std: Tweak stack overflow printing for robustness

The printing of the error message on stack overflow had two sometimes false
assumptions previously. The first is that a local task was always available (it
called Local::take) and the second is that it used println! instead of
manually writing.

The first assumption isn't necessarily true because while stack overflow will
likely only be detected in situations that a local task is available, it's not
guaranteed to always be in TLS. For example, during a println! call a task
may be blocking, causing it to be unavailable. By using Local::try_take(), we
can be resilient against these occurrences.

The second assumption could lead to odd behavior because the stdout logger can
be overwritten to run arbitrary code. Currently this should be possible, but the
utility is much diminished because a stack overflow translates to an abort()
instead of a failure.

1 files changed, 10 insertions, 5 deletions

diff --git a/src/libstd/rt/stack.rs b/src/libstd/rt/stack.rs
index 655c209fec8..963ff000c4a 100644
--- a/src/libstd/rt/stack.rs
+++ b/src/libstd/rt/stack.rs
@@ -36,7 +36,7 @@ pub static RED_ZONE: uint = 20 * 1024;
                   //   irrelevant for documentation purposes.
 #[cfg(not(test))] // in testing, use the original libstd's version
 pub extern "C" fn rust_stack_exhausted() {
-    use option::None;
+    use option::{Option, None, Some};
     use rt::local::Local;
     use rt::task::Task;
     use str::Str;
@@ -85,16 +85,21 @@ pub extern "C" fn rust_stack_exhausted() {
         //  #9854 - unwinding on windows through __morestack has never worked
         //  #2361 - possible implementation of not using landing pads
 
-        let mut task = Local::borrow(None::<Task>);
-        let n = task.get().name.as_ref()
-                    .map(|n| n.as_slice()).unwrap_or("<unnamed>");
+        let task: Option<~Task> = Local::try_take();
+        let name = match task {
+            Some(ref task) => {
+                task.name.as_ref().map(|n| n.as_slice())
+            }
+            None => None
+        };
+        let name = name.unwrap_or("<unknown>");
 
         // See the message below for why this is not emitted to the
         // task's logger. This has the additional conundrum of the
         // logger may not be initialized just yet, meaning that an FFI
         // call would happen to initialized it (calling out to libuv),
         // and the FFI call needs 2MB of stack when we just ran out.
-        println!("task '{}' has overflowed its stack", n);
+        rterrln!("task '{}' has overflowed its stack", name);
 
         intrinsics::abort();
     }