[gdbserver/lynx] spurious failure to write in inferior memory

[gdbserver/lynx] spurious failure to write in inferior memory

We noticed the following error on ppc-lynx178, using just about
any program:

        (gdb) tar remote mytarget:4444
        Remote debugging using mytarget:4444
        0x000100c8 in _start ()
        (gdb) b try
        Breakpoint 1 at 0x10844: file try.adb, line 11.
        (gdb) cont
        Continuing.
 !!!->  Cannot remove breakpoints because program is no longer writable.
 !!!->  Further execution is probably impossible.

        Breakpoint 1, try () at try.adb:11
        11          Local : Integer := 18;

And, of course, trying to continue yielded the expected outcome:

       (gdb) c
       Continuing.
       warning: Error removing breakpoint 1
       Cannot remove breakpoints because program is no longer writable.
       Further execution is probably impossible.

It turns out that the problem is caused by an intentional test
against a variable with an undefined value. After GDB receives
notification of the inferior stopping, it tries to remove the
breakpoint by sending a memory-write packet ("X10844,4:9 ").
This leads us to lynx_write_memory, where it tries to split
the memory-write into chunks of 4 bytes. And, in order to handle
writes which are not aligned on word boundaries, we have the
following code:

      if (skip > 0 || truncate > 0)
        /* We need to read the memory at this address in order to preserve
           the data that we are not overwriting.  */
        lynx_read_memory (addr, (unsigned char *) &buf, xfer_size);
        if (errno)
          return errno;

(the comment explains what the code is about).

Unfortunately, the not-so-glaring error that we've made here is
that we're checking ERRNO regardless of whether we've called
lynx_read_memory. In our case, because we are writing 4 bytes
aligned on a word boundary, we do not call lynx_read_memory and
therefore test an ERRNO with an undefined value.

gdb/gdbserver/ChangeLog:

        * lynx-low.c (lynx_write_memory): Put lynx_read_memory and
        corresponding ERRNO check in same block.