Comment 19 for bug 1711602

Via a watchpoints I found that the error is set by qemu_fill_buffer.

b qemu_loadvm_state
handle SIGUSR1 nostop noprint pass
c
# on the break check and watch the status
(gdb) p f
$1 = (QEMUFile *) 0xb9babb3c00
(gdb) p *f
$2 = {ops = 0xb9b89880a0 <channel_input_ops>, hooks = 0x0, opaque = 0xb9bbabfe00, bytes_xfer = 0, xfer_limit = 0, pos = 0, buf_index = 0,
  buf_size = 0, buf = '\000' <repeats 32767 times>, may_free = {0}, iov = {{iov_base = 0x0, iov_len = 0} <repeats 64 times>}, iovcnt = 0,
  last_error = 0}

# ok still no err, set watchpoint
(gdb) p &(f->last_error)
$4 = (int *) 0xb9babbc044
(gdb) watch *(int *) 0xb9babbc044
Hardware watchpoint 2: *(int *) 0xb9babbc044

# This catches the following
Thread 1 "qemu-system-x86" hit Hardware watchpoint 2: *(int *) 0xb9babbc044

Old value = 0
New value = -5
0x000000b9b82bd0ec in qemu_file_set_error (ret=-5, f=0xb9babb3c00) at ./migration/qemu-file.c:125
warning: Source file is more recent than executable.
125 f->last_error = ret;
(gdb) bt
#0 0x000000b9b82bd0ec in qemu_file_set_error (ret=-5, f=0xb9babb3c00) at ./migration/qemu-file.c:125
#1 qemu_fill_buffer (f=0xb9babb3c00) at ./migration/qemu-file.c:299
#2 0x000000b9b82bdbb1 in qemu_peek_byte (f=0xb9babb3c00, offset=0) at ./migration/qemu-file.c:553
#3 0x000000b9b82bdc1b in qemu_get_byte (f=f@entry=0xb9babb3c00) at ./migration/qemu-file.c:566
#4 0x000000b9b82b5853 in qemu_loadvm_state_main (f=f@entry=0xb9babb3c00, mis=0xb9b8a4f700 <mis_current>) at ./migration/savevm.c:1947
#5 0x000000b9b82b864f in qemu_loadvm_state (f=f@entry=0xb9babb3c00) at ./migration/savevm.c:2032
#6 0x000000b9b82af5c3 in process_incoming_migration_co (opaque=0xb9babb3c00) at ./migration/migration.c:320
#7 0x000000b9b83e42a6 in coroutine_trampoline (i0=<optimized out>, i1=<optimized out>) at ./util/coroutine-ucontext.c:79
#8 0x00007fbf3702fac0 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#9 0x00007fffe3f9f800 in ?? ()
#10 0x0000000000000000 in ?? ()