwrap and Is methods to various error types

Add Unwrap methods to types which wrap an underlying error:

  "encodinc/csv".ParseError
  "encoding/json".MarshalerError
  "net/http".transportReadFromServerError
  "net".OpError
  "net".DNSConfigError
  "net/url".Error
  "os/exec".Error
  "signal/internal/pty".PtyError
  "text/template".ExecError

Add os.ErrTemporary. A case could be made for putting this error
value in package net, since no exported error types in package os
include a Temporary method. However, syscall errors returned from
the os package do include this method.

Add Is methods to error types with a Timeout or Temporary method,
making errors.Is(err, os.Err{Timeout,Temporary}) equivalent to
testing the corresponding method:

  "context".DeadlineExceeded
  "internal/poll".TimeoutError
  "net".adrinfoErrno
  "net".OpError
  "net".DNSError
  "net/http".httpError
  "net/http".tlsHandshakeTimeoutError
  "net/pipe".timeoutError
  "net/url".Error

Updates #30322
Updates #29934

Change-Id: I409fb20c072ea39116ebfb8c7534d493483870dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/170037
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

2 files changed, 9 insertions, 0 deletions

diff --git a/src/context/context.go b/src/context/context.go
index 93bf5b627d..0f36881b1e 100644
--- a/src/context/context.go
+++ b/src/context/context.go
@@ -49,6 +49,7 @@ package context
 
 import (
 	"errors"
+	"internal/oserror"
 	"internal/reflectlite"
 	"sync"
 	"time"
@@ -162,6 +163,9 @@ type deadlineExceededError struct{}
 func (deadlineExceededError) Error() string   { return "context deadline exceeded" }
 func (deadlineExceededError) Timeout() bool   { return true }
 func (deadlineExceededError) Temporary() bool { return true }
+func (deadlineExceededError) Is(target error) bool {
+	return target == oserror.ErrTimeout || target == oserror.ErrTemporary
+}
 
 // An emptyCtx is never canceled, has no values, and has no deadline. It is not
 // struct{}, since vars of this type must have distinct addresses.
diff --git a/src/context/context_test.go b/src/context/context_test.go
index 0e69e2f6fd..9991c5b09c 100644
--- a/src/context/context_test.go
+++ b/src/context/context_test.go
@@ -5,8 +5,10 @@
 package context
 
 import (
+	"errors"
 	"fmt"
 	"math/rand"
+	"os"
 	"runtime"
 	"strings"
 	"sync"
@@ -647,4 +649,7 @@ func XTestDeadlineExceededSupportsTimeout(t testingT) {
 	if !i.Timeout() {
 		t.Fatal("wrong value for timeout")
 	}
+	if !errors.Is(DeadlineExceeded, os.ErrTimeout) {
+		t.Fatal("errors.Is(DeadlineExceeded, os.ErrTimeout) = false, want true")
+	}
 }