return a Result from StackSource::get_stack with an associated Error

fix #7

benches/swap.rs

@@ -8,7 +8,7 @@ static mut ctx_slot: *mut Context<'static, lwkt::os::Stack> = 0 as *mut Context<
 #[bench]
 fn swap(b: &mut test::Bencher) {
   unsafe {
-    let stack = lwkt::os::StackSource::get_stack(4 << 20);
+    let stack = lwkt::os::StackSource::get_stack(4 << 20).unwrap();
 
     let mut ctx = Context::new(stack, move || {
       let ctx_ptr = ctx_slot;

examples/basic.rs

@@ -7,7 +7,7 @@ static mut ctx_slot: *mut Context<'static, lwkt::os::Stack> = 0 as *mut Context<
 
 fn main() {
   unsafe {
-    let stack = lwkt::os::StackSource::get_stack(4 << 20);
+    let stack = lwkt::os::StackSource::get_stack(4 << 20).unwrap();
 
     let mut ctx = Context::new(stack, move || {
       println!("it's alive!");

src/os.rs

@@ -17,7 +17,9 @@ pub struct StackSource;
 
 impl stack::StackSource for StackSource {
   type Output = Stack;
-  fn get_stack(size: usize) -> Stack {
+  type Error = IoError;
+
+  fn get_stack(size: usize) -> Result<Stack, IoError> {
     Stack::new(size)
   }
 }
@@ -37,7 +39,7 @@ impl stack::Stack for Stack {
 }
 
 impl Stack {
-  fn new(size: usize) -> Stack {
+  fn new(size: usize) -> Result<Stack, IoError> {
     let page_size = sys::page_size();
 
     // round the page size up,
@@ -45,25 +47,20 @@ impl Stack {
     let len = (size + page_size - 1) & !(page_size - 1);
 
     let stack = unsafe {
-      let ptr = match sys::map_stack(size) {
-        None => {
-          panic!("mmap for stack of size {} failed: {}",
-                 len, IoError::last_os_error())
-        }
-        Some(ptr) => ptr
-      };
+      let ptr = try!(match sys::map_stack(size) {
+        None => Err(IoError::last_os_error()),
+        Some(ptr) => Ok(ptr)
+      });
 
       Stack { ptr: ptr as *mut u8, len: len }
     };
 
-    unsafe {
-      if !sys::protect_stack(stack.ptr) {
-        panic!("mprotect for guard page of stack {:p} failed: {}",
-               stack.ptr, IoError::last_os_error());
-      }
-    }
+    try!(unsafe {
+      if sys::protect_stack(stack.ptr) { Ok(()) }
+      else { Err(IoError::last_os_error()) }
+    });
 
-    stack
+    Ok(stack)
   }
 }
 

src/stack.rs

@@ -1,4 +1,5 @@
 //! Traits for stacks.
+use core::prelude::*;
 
 /// A trait for objects that hold ownership of a stack.
 pub trait Stack {
@@ -15,5 +16,6 @@ pub trait Stack {
 /// A trait for objects that provide stacks of arbitrary size.
 pub trait StackSource {
   type Output: Stack;
-  fn get_stack(size: usize) -> Self::Output;
+  type Error = ();
+  fn get_stack(size: usize) -> Result<Self::Output, Self::Error>;
 }