syscall: don't allocate when boxing common Errno values into errors

Fixes #8859

Change-Id: If8098e6edad1f83dc35fa84f4431689e8ee97678
Reviewed-on: https://go-review.googlesource.com/6701Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/mksyscall.pl

@@ -292,7 +292,7 @@ while(<>) {
 		$text .= "\t}\n";
 	} elsif ($do_errno) {
 		$text .= "\tif e1 != 0 {\n";
-		$text .= "\t\terr = e1\n";
+		$text .= "\t\terr = errnoErr(e1)\n";
 		$text .= "\t}\n";
 	}
 	$text .= "\treturn\n";

src/syscall/syscall_unix.go

@@ -116,6 +116,30 @@ func (e Errno) Timeout() bool {
 	return e == EAGAIN || e == EWOULDBLOCK || e == ETIMEDOUT
 }
 
+// Do the interface allocations only once for common
+// Errno values.
+var (
+	errEAGAIN error = EAGAIN
+	errEINVAL error = EINVAL
+	errENOENT error = ENOENT
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e Errno) error {
+	switch e {
+	case 0:
+		return nil
+	case EAGAIN:
+		return errEAGAIN
+	case EINVAL:
+		return errEINVAL
+	case ENOENT:
+		return errENOENT
+	}
+	return e
+}
+
 // A Signal is a number describing a process signal.
 // It implements the os.Signal interface.
 type Signal int