[PATCH] player: reduce syscalls when splicing to single target

[Eric Wong <e@80x24.org>]

splice(2) alone does not give enough information as to whether
the source or destination is blocking.  However, as far as audio
playback chain goes, the sink should ALWAYS be the limiting
factor as decoder sources need to be able to produce data at
least as fast as the audio is being played (otherwise there'll
be audible drops).

Thus, we bias the select(2) into waiting on a targets on if we
splice(2) less than the data we requested.
---
 lib/dtas/buffer/fiddle_splice.rb | 3 ++-
 lib/dtas/buffer/splice.rb        | 3 ++-
 test/test_buffer.rb              | 2 +-
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/lib/dtas/buffer/fiddle_splice.rb b/lib/dtas/buffer/fiddle_splice.rb
index ad007eb..d9232cd 100644
--- a/lib/dtas/buffer/fiddle_splice.rb
+++ b/lib/dtas/buffer/fiddle_splice.rb
@@ -84,7 +84,8 @@ def broadcast_one(targets, limit = nil)
       targets # our one and only target blocked on write
     else
       @bytes_xfer += s
-      :wait_readable # we want to read more from @to_io soon
+      # s < limit means targets[0] is full
+      s < limit ? targets : :wait_readable
     end
   rescue Errno::EPIPE, IOError => e
     __dst_error(targets[0], e)
diff --git a/lib/dtas/buffer/splice.rb b/lib/dtas/buffer/splice.rb
index e5d17ab..b9957ce 100644
--- a/lib/dtas/buffer/splice.rb
+++ b/lib/dtas/buffer/splice.rb
@@ -39,7 +39,8 @@ def broadcast_one(targets, limit = nil)
       targets # our one and only target blocked on write
     else
       @bytes_xfer += s
-      :wait_readable # we want to read more from @to_io soon
+      # s < limit means targets[0] is full
+      s < limit ? targets : :wait_readable
     end
   rescue Errno::EPIPE, IOError => e
     __dst_error(targets[0], e)
diff --git a/test/test_buffer.rb b/test/test_buffer.rb
index 54ee584..b89a090 100644
--- a/test/test_buffer.rb
+++ b/test/test_buffer.rb
@@ -62,7 +62,7 @@ def test_broadcast_1
     buf = new_buffer
     r, w = IO.pipe
     buf.wr.write "HIHI"
-    assert_equal :wait_readable, buf.broadcast([w])
+    assert_equal [w], buf.broadcast([w])
     assert_equal 4, buf.bytes_xfer
     tmp = [w]
     r.close