The current implementation does not read() from each Task's pipe until execution
finishes. This causes subtasks to block in write() if the pipe is full, which
only happens if there is a large amount of output (such as in -dump-ast mode).
Fixing output buffering and reenabling the Unix TaskQueue implementation is
tracked by <rdar://problem/15795234>.
Swift SVN r12135
Due to the nature of this class, there are two implementations of TaskQueue:
a Unix-specific implementation which supports both parallel execution and output
buffering, and a default implementation which supports neither of these features.
(The default implementation uses the functions from llvm/Support/Program.h for execution.)
TaskQueue allows clients to provide a TaskBeganCallback and a TaskFinishedCallback,
each of which will be called when a new task begins or when a task finishes execution,
respectively. Clients may add tasks to the TaskQueue in either of these callbacks,
and clients can stop further execution by returning TaskFinishedResponse::StopExecution
from the TaskFinishedCallback.
Swift SVN r12059
