Auto merge of #36341 - sagebind:thread_id, r=alexcrichton

Add ThreadId for comparing threads

This adds the capability to store and compare threads with the current calling thread via a new struct, `std:🧵:ThreadId`. Addresses the need outlined in issue #21507.

This avoids the need to add any special checks to the existing thread structs and does not rely on the system to provide an identifier for a thread, since it seems that this approach is unreliable and undesirable. Instead, this simply uses a lazily-created, thread-local `usize` whose value is copied from a global atomic counter. The code should be simple enough that it should be as much reliable as the `#[thread_local]` attribute it uses (however much that is).

`ThreadId`s can be compared directly for equality and have copy semantics.

Also see these other attempts:
- rust-lang/rust#29457
- rust-lang/rust#29448
- rust-lang/rust#29447

And this in the RFC repo: rust-lang/rfcs#1435

src/libstd/thread/mod.rs

@@ -166,6 +166,7 @@ use panicking;
 use str;
 use sync::{Mutex, Condvar, Arc};
 use sys::thread as imp;
+use sys_common::mutex;
 use sys_common::thread_info;
 use sys_common::util;
 use sys_common::{AsInner, IntoInner};
@@ -524,6 +525,45 @@ pub fn park_timeout(dur: Duration) {
     *guard = false;
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// ThreadId
+////////////////////////////////////////////////////////////////////////////////
+
+/// A unique identifier for a running thread.
+///
+/// A `ThreadId` is an opaque object that has a unique value for each thread
+/// that creates one. `ThreadId`s do not correspond to a thread's system-
+/// designated identifier.
+#[unstable(feature = "thread_id", issue = "21507")]
+#[derive(Eq, PartialEq, Copy, Clone)]
+pub struct ThreadId(u64);
+
+impl ThreadId {
+    // Generate a new unique thread ID.
+    fn new() -> ThreadId {
+        static GUARD: mutex::Mutex = mutex::Mutex::new();
+        static mut COUNTER: u64 = 0;
+
+        unsafe {
+            GUARD.lock();
+
+            // If we somehow use up all our bits, panic so that we're not
+            // covering up subtle bugs of IDs being reused.
+            if COUNTER == ::u64::MAX {
+                GUARD.unlock();
+                panic!("failed to generate unique thread ID: bitspace exhausted");
+            }
+
+            let id = COUNTER;
+            COUNTER += 1;
+
+            GUARD.unlock();
+
+            ThreadId(id)
+        }
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Thread
 ////////////////////////////////////////////////////////////////////////////////
@@ -531,6 +571,7 @@ pub fn park_timeout(dur: Duration) {
 /// The internal representation of a `Thread` handle
 struct Inner {
     name: Option<CString>,      // Guaranteed to be UTF-8
+    id: ThreadId,
     lock: Mutex<bool>,          // true when there is a buffered unpark
     cvar: Condvar,
 }
@@ -551,6 +592,7 @@ impl Thread {
         Thread {
             inner: Arc::new(Inner {
                 name: cname,
+                id: ThreadId::new(),
                 lock: Mutex::new(false),
                 cvar: Condvar::new(),
             })
@@ -569,6 +611,12 @@ impl Thread {
         }
     }
 
+    /// Gets the thread's unique identifier.
+    #[unstable(feature = "thread_id", issue = "21507")]
+    pub fn id(&self) -> ThreadId {
+        self.inner.id
+    }
+
     /// Gets the thread's name.
     ///
     /// # Examples
@@ -977,6 +1025,17 @@ mod tests {
         thread::sleep(Duration::from_millis(2));
     }
 
+    #[test]
+    fn test_thread_id_equal() {
+        assert!(thread::current().id() == thread::current().id());
+    }
+
+    #[test]
+    fn test_thread_id_not_equal() {
+        let spawned_id = thread::spawn(|| thread::current().id()).join().unwrap();
+        assert!(thread::current().id() != spawned_id);
+    }
+
     // NOTE: the corresponding test for stderr is in run-pass/thread-stderr, due
     // to the test harness apparently interfering with stderr configuration.
 }