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>

1 files changed, 4 insertions, 0 deletions

diff --git a/src/text/template/exec.go b/src/text/template/exec.go
index 62cf19d30c..0e2ab0e211 100644
--- a/src/text/template/exec.go
+++ b/src/text/template/exec.go
@@ -121,6 +121,10 @@ func (e ExecError) Error() string {
 	return e.Err.Error()
 }
 
+func (e ExecError) Unwrap() error {
+	return e.Err
+}
+
 // errorf records an ExecError and terminates processing.
 func (s *state) errorf(format string, args ...interface{}) {
 	name := doublePercent(s.tmpl.Name())