Uninitialized read of "errno" in handle_event() in src/modules/stdin-util.c

There is potentially possibility of uninitialized read of "errno" in handle_event() in src/modules/stdin-util.c

The function handle_event() tests errno if read_byte fails (returns -1):

...
if ((opcode = read_byte(u)) < 0) {
   if (errno == EINTR || errno == EAGAIN)
      break;
...

The problem with above is that read_byte() can fail (return -1) without any guarantee that 'errno' was actually set.

The uninitialized 'errno' read can occur on EOF.

The call flow looks like:

-> read_byte()
  -> fill_buf()
    -> pa_read() 
       -> read()
         - reads 0 bytes, EOF
       <- read() returns 0
    <- pa_read() return 0
    - fill_buf thinks it's a failure when read returned 0 bytes
  <- fill_buf() returns -1  [1]
<- read_byte() returns -1

The 'errno' is neither initialized by read() which returned 0, nor pa_read(), nor other function in the above flow.

The function handle_event() may get -1 from read_byte() and think that the call failed. Subsequently handle_event() reads 'errno', but it might read 'errno' of completely different / unrelated failure in one of previous calls to *libc.

To fix it, one can e.g. set 'errno' to 0 either in fill_buf() before pa_read() is called or in pa_read() itself before read() is called.