Support arbitrary stdout/stderr/logger handles

This will allow capturing of common things like logging messages, stdout prints
(using stdio println), and failure messages (printed to stderr).  Any new prints
added to libstd should be funneled through these task handles to allow capture
as well.

Additionally, this commit redirects logging back through a `Logger` trait so the
log level can be usefully consumed by an arbitrary logger.

This commit also introduces methods to set the task-local stdout handles:

* std::io::stdio::set_stdout
* std::io::stdio::set_stderr
* std::io::logging::set_logger

These methods all return the previous logger just in case it needs to be used
for inspection.

I plan on using this infrastructure for extra::test soon, but we don't quite
have the primitives that I'd like to use for it, so it doesn't migrate
extra::test at this time.

Closes #6369

src/libstd/io/stdio.rs

@@ -32,8 +32,12 @@ use io::{Reader, Writer, io_error, IoError, OtherIoError,
          standard_error, EndOfFile};
 use libc;
 use option::{Option, Some, None};
+use prelude::drop;
 use result::{Ok, Err};
+use rt::local::Local;
 use rt::rtio::{DontClose, IoFactory, LocalIo, RtioFileStream, RtioTTY};
+use rt::task::Task;
+use util;
 
 // And so begins the tale of acquiring a uv handle to a stdio stream on all
 // platforms in all situations. Our story begins by splitting the world into two
@@ -101,6 +105,44 @@ pub fn stderr() -> StdWriter {
     src(libc::STDERR_FILENO, false, |src| StdWriter { inner: src })
 }
 
+fn reset_helper(w: ~Writer,
+                f: |&mut Task, ~Writer| -> Option<~Writer>) -> Option<~Writer> {
+    let mut t = Local::borrow(None::<Task>);
+    // Be sure to flush any pending output from the writer
+    match f(t.get(), w) {
+        Some(mut w) => {
+            drop(t);
+            w.flush();
+            Some(w)
+        }
+        None => None
+    }
+}
+
+/// Resets the task-local stdout handle to the specified writer
+///
+/// This will replace the current task's stdout handle, returning the old
+/// handle. All future calls to `print` and friends will emit their output to
+/// this specified handle.
+///
+/// Note that this does not need to be called for all new tasks; the default
+/// output handle is to the process's stdout stream.
+pub fn set_stdout(stdout: ~Writer) -> Option<~Writer> {
+    reset_helper(stdout, |t, w| util::replace(&mut t.stdout, Some(w)))
+}
+
+/// Resets the task-local stderr handle to the specified writer
+///
+/// This will replace the current task's stderr handle, returning the old
+/// handle. Currently, the stderr handle is used for printing failure messages
+/// during task failure.
+///
+/// Note that this does not need to be called for all new tasks; the default
+/// output handle is to the process's stderr stream.
+pub fn set_stderr(stderr: ~Writer) -> Option<~Writer> {
+    reset_helper(stderr, |t, w| util::replace(&mut t.stderr, Some(w)))
+}
+
 // Helper to access the local task's stdout handle
 //
 // Note that this is not a safe function to expose because you can create an
@@ -112,38 +154,49 @@ pub fn stderr() -> StdWriter {
 //      })
 //  })
 fn with_task_stdout(f: |&mut Writer|) {
-    use rt::local::Local;
-    use rt::task::Task;
+    let task: Option<~Task> = Local::try_take();
+    match task {
+        Some(mut task) => {
+            // Printing may run arbitrary code, so ensure that the task is in
+            // TLS to allow all std services. Note that this means a print while
+            // printing won't use the task's normal stdout handle, but this is
+            // necessary to ensure safety (no aliasing).
+            let mut my_stdout = task.stdout.take();
+            Local::put(task);
 
-    unsafe {
-        let task: Option<*mut Task> = Local::try_unsafe_borrow();
-        match task {
-            Some(task) => {
-                match (*task).stdout_handle {
-                    Some(ref mut handle) => f(*handle),
-                    None => {
-                        let handle = ~LineBufferedWriter::new(stdout());
-                        let mut handle = handle as ~Writer;
-                        f(handle);
-                        (*task).stdout_handle = Some(handle);
+            if my_stdout.is_none() {
+                my_stdout = Some(~LineBufferedWriter::new(stdout()) as ~Writer);
+            }
+            f(*my_stdout.get_mut_ref());
+
+            // Note that we need to be careful when putting the stdout handle
+            // back into the task. If the handle was set to `Some` while
+            // printing, then we can run aribitrary code when destroying the
+            // previous handle. This means that the local task needs to be in
+            // TLS while we do this.
+            //
+            // To protect against this, we do a little dance in which we
+            // temporarily take the task, swap the handles, put the task in TLS,
+            // and only then drop the previous handle.
+            let mut t = Local::borrow(None::<Task>);
+            let prev = util::replace(&mut t.get().stdout, my_stdout);
+            drop(t);
+            drop(prev);
+        }
+
+        None => {
+            struct Stdout;
+            impl Writer for Stdout {
+                fn write(&mut self, data: &[u8]) {
+                    unsafe {
+                        libc::write(libc::STDOUT_FILENO,
+                                    data.as_ptr() as *libc::c_void,
+                                    data.len() as libc::size_t);
                     }
                 }
             }
-
-            None => {
-                struct Stdout;
-                impl Writer for Stdout {
-                    fn write(&mut self, data: &[u8]) {
-                        unsafe {
-                            libc::write(libc::STDOUT_FILENO,
-                                        data.as_ptr() as *libc::c_void,
-                                        data.len() as libc::size_t);
-                        }
-                    }
-                }
-                let mut io = Stdout;
-                f(&mut io as &mut Writer);
-            }
+            let mut io = Stdout;
+            f(&mut io as &mut Writer);
         }
     }
 }
@@ -313,4 +366,29 @@ mod tests {
         stdout();
         stderr();
     })
+
+    iotest!(fn capture_stdout() {
+        use io::comm_adapters::{PortReader, ChanWriter};
+
+        let (p, c) = Chan::new();
+        let (mut r, w) = (PortReader::new(p), ChanWriter::new(c));
+        do spawn {
+            set_stdout(~w as ~Writer);
+            println!("hello!");
+        }
+        assert_eq!(r.read_to_str(), ~"hello!\n");
+    })
+
+    iotest!(fn capture_stderr() {
+        use io::comm_adapters::{PortReader, ChanWriter};
+
+        let (p, c) = Chan::new();
+        let (mut r, w) = (PortReader::new(p), ChanWriter::new(c));
+        do spawn {
+            set_stderr(~w as ~Writer);
+            fail!("my special message");
+        }
+        let s = r.read_to_str();
+        assert!(s.contains("my special message"));
+    })
 }