test: add test that variables captured by deferred funcs are current on fault

This came up in private mail.
It works today and I want to make sure it stays working.

Change-Id: I13ebdc2dfadb3c72d7f179be89883137320c05d0
Reviewed-on: https://go-review.googlesource.com/7390
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Rob Pike <r@golang.org>

diff --git a/test/recover4.go b/test/recover4.go
new file mode 100644 (file)
index 0000000..115d5a0
--- /dev/null
+++ b/test/recover4.go
@@ -0,0 +1,71 @@
+// +build linux darwin
+// run
+
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that if a slice access causes a fault, a deferred func
+// sees the most recent value of the variables it accesses.
+// This is true today; the role of the test is to ensure it stays true.
+//
+// In the test, memcopy is the function that will fault, during dst[i] = src[i].
+// The deferred func recovers from the error and returns, making memcopy
+// return the current value of n. If n is not being flushed to memory
+// after each modification, the result will be a stale value of n.
+//
+// The test is set up by mmapping a 64 kB block of memory and then
+// unmapping a 16 kB hole in the middle of it. Running memcopy
+// on the resulting slice will fault when it reaches the hole.
+
+package main
+
+import (
+       "log"
+       "runtime/debug"
+       "syscall"
+       "unsafe"
+)
+
+func memcopy(dst, src []byte) (n int, err error) {
+       defer func() {
+               err = recover().(error)
+       }()
+
+       for i := 0; i < len(dst) && i < len(src); i++ {
+               dst[i] = src[i]
+               n++
+       }
+       return
+}
+
+func main() {
+       // Turn the eventual fault into a panic, not a program crash,
+       // so that memcopy can recover.
+       debug.SetPanicOnFault(true)
+
+       // Map 64 kB block of data with 16 kB hole in middle.
+       data, err := syscall.Mmap(-1, 0, 64*1024, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
+       if err != nil {
+               log.Fatalf("mmap: %v", err)
+       }
+
+       // Note: Cannot call syscall.Munmap, because Munmap checks
+       // that you are unmapping a whole region returned by Mmap.
+       // We are trying to unmap just a hole in the middle.
+       if _, _, err := syscall.Syscall(syscall.SYS_MUNMAP, uintptr(unsafe.Pointer(&data[32*1024])), 16*1024, 0); err != 0 {
+               log.Fatalf("munmap: %v", err)
+       }
+
+       other := make([]byte, 64*1024)
+
+       // Check that memcopy returns the actual amount copied
+       // before the fault (32kB - 5, the offset we skip in the argument).
+       n, err := memcopy(data[5:], other)
+       if err == nil {
+               log.Fatal("no error from memcopy across memory hole")
+       }
+       if n != 32*1024-5 {
+               log.Fatal("memcopy returned %d, want %d", n, 32*1024-5)
+       }
+}