auto merge of #19094 : alexcrichton/rust/rm-std-local-data, r=aturon

This commit removes the `std::local_data` module in favor of a new `std::thread_local`
module providing thread local storage. The module provides two variants of TLS:
one which owns its contents and one which is based on scoped references. Each
implementation has pros and cons listed in the documentation.

Both flavors have accessors through a function called `with` which yield a
reference to a closure provided. Both flavors also panic if a reference cannot
be yielded and provide a function to test whether an access would panic or not.
This is an implementation of [RFC 461][rfc] and full details can be found in
that RFC.

This is a breaking change due to the removal of the `std::local_data` module.
All users can migrate to the new tls system like so:

    thread_local!(static FOO: Rc<RefCell<Option<T>>> = Rc::new(RefCell::new(None)))

The old `local_data` module inherently contained the `Rc<RefCell<Option<T>>>` as
an implementation detail which must now be explicitly stated by users.

[rfc]: https://github.com/rust-lang/rfcs/pull/461
[breaking-change]

2 files changed, 242 insertions, 3 deletions

diff --git a/src/libstd/sys/windows/mod.rs b/src/libstd/sys/windows/mod.rs
index 815ace21f87..a785ccfe804 100644
--- a/src/libstd/sys/windows/mod.rs
+++ b/src/libstd/sys/windows/mod.rs
@@ -35,14 +35,15 @@ macro_rules! helper_init( (static $name:ident: Helper<$m:ty>) => (
 
 pub mod c;
 pub mod fs;
+pub mod helper_signal;
 pub mod os;
-pub mod tcp;
-pub mod udp;
 pub mod pipe;
-pub mod helper_signal;
 pub mod process;
+pub mod tcp;
+pub mod thread_local;
 pub mod timer;
 pub mod tty;
+pub mod udp;
 
