chore: add docs for the global re-entrant lock (#609)

Author: Gankra

src/lib.rs

@@ -145,32 +145,95 @@ mod lock {
     use std::ptr;
     use std::sync::{Mutex, MutexGuard, Once};
 
+    /// A "Maybe" LockGuard
     pub struct LockGuard(Option<MutexGuard<'static, ()>>);
 
+    /// The global lock, lazily allocated on first use
     static mut LOCK: *mut Mutex<()> = ptr::null_mut();
     static INIT: Once = Once::new();
+    // Whether this thread is the one that holds the lock
     thread_local!(static LOCK_HELD: Cell<bool> = Cell::new(false));
 
     impl Drop for LockGuard {
         fn drop(&mut self) {
+            // Don't do anything if we're a LockGuard(None)
             if self.0.is_some() {
                 LOCK_HELD.with(|slot| {
+                    // Immediately crash if we somehow aren't the thread holding this lock
                     assert!(slot.get());
+                    // We are no longer the thread holding this lock
                     slot.set(false);
                 });
             }
+            // lock implicitly released here, if we're a LockGuard(Some(..))
         }
     }
 
+    /// Acquire a partially unsound(!!!) global re-entrant lock over
+    /// backtrace's internals.
+    ///
+    /// That is, this lock can be acquired as many times as you want
+    /// on a single thread without deadlocking, allowing one thread
+    /// to acquire exclusive access to the ability to make backtraces.
+    /// Calls to this locking function are freely sprinkled in every place
+    /// where that needs to be enforced.
+    ///
+    ///
+    /// # Why
+    ///
+    /// This was first introduced to guard uses of Windows' dbghelp API,
+    /// which isn't threadsafe. It's unclear if other things now rely on
+    /// this locking.
+    ///
+    ///
+    /// # How
+    ///
+    /// The basic idea is to have a single global mutex, and a thread_local
+    /// boolean saying "yep this is the thread that acquired the mutex".
+    ///
+    /// The first time a thread acquires the lock, it is handed a
+    /// `LockGuard(Some(..))` that will actually release the lock on Drop.
+    /// All subsequence attempts to lock on the same thread will see
+    /// that their thread acquired the lock, and get `LockGuard(None)`
+    /// which will do nothing when dropped.
+    ///
+    ///
+    /// # Safety
+    ///
+    /// As long as you only ever assign the returned LockGuard to a freshly
+    /// declared local variable, it will do its job correctly, as the "first"
+    /// LockGuard will strictly outlive all subsequent LockGuards and
+    /// properly release the lock when the thread is done with backtracing.
+    ///
+    /// However if you ever attempt to store a LockGuard beyond the scope
+    /// it was acquired in, it might actually be a `LockGuard(None)` that
+    /// doesn't actually hold the lock! In this case another thread might
+    /// acquire the lock and you'll get races this system was intended to
+    /// avoid!
+    ///
+    /// This is why this is "partially unsound". As a public API this would
+    /// be unacceptable, but this is crate-private, and if you use this in
+    /// the most obvious and simplistic way it Just Works™.
+    ///
+    /// Note however that std specifically bypasses this lock, and uses
+    /// the `*_unsynchronized` backtrace APIs. This is "fine" because
+    /// it wraps its own calls to backtrace in a non-reentrant Mutex
+    /// that prevents two backtraces from getting interleaved during printing.
     pub fn lock() -> LockGuard {
+        // If we're the thread holding this lock, pretend to acquire the lock
+        // again by returning a LockGuard(None)
         if LOCK_HELD.with(|l| l.get()) {
             return LockGuard(None);
         }
+        // Insist that we totally are the thread holding the lock
+        // (our thread will block until we are)
         LOCK_HELD.with(|s| s.set(true));
         unsafe {
+            // lazily allocate the lock if necessary
             INIT.call_once(|| {
                 LOCK = Box::into_raw(Box::new(Mutex::new(())));
             });
+            // ok *actually* try to acquire the lock, blocking as necessary
             LockGuard(Some((*LOCK).lock().unwrap()))
         }
     }