core::rt: Add more I/O docs

Author: brson

src/libcore/rt/io/mod.rs

@@ -10,12 +10,14 @@
 
 /*! Synchronous I/O
 
-This module defines the Rust interface for synchronous I/O.  It is
-build around Reader and Writer traits that define byte stream sources
-and sinks.  Implementations are provided for common I/O streams like
-file, TCP, UDP, Unix domain sockets, multiple types of memory bufers.
-Readers and Writers may be composed to add things like string parsing,
-and compression.
+This module defines the Rust interface for synchronous I/O.
+It models byte-oriented input and output with the Reader and Writer traits.
+Types that implement both `Reader` and `Writer` and called 'streams',
+and automatically implement trait `Stream`.
+Implementations are provided for common I/O streams like
+file, TCP, UDP, Unix domain sockets.
+Readers and Writers may be composed to add capabilities like string
+parsing, encoding, and compression.
 
 This will likely live in core::io, not core::rt::io.
 
@@ -31,22 +33,22 @@ Some examples of obvious things you might want to do
 
 * Read a complete file to a string, (converting newlines?)
 
-    let contents = FileStream::open("message.txt").read_to_str(); // read_to_str??
+    let contents = File::open("message.txt").read_to_str(); // read_to_str??
 
 * Write a line to a file
 
-    let file = FileStream::open("message.txt", Create, Write);
+    let file = File::open("message.txt", Create, Write);
     file.write_line("hello, file!");
 
 * Iterate over the lines of a file
 
-    do FileStream::open("message.txt").each_line |line| {
+    do File::open("message.txt").each_line |line| {
         println(line)
     }
 
 * Pull the lines of a file into a vector of strings
 
-    let lines = FileStream::open("message.txt").line_iter().to_vec();
+    let lines = File::open("message.txt").line_iter().to_vec();
 
 * Make an simple HTTP request
 
@@ -63,25 +65,145 @@ Some examples of obvious things you might want to do
 
 # Terms
 
-* reader
-* writer
-* stream
-* Blocking vs. non-blocking
-* synchrony and asynchrony
-
-I tend to call this implementation non-blocking, because performing I/O
-doesn't block the progress of other tasks. Is that how we want to present
-it, 'synchronous but non-blocking'?
+* Reader - An I/O source, reads bytes into a buffer
+* Writer - An I/O sink, writes bytes from a buffer
+* Stream - Typical I/O sources like files and sockets are both Readers and Writers,
+  and are collectively referred to a `streams`.
+* Decorator - A Reader or Writer that composes with others to add additional capabilities
+  such as encoding or decoding
+
+# Blocking and synchrony
+
+When discussing I/O you often hear the terms 'synchronous' and
+'asynchronous', along with 'blocking' and 'non-blocking' compared and
+contrasted. A synchronous I/O interface performs each I/O operation to
+completion before proceeding to the next. Synchronous interfaces are
+usually used in imperative style as a sequence of commands. An
+asynchronous interface allows multiple I/O requests to be issued
+simultaneously, without waiting for each to complete before proceeding
+to the next.
+
+Asynchronous interfaces are used to achieve 'non-blocking' I/O. In
+traditional single-threaded systems, performing a synchronous I/O
+operation means that the program stops all activity (it 'blocks')
+until the I/O is complete. Blocking is bad for performance when
+there are other computations that could be done.
+
+Asynchronous interfaces are most often associated with the callback
+(continuation-passing) style popularised by node.js. Such systems rely
+on all computations being run inside an event loop which maintains a
+list of all pending I/O events; when one completes the registered
+callback is run and the code that made the I/O request continiues.
+Such interfaces achieve non-blocking at the expense of being more
+difficult to reason about.
+
+Rust's I/O interface is synchronous - easy to read - and non-blocking by default.
+
+Remember that Rust tasks are 'green threads', lightweight threads that
+are multiplexed onto a single operating system thread. If that system
+thread blocks then no other task may proceed. Rust tasks are
+relatively cheap to create, so as long as other tasks are free to
+execute then non-blocking code may be written by simply creating a new
+task.
+
+When discussing blocking in regards to Rust's I/O model, we are
+concerned with whether performing I/O blocks other Rust tasks from
+proceeding. In other words, when a task calls `read`, it must then
+wait (or 'sleep', or 'block') until the call to `read` is complete.
+During this time, other tasks may or may not be executed, depending on
+how `read` is implemented.
+
+
+Rust's default I/O implementation is non-blocking; by cooperating
+directly with the task scheduler it arranges to never block progress
+of *other* tasks. Under the hood, Rust uses asynchronous I/O via a
+per-scheduler (and hence per-thread) event loop. Synchronous I/O
+requests are implemented by descheduling the running task and
+performing an asynchronous request; the task is only resumed once the
+asynchronous request completes.
+
+For blocking (but possibly more efficient) implementations, look
+in the `io::native` module.
 
