Explicitly count the number of panics

Move the panic handling logic from the `unwind` module to `panicking`
and use a panic counter to distinguish between normal state, panics
and double panics.

src/libstd/panicking.rs

@@ -12,11 +12,15 @@
 use io::prelude::*;
 
 use any::Any;
+use cell::Cell;
 use cell::RefCell;
+use intrinsics;
 use sys::stdio::Stderr;
 use sys_common::backtrace;
 use sys_common::thread_info;
-use sys_common::unwind;
+use sys_common::util;
+
+thread_local! { pub static PANIC_COUNT: Cell<usize> = Cell::new(0) }
 
 thread_local! {
     pub static LOCAL_STDERR: RefCell<Option<Box<Write + Send>>> = {
@@ -61,8 +65,24 @@ fn log_panic(obj: &(Any+Send), file: &'static str, line: u32,
 }
 
 pub fn on_panic(obj: &(Any+Send), file: &'static str, line: u32) {
+    let panics = PANIC_COUNT.with(|s| {
+        let count = s.get() + 1;
+        s.set(count);
+        count
+    });
+
     // If this is a double panic, make sure that we print a backtrace
     // for this panic. Otherwise only print it if logging is enabled.
-    let log_backtrace = unwind::panicking() || backtrace::log_enabled();
+    let log_backtrace = panics >= 2 || backtrace::log_enabled();
     log_panic(obj, file, line, log_backtrace);
+
+    if panics >= 2 {
+        // If a thread panics while it's already unwinding then we
+        // have limited options. Currently our preference is to
+        // just abort. In the future we may consider resuming
+        // unwinding or otherwise exiting the thread cleanly.
+        util::dumb_print(format_args!("thread panicked while panicking. \
+                                       aborting."));
+        unsafe { intrinsics::abort() }
+    }
 }

src/libstd/sys/common/unwind/mod.rs

@@ -64,9 +64,8 @@
 
 use any::Any;
 use boxed;
-use cell::Cell;
 use cmp;
-use panicking;
+use panicking::{self,PANIC_COUNT};
 use fmt;
 use intrinsics;
 use mem;
@@ -92,8 +91,6 @@
 #[path = "gcc.rs"] #[doc(hidden)]
 pub mod imp;
 
-thread_local! { static PANICKING: Cell<bool> = Cell::new(false) }
-
 /// Invoke a closure, capturing the cause of panic if one occurs.
 ///
 /// This function will return `Ok(())` if the closure did not panic, and will
@@ -131,9 +128,9 @@ pub unsafe fn try<F: FnOnce()>(f: F) -> Result<(), Box<Any + Send>> {
     // care of exposing correctly.
     unsafe fn inner_try(f: fn(*mut u8), data: *mut u8)
                         -> Result<(), Box<Any + Send>> {
-        PANICKING.with(|s| {
+        PANIC_COUNT.with(|s| {
             let prev = s.get();
-            s.set(false);
+            s.set(0);
             let ep = intrinsics::try(f, data);
             s.set(prev);
             if ep.is_null() {
@@ -161,7 +158,7 @@ fn rust_try(f: extern fn(*mut u8),
 
 /// Determines whether the current thread is unwinding because of panic.
 pub fn panicking() -> bool {
-    PANICKING.with(|s| s.get())
+    PANIC_COUNT.with(|s| s.get() != 0)
 }
 
 // An uninlined, unmangled function upon which to slap yer breakpoints
@@ -234,17 +231,6 @@ fn begin_unwind_inner(msg: Box<Any + Send>,
     // First, invoke the default panic handler.
     panicking::on_panic(&*msg, file, line);
 
-    if panicking() {
-        // If a thread panics while it's already unwinding then we
-        // have limited options. Currently our preference is to
-        // just abort. In the future we may consider resuming
-        // unwinding or otherwise exiting the thread cleanly.
-        super::util::dumb_print(format_args!("thread panicked while panicking. \
-                                              aborting."));
-        unsafe { intrinsics::abort() }
-    }
-    PANICKING.with(|s| s.set(true));
-
     // Finally, perform the unwinding.
     rust_panic(msg);
 }