Edit README for clarity (#27)

Recent changes (e.g., #17) introduced new concepts necessary for the
specification. This change migrates this new text to the WASI proposal
template style by:
- linking each section in the document index
- using paragraph style more extensively
- clarifying and compacting several sentences

Author: abrown

README.md

@@ -24,6 +24,8 @@ _TODO before entering Phase 2._
   - [Use case: support various languages](#use-case-support-various-languages)
   - [Use case: support thread-local storage](#use-case-support-thread-local-storage)
 - [Detailed design discussion](#detailed-design-discussion)
+  - [Design choice: thread IDs](#design-choice-thread-ids)
+  - [Design choice: termination](#design-choice-termination)
   - [Design choice: pthreads](#design-choice-pthreads)
   - [Design choice: instance-per-thread](#design-choice-instance-per-thread)
 - [Considered alternatives](#considered-alternatives)
@@ -90,9 +92,9 @@ The API consists of a single function. In pseudo-code:
 status wasi_thread_spawn(thread_start_arg* start_arg);
 ```
 
-where the `status` is a unique non-negative integer thread ID of the new
-thread or negative number representing an error in case the thread failed to
-start.
+where the `status` is a unique non-negative integer thread ID (TID) of the new
+thread (see [Design choice: thread IDs](#design-choice-thread-ids)) or a
+negative number representing an error if the host failed to spawn the thread.
 The host implementing `wasi_thread_spawn` will call a predetermined function
 export (`wasi_thread_start`) in a new WebAssembly instance. Any necessary
 locking/signaling/thread-local storage will be implemented using existing
@@ -127,13 +129,10 @@ TLS bookkeeping (this is not much different than how C starts threads natively).
 
 ### Detailed design discussion
 
-Threads are tricky to implement. This proposal relies on a specific WebAssembly
-convention in order to work correctly.
-
-When instantiating a module which is expected to run with wasi-threads,
-the WASI host must:
-
-1. create and provide shared memories to satisfy the module's imports.
+Threads are tricky to implement. This proposal relies on a specific convention
+in order to work correctly. When instantiating a module which is expected to run
+with `wasi-threads`, the WASI host must first allocate shared memories to
+satisfy the module's imports.
 
 Upon a call to `wasi_thread_spawn`, the WASI host must:
 
@@ -152,69 +151,32 @@ Upon a call to `wasi_thread_spawn`, the WASI host must:
 A WASI host that implements the above should be able to spawn threads for a
 variety of languages.
 
-### TID (thread ID)
-
-TID is a 32-bit integer to identify threads created with `wasi_thread_spawn`.
-
-* TIDs are managed and provided by host.
-
-* TID 0 is reserved. `wasi_thread_spawn` should not return this value.
-
-  * It's widely assumed in musl/wasi-libc.
-
-* The upper-most 3-bits of TID are always 0.
-  `wasi_thread_spawn` should not return values with these bits set.
-
-  * The most significant bit is the sign bit. As `wasi_thread_spawn` uses
-    signed integer and uses negative values to indicate errors, a TID needs
-    to be positive.
-
-  * The second bit need to be 0 in order to be compatible with the TID-based
-    locking implementation in musl/wasi-libc.
-
-  * The third bit need to be 0 in order to make an extra room for other
-    reserved values in wasi-libc.
-    For example, it can be used to indicate the main thread, which doesn't
-    have a TID in the current version of this proposal.
-
-### Process
-
-* A process is a group of threads.
-
-* The main thread starts with a process which only contains
-  the main thread.
-
-* Threads created by a thread in a process using `wasi_thread_spawn`
-  are added to the process.
-
-* When a thread is terminated, it's removed from the process.
-
-### Voluntary thread termination
-
-A thread can terminate itself voluntarily by returning from
-`wasi_thread_start`.
-
-### Changes to WASI `proc_exit`
-
-With this proposal, the `proc_exit` function takes extra responsibility
-to terminate all threads in the process, not only the calling one.
-
-Any of the threads in the process can call `proc_exit`.
+#### Design choice: thread IDs
 
-### Traps
+When `wasi_thread_spawn` successfully spawns a thread, it returns a thread ID
+(TID) — 32-bit integer with several restrictions. TIDs are managed and
+provided by the WASI host. To avoid leaking information, the host may choose to
+return arbitrary TIDs (as opposed to leaking OS TIDs).
 
-When a trap occurs in any thread, the entire process is terminated.
+Valid TIDs fall in the range $[1, 2^{29})$. Some considerations apply:
+- `0` is reserved for compatibility reasons with existing libraries (e.g.,
+  wasi-libc) and must not be returned by `wasi_thread_spawn`
+- the uppermost three bits of a valid TID must always be `0`. The most
+  significant bit is the sign bit and recall that `wasi_thread_spawn` uses
+  negative values to indicate errors. The remaining bits are reserved for
+  compatibility with existing language implementations.
 
-### Process exit status
+#### Design choice: termination
 
-If one or more threads call WASI `proc_exit` or raise a trap,
-one of them is chosen by the runtime to represent the exit status
-of the process.
-It's non deterministic which one is chosen.
+A `wasi-threads` module initially executes a single thread — the main
+thread. As `wasi_thread_spawn` is called, more threads begin to execute. Threads
+terminate in the following ways:
 
-If all the threads in the process have been terminated without calling
-`proc_exit` or raising a trap, it's treated as if the last thread called
-`proc_exit` with exit code 0.
+- __upon return__ from `wasi_thread_start`, and other threads continue to
+  execute
+- __upon a trap__ in any thread; all threads are immediately terminated
+- __upon a `proc_exit` call__ in any thread; all threads are immediately
+  terminated.
 
 #### Design choice: pthreads