 # Error Handling
 
+I/O is an area where nearly every operation can result in unexpected
+errors. It should allow errors to be handled efficiently.
+It needs to be convenient to use I/O when you don't care
+about dealing with specific errors.
+
+Rust's I/O employs a combination of techniques to reduce boilerplate
+while still providing feedback about errors. The basic strategy:
+
+* Errors are fatal by default, resulting in task failure
+* Errors raise the `io_error` conditon which provides an opportunity to inspect
+  an IoError object containing details.
+* Return values must have a sensible null or zero value which is returned
+  if a condition is handled successfully. This may be an `Option`, an empty
+  vector, or other designated error value.
+* Common traits are implemented for `Option`, e.g. `impl<R: Reader> Reader for Option<R>`,
+  so that nullable values do not have to be 'unwrapped' before use.
+
+These features combine in the API to allow for expressions like
+`File::new("diary.txt").write_line("met a girl")` without having to
+worry about whether "diary.txt" exists or whether the write
+succeeds. As written, if either `new` or `write_line` encounters
+an error the task will fail.
+
+If you wanted to handle the error though you might write
+
+    let mut error = None;
+    do io_error::cond(|e: IoError| {
+        error = Some(e);
+    }).in {
+        File::new("diary.txt").write_line("met a girl");
+    }
+
+    if error.is_some() {
+        println("failed to write my diary");
+    }
+
+XXX: Need better condition handling syntax
+
+In this case the condition handler will have the opportunity to
+inspect the IoError raised by either the call to `new` or the call to
+`write_line`, but then execution will continue.
+
+So what actually happens if `new` encounters an error? To understand
+that it's important to know that what `new` returns is not a `File`
+but an `Option<File>`.  If the file does not open, and the condition
+is handled, then `new` will simply return `None`. Because there is an
+implementation of `Writer` (the trait required ultimately required for
+types to implement `write_line`) there is no need to inspect or unwrap
+the `Option<File>` and we simply call `write_line` on it.  If `new`
+returned a `None` then the followup call to `write_line` will also
+raise an error.
+
+## Concerns about this strategy
+
+This structure will encourage a programming style that is prone
+to errors similar to null pointer dereferences.
+In particular code written to ignore errors and expect conditions to be unhandled
+will start passing around null or zero objects when wrapped in a condition handler.
+
+* XXX: How should we use condition handlers that return values?
+
+
+# Issues withi/o scheduler affinity, work stealing, task pinning
+
 # Resource management
 
 * `close` vs. RAII
 
-# Paths and URLs
+# Paths, URLs and overloaded constructors
+
+
 
-# std
+# Scope
+
+In scope for core
+
+* Url?
 
 Some I/O things don't belong in core
 
@@ -90,7 +212,12 @@ Some I/O things don't belong in core
     - http
   - flate
 
-# XXX
+Out of scope
+
+* Async I/O. We'll probably want it eventually
+
+
+# XXX Questions and issues
 
 * Should default constructors take `Path` or `&str`? `Path` makes simple cases verbose.
   Overloading would be nice.
@@ -100,6 +227,7 @@ Some I/O things don't belong in core
 * fsync
 * relationship with filesystem querying, Directory, File types etc.
 * Rename Reader/Writer to ByteReader/Writer, make Reader/Writer generic?
+* Can Port and Chan be implementations of a generic Reader<T>/Writer<T>?
 * Trait for things that are both readers and writers, Stream?
 * How to handle newline conversion
 * String conversion
@@ -109,6 +237,7 @@ Some I/O things don't belong in core
 * Do we need `close` at all? dtors might be good enough
 * How does I/O relate to the Iterator trait?
 * std::base64 filters
+* Using conditions is a big unknown since we don't have much experience with them
 
 */
 

src/libcore/rt/mod.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+/*! The Rust runtime, including the scheduler and I/O interface */
+
 #[doc(hidden)];
 
 use libc::c_char;