client buffer

##########++++++++A++++++A+++A++..........
^         ^       ^      ^   ^  ^         ^
|         |       |      |   |  |         |
|         |       |      |   |  |         +-- end of buffer
|         |       |      |   |  |
|         |       |      |   |  +-- end of read(2) bytes
|         |       |      |   |
|         |       +-- newlines
|         |              
|         +-- start of line scanning
|
+-- start of line


# original idea

- consume many lines in client__pollin
- problem: in the echo server case, we want to write each line but a call to
  write(2) may block, so we can't do line processing in client__pollin
- solution: process lines in client__pollout

i = scan_start
while i < buffer_end:
    byte = mem[i]
    i += 1
    if byte == '\n':
        handle(line_start, i - line_start)
        line_start = i
        scan_start = i

if line_start + read(2)_length >= buffer_end:
    out_of_memory()
    return

i = line_start
while i < line_start + read(2)_length:
    mem[i - line_start + buffer_start] = mem[i]
    i += 1


# better idea

in pollin handler, just read once
indicate to the caller:
- we read a complete line (ie 0x0a is present)
- we read an incomplete non-empty line
- we read nothing (EOF)
- error from read(2)