IO: remove flush_on_newline and only sync on TTY devices

spec/std/io/buffered_spec.cr

@@ -262,28 +262,6 @@ describe "IO::Buffered" do
     str.to_s.should eq("hello" * 10_000)
   end
 
-  it "flushes on \n" do
-    str = IO::Memory.new
-    io = BufferedWrapper.new(str)
-    io.flush_on_newline = true
-
-    io << "hello\nworld"
-    str.to_s.should eq("hello\n")
-    io.flush
-    str.to_s.should eq("hello\nworld")
-  end
-
-  it "doesn't write past count" do
-    str = IO::Memory.new
-    io = BufferedWrapper.new(str)
-    io.flush_on_newline = true
-
-    slice = Slice.new(10) { |i| i == 9 ? '\n'.ord.to_u8 : ('a'.ord + i).to_u8 }
-    io.write slice[0, 4]
-    io.flush
-    str.to_s.should eq("abcd")
-  end
-
   describe "sync" do
     it "syncs (write)" do
       str = IO::Memory.new

src/http/server/response.cr

@@ -158,7 +158,6 @@ class HTTP::Server
         @in_buffer_rem = Bytes.empty
         @out_count = 0
         @sync = false
-        @flush_on_newline = false
         @chunked = false
         @closed = false
       end

src/io/buffered.cr

@@ -12,7 +12,6 @@ module IO::Buffered
   @out_count = 0
   @sync = false
   @read_buffering = true
-  @flush_on_newline = false
 
   # Reads at most *slice.size* bytes from the wrapped `IO` into *slice*.
   # Returns the number of bytes read.
@@ -123,17 +122,6 @@ module IO::Buffered
       return unbuffered_write(slice)
     end
 
-    if flush_on_newline?
-      index = slice[0, count.to_i32].rindex('\n'.ord.to_u8)
-      if index
-        flush
-        index += 1
-        unbuffered_write slice[0, index]
-        slice += index
-        count -= index
-      end
-    end
-
     if count >= BUFFER_SIZE
       flush
       return unbuffered_write slice[0, count]
@@ -161,20 +149,6 @@ module IO::Buffered
     end
     out_buffer[@out_count] = byte
     @out_count += 1
-
-    if flush_on_newline? && byte === '\n'
-      flush
-    end
-  end
-
-  # Turns on/off flushing the underlying `IO` when a newline is written.
-  def flush_on_newline=(flush_on_newline)
-    @flush_on_newline = !!flush_on_newline
-  end
-
-  # Determines if this `IO` flushes automatically when a newline is written.
-  def flush_on_newline?
-    @flush_on_newline
   end
 
   # Turns on/off `IO` **write** buffering. When *sync* is set to `true`, no buffering

src/io/file_descriptor.cr

@@ -31,7 +31,11 @@ class IO::FileDescriptor < IO
     clone_fd = LibC.open(path, LibC::O_RDWR)
     return new(fd, blocking: true) if clone_fd == -1
 
-    new(clone_fd).tap(&.close_on_exec = true)
+    # We don't buffer output for TTY devices to see their output right away
+    io = new(clone_fd)
+    io.close_on_exec = true
+    io.sync = true
+    io
   end
 
   def blocking

src/kernel.cr

@@ -5,16 +5,12 @@
   # The standard output file descriptor.
   #
   # Typically used to output data and information.
-  #
-  # NOTE: Gets flushed when a newline is written.
-  STDOUT = IO::FileDescriptor.new(1).tap { |f| f.flush_on_newline = true }
+  STDOUT = IO::FileDescriptor.new(1)
 
   # The standard error file descriptor.
   #
   # Typically used to output error messages and diagnostics.
-  #
-  # NOTE: Gets flushed when a newline is written.
-  STDERR = IO::FileDescriptor.new(2).tap { |f| f.flush_on_newline = true }
+  STDERR = IO::FileDescriptor.new(2)
 {% else %}
   require "c/unistd"
 
@@ -25,15 +21,17 @@
   #
   # Typically used to output data and information.
   #
-  # NOTE: Gets flushed when a newline is written.
-  STDOUT = IO::FileDescriptor.from_stdio(1).tap { |f| f.flush_on_newline = true }
+  # When this is a TTY device, `sync` will be true for it
+  # at the start of the program.
+  STDOUT = IO::FileDescriptor.from_stdio(1)
 
   # The standard error file descriptor.
   #
   # Typically used to output error messages and diagnostics.
   #
-  # NOTE: Gets flushed when a newline is written.
-  STDERR = IO::FileDescriptor.from_stdio(2).tap { |f| f.flush_on_newline = true }
+  # When this is a TTY device, `sync` will be true for it
+  # at the start of the program.
+  STDERR = IO::FileDescriptor.from_stdio(2)
 {% end %}
 
 # The name, the program was called with.
@@ -118,7 +116,6 @@ end
 # See also: `IO#print`.
 def print(*objects : _) : Nil
   STDOUT.print *objects
-  STDOUT.flush
 end
 
 # Prints a formatted string to `STDOUT`.

src/process.cr

@@ -430,8 +430,8 @@ class Process
   end
 
   ORIGINAL_STDIN  = IO::FileDescriptor.new(0, blocking: true)
-  ORIGINAL_STDOUT = IO::FileDescriptor.new(1, blocking: true).tap { |f| f.flush_on_newline = true }
-  ORIGINAL_STDERR = IO::FileDescriptor.new(2, blocking: true).tap { |f| f.flush_on_newline = true }
+  ORIGINAL_STDOUT = IO::FileDescriptor.new(1, blocking: true)
+  ORIGINAL_STDERR = IO::FileDescriptor.new(2, blocking: true)
 
   # :nodoc:
   protected def self.exec_internal(command, args, env, clear_env, input, output, error, chdir) : NoReturn