 pub mod addrinfo {
     pub use sys_common::net::get_host_addresses;
diff --git a/src/libstd/sys/windows/thread_local.rs b/src/libstd/sys/windows/thread_local.rs
new file mode 100644
index 00000000000..b841f6d3a2b
--- /dev/null
+++ b/src/libstd/sys/windows/thread_local.rs
@@ -0,0 +1,238 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use prelude::*;
+
+use libc::types::os::arch::extra::{DWORD, LPVOID, BOOL};
+
+use mem;
+use rustrt;
+use rustrt::exclusive::Exclusive;
+use sync::{ONCE_INIT, Once};
+
+pub type Key = DWORD;
+pub type Dtor = unsafe extern fn(*mut u8);
+
+// Turns out, like pretty much everything, Windows is pretty close the
+// functionality that Unix provides, but slightly different! In the case of
+// TLS, Windows does not provide an API to provide a destructor for a TLS
+// variable. This ends up being pretty crucial to this implementation, so we
+// need a way around this.
+//
+// The solution here ended up being a little obscure, but fear not, the
+// internet has informed me [1][2] that this solution is not unique (no way
+// I could have thought of it as well!). The key idea is to insert some hook
+// somewhere to run arbitrary code on thread termination. With this in place
+// we'll be able to run anything we like, including all TLS destructors!
+//
+// To accomplish this feat, we perform a number of tasks, all contained
+// within this module:
+//
+// * All TLS destructors are tracked by *us*, not the windows runtime. This
+//   means that we have a global list of destructors for each TLS key that
+//   we know about.
+// * When a TLS key is destroyed, we're sure to remove it from the dtor list
+//   if it's in there.
+// * When a thread exits, we run over the entire list and run dtors for all
+//   non-null keys. This attempts to match Unix semantics in this regard.
+//
+// This ends up having the overhead of using a global list, having some
+// locks here and there, and in general just adding some more code bloat. We
+// attempt to optimize runtime by forgetting keys that don't have
+// destructors, but this only gets us so far.
+//
+// For more details and nitty-gritty, see the code sections below!
+//
+// [1]: http://www.codeproject.com/Articles/8113/Thread-Local-Storage-The-C-Way
+// [2]: https://github.com/ChromiumWebApps/chromium/blob/master/base
+//                        /threading/thread_local_storage_win.cc#L42
+
+static INIT_DTORS: Once = ONCE_INIT;
+static mut DTORS: *mut Exclusive<Vec<(Key, Dtor)>> = 0 as *mut _;
+
+// -------------------------------------------------------------------------
+// Native bindings
+//
+// This section is just raw bindings to the native functions that Windows
+// provides, There's a few extra calls to deal with destructors.
+
+#[inline]
+pub unsafe fn create(dtor: Option<Dtor>) -> Key {
+    const TLS_OUT_OF_INDEXES: DWORD = 0xFFFFFFFF;
+    let key = TlsAlloc();
+    assert!(key != TLS_OUT_OF_INDEXES);
+    match dtor {
+        Some(f) => register_dtor(key, f),
+        None => {}
+    }
+    return key;
+}
+
+#[inline]
+pub unsafe fn set(key: Key, value: *mut u8) {
+    let r = TlsSetValue(key, value as LPVOID);
+    debug_assert!(r != 0);
+}
+
+#[inline]
+pub unsafe fn get(key: Key) -> *mut u8 {
+    TlsGetValue(key) as *mut u8
+}
+
+#[inline]
+pub unsafe fn destroy(key: Key) {
+    if unregister_dtor(key) {
+        // FIXME: Currently if a key has a destructor associated with it we
+        // can't actually ever unregister it. If we were to
+        // unregister it, then any key destruction would have to be
+        // serialized with respect to actually running destructors.
+        //
+        // We want to avoid a race where right before run_dtors runs
+        // some destructors TlsFree is called. Allowing the call to
+        // TlsFree would imply that the caller understands that *all
+        // known threads* are not exiting, which is quite a difficult
+        // thing to know!
+        //
+        // For now we just leak all keys with dtors to "fix" this.
+        // Note that source [2] above shows precedent for this sort
+        // of strategy.
+    } else {
+        let r = TlsFree(key);
+        debug_assert!(r != 0);
+    }
+}
+
+extern "system" {
+    fn TlsAlloc() -> DWORD;
+    fn TlsFree(dwTlsIndex: DWORD) -> BOOL;
+    fn TlsGetValue(dwTlsIndex: DWORD) -> LPVOID;
+    fn TlsSetValue(dwTlsIndex: DWORD, lpTlsvalue: LPVOID) -> BOOL;
+}
+
+// -------------------------------------------------------------------------
+// Dtor registration
+//
+// These functions are associated with registering and unregistering
+// destructors. They're pretty simple, they just push onto a vector and scan
+// a vector currently.
+//
+// FIXME: This could probably be at least a little faster with a BTree.
+
+fn init_dtors() {
+    let dtors = box Exclusive::new(Vec::<(Key, Dtor)>::new());
+    unsafe {
+        DTORS = mem::transmute(dtors);
+    }
+
+    rustrt::at_exit(proc() unsafe {
+        mem::transmute::<_, Box<Exclusive<Vec<(Key, Dtor)>>>>(DTORS);
+        DTORS = 0 as *mut _;
+    });
+}
+
+unsafe fn register_dtor(key: Key, dtor: Dtor) {
+    INIT_DTORS.doit(init_dtors);
+    let mut dtors = (*DTORS).lock();
+    dtors.push((key, dtor));
+}
+
+unsafe fn unregister_dtor(key: Key) -> bool {
+    if DTORS.is_null() { return false }
+    let mut dtors = (*DTORS).lock();
+    let before = dtors.len();
+    dtors.retain(|&(k, _)| k != key);
+    dtors.len() != before
+}
+
+// -------------------------------------------------------------------------
+// Where the Magic (TM) Happens
+//
+// If you're looking at this code, and wondering "what is this doing?",
+// you're not alone! I'll try to break this down step by step:
+//
+// # What's up with CRT$XLB?
+//
+// For anything about TLS destructors to work on Windows, we have to be able
+// to run *something* when a thread exits. To do so, we place a very special
+// static in a very special location. If this is encoded in just the right
+// way, the kernel's loader is apparently nice enough to run some function
+// of ours whenever a thread exits! How nice of the kernel!
+//
+// Lots of detailed information can be found in source [1] above, but the
+// gist of it is that this is leveraging a feature of Microsoft's PE format
+// (executable format) which is not actually used by any compilers today.
+// This apparently translates to any callbacks in the ".CRT$XLB" section
+// being run on certain events.
+//
+// So after all that, we use the compiler's #[link_section] feature to place
+// a callback pointer into the magic section so it ends up being called.
+//
+// # What's up with this callback?
+//
+// The callback specified receives a number of parameters from... someone!
+// (the kernel? the runtime? I'm not qute sure!) There are a few events that
+// this gets invoked for, but we're currentl only interested on when a
+// thread or a process "detaches" (exits). The process part happens for the
+// last thread and the thread part happens for any normal thread.
+//
+// # Ok, what's up with running all these destructors?
+//
+// This will likely need to be improved over time, but this function
+// attempts a "poor man's" destructor callback system. To do this we clone a
+// local copy of the dtor list to start out with. This is our fudgy attempt
+// to not hold the lock while destructors run and not worry about the list
+// changing while we're looking at it.
+//
+// Once we've got a list of what to run, we iterate over all keys, check
+// their values, and then run destructors if the values turn out to be non
+// null (setting them to null just beforehand). We do this a few times in a
+// loop to basically match Unix semantics. If we don't reach a fixed point
+// after a short while then we just inevitably leak something most likely.
+//
+// # The article mentions crazy stuff about "/INCLUDE"?
+//
+// It sure does! This seems to work for now, so maybe we'll just run into
+// that if we start linking with msvc?
+
+#[link_section = ".CRT$XLB"]
+#[linkage = "external"]
+#[allow(warnings)]
+pub static p_thread_callback: unsafe extern "system" fn(LPVOID, DWORD,
+                                                        LPVOID) =
+        on_tls_callback;
+
+#[allow(warnings)]
+unsafe extern "system" fn on_tls_callback(h: LPVOID,
+                                          dwReason: DWORD,
+                                          pv: LPVOID) {
+    const DLL_THREAD_DETACH: DWORD = 3;
+    const DLL_PROCESS_DETACH: DWORD = 0;
+    if dwReason == DLL_THREAD_DETACH || dwReason == DLL_PROCESS_DETACH {
+        run_dtors();
+    }
+}
+
+unsafe fn run_dtors() {
+    if DTORS.is_null() { return }
+    let mut any_run = true;
+    for _ in range(0, 5i) {
+        if !any_run { break }
+        any_run = false;
+        let dtors = (*DTORS).lock().iter().map(|p| *p).collect::<Vec<_>>();
+        for &(key, dtor) in dtors.iter() {
+            let ptr = TlsGetValue(key);
+            if !ptr.is_null() {
+                TlsSetValue(key, 0 as *mut _);
+                dtor(ptr as *mut _);
+                any_run = true;
+            }
+        }
+    }
+}