From 65e624e7b9c9bbeb04a3c3a29a88986a4b8b816c Mon Sep 17 00:00:00 2001
From: Jonathan Amsterdam <jba@google.com>
Date: Thu, 22 Aug 2019 12:40:52 -0400
Subject: [PATCH] syscall: document relationship among Errno, errors.Is and
 os.Err*

- Add doc to syscall.Errno (and syscall.ErrorString for plan9).

- Mention under `syscall` in release notes.

Fixes #33436.

Change-Id: I032ffebaa76ed67eb9d748e7645ca73f26144ea0
Reviewed-on: https://go-review.googlesource.com/c/go/+/191337
Reviewed-by: Ian Lance Taylor <iant@golang.org>
---
 doc/go1.13.html                | 7 +++++++
 src/syscall/syscall_js.go      | 6 ++++++
 src/syscall/syscall_nacl.go    | 6 ++++++
 src/syscall/syscall_plan9.go   | 6 ++++++
 src/syscall/syscall_unix.go    | 6 ++++++
 src/syscall/syscall_windows.go | 6 ++++++
 6 files changed, 37 insertions(+)

diff --git a/doc/go1.13.html b/doc/go1.13.html
index 6cee0b28e6..5760669fe5 100644
--- a/doc/go1.13.html
+++ b/doc/go1.13.html
@@ -961,6 +961,13 @@ godoc
       <a href="/pkg/syscall/?GOOS=windows#Chmod"><code>Chmod</code></a> mode on Windows.
     </p>
 
+    <p><!-- CL 191337 -->
+      Values of type <code>Errno</code> can be tested against error values in
+      the <code>os</code> package,
+      like <a href="/pkg/os/#ErrExist"><code>ErrExist</code></a>, using
+      <a href="/pkg/errors/#Is"><code>errors.Is</code></a>.
+    </p>
+
 </dl><!-- syscall -->
 
 <dl id="syscall/js"><dt><a href="/pkg/syscall/js/">syscall/js</a></dt>
diff --git a/src/syscall/syscall_js.go b/src/syscall/syscall_js.go
index 175fe47fca..24fbd51189 100644
--- a/src/syscall/syscall_js.go
+++ b/src/syscall/syscall_js.go
@@ -44,6 +44,12 @@ const PathMax = 256
 //	if errno != 0 {
 //		err = errno
 //	}
+//
+// Errno values can be tested against error values from the the os package
+// using errors.Is. For example:
+//
+//	_, _, err := syscall.Syscall(...)
+//	if errors.Is(err, os.ErrNotExist) ...
 type Errno uintptr
 
 func (e Errno) Error() string {
diff --git a/src/syscall/syscall_nacl.go b/src/syscall/syscall_nacl.go
index e887b1e04e..efc986a6f4 100644
--- a/src/syscall/syscall_nacl.go
+++ b/src/syscall/syscall_nacl.go
@@ -51,6 +51,12 @@ const PathMax = 256
 //	if errno != 0 {
 //		err = errno
 //	}
+//
+// Errno values can be tested against error values from the the os package
+// using errors.Is. For example:
+//
+//	_, _, err := syscall.Syscall(...)
+//	if errors.Is(err, os.ErrNotExist) ...
 type Errno uintptr
 
 func (e Errno) Error() string {
diff --git a/src/syscall/syscall_plan9.go b/src/syscall/syscall_plan9.go
index c11f030531..52a9d2aabe 100644
--- a/src/syscall/syscall_plan9.go
+++ b/src/syscall/syscall_plan9.go
@@ -20,6 +20,12 @@ const ImplementsGetwd = true
 const bitSize16 = 2
 
 // ErrorString implements Error's String method by returning itself.
+//
+// ErrorString values can be tested against error values from the the os package
+// using errors.Is. For example:
+//
+//	_, _, err := syscall.Syscall(...)
+//	if errors.Is(err, os.ErrNotExist) ...
 type ErrorString string
 
 func (e ErrorString) Error() string { return string(e) }
diff --git a/src/syscall/syscall_unix.go b/src/syscall/syscall_unix.go
index 59c8c34933..4a6305e4c5 100644
--- a/src/syscall/syscall_unix.go
+++ b/src/syscall/syscall_unix.go
@@ -107,6 +107,12 @@ func (m *mmapper) Munmap(data []byte) (err error) {
 //	if errno != 0 {
 //		err = errno
 //	}
+//
+// Errno values can be tested against error values from the the os package
+// using errors.Is. For example:
+//
+//	_, _, err := syscall.Syscall(...)
+//	if errors.Is(err, os.ErrNotExist) ...
 type Errno uintptr
 
 func (e Errno) Error() string {
diff --git a/src/syscall/syscall_windows.go b/src/syscall/syscall_windows.go
index 2e8edc7acc..cfa9d9c5d0 100644
--- a/src/syscall/syscall_windows.go
+++ b/src/syscall/syscall_windows.go
@@ -77,6 +77,12 @@ func UTF16PtrFromString(s string) (*uint16, error) {
 }
 
 // Errno is the Windows error number.
+//
+// Errno values can be tested against error values from the the os package
+// using errors.Is. For example:
+//
+//	_, _, err := syscall.Syscall(...)
+//	if errors.Is(err, os.ErrNotExist) ...
 type Errno uintptr
 
 func langid(pri, sub uint16) uint32 { return uint32(sub)<<10 | uint32(pri) }
-- 
2.50.0