Some time ago, when testing gdb master on Solaris again after several
months, I discovered that gdb couldn't execute even a trivial program
anymore.  This had gone unnoticed by the Solaris buildbots since the
code continued to compile just fine.  Those bots are build-only since
many tests (especially thread tests) are either flaky or time out.

A reghunt identified the multi-target merge as the culprit.

Here's what I see:

$ ./gdb -D ./data-directory ./hello
GNU gdb (GDB) 10.0.50.20200106-git
[...]
Reading symbols from ./hello...
(gdb) run
Starting program: /vol/obj/gnu/gdb/gdb/reghunt/no-resync/122448/gdb/hello 
/vol/src/gnu/gdb/hg/master/reghunt/gdb/thread.c:336: internal-error: thread_info::thread_info(inferior*, ptid_t): Assertion `inf_ != NULL' failed.

Here's the start of the corresponding stack trace:

#0  internal_error (
    file=file@entry=0x966150 "/vol/src/gnu/gdb/hg/master/reghunt/gdb/thread.c", line=line@entry=336, fmt=0x9ddb94 "%s: Assertion `%s' failed.")
    at /vol/src/gnu/gdb/hg/master/reghunt/gdb/gdbsupport/errors.c:51
#1  0x0000000000ef81f4 in thread_info::thread_info (this=0x1212020, 
    inf_=<optimized out>, ptid_=...)
    at /vol/src/gnu/gdb/hg/master/reghunt/gdb/thread.c:344
#2  0x0000000000ef82cd in new_thread (inf=inf@entry=0x0, ptid=...)
    at /vol/src/gnu/gdb/hg/master/reghunt/gdb/thread.c:239
#3  0x0000000000efac3c in add_thread_silent (
    targ=targ@entry=0x11b0940 <the_procfs_target>, ptid=...)
    at /vol/src/gnu/gdb/hg/master/reghunt/gdb/thread.c:304
#4  0x0000000000d90692 in procfs_target::create_inferior (
    this=0x11b0940 <the_procfs_target>, 
    exec_file=0x13dbef0 "/vol/obj/gnu/gdb/gdb/reghunt/no-resync/122448/gdb/hello", allargs="", env=0x13c48f0, from_tty=<optimized out>)
    at /vol/src/gnu/gdb/hg/master/reghunt/gdb/gdbsupport/ptid.h:47
#5  0x0000000000c84e64 in run_command_1 (args=<optimized out>, from_tty=1, 
    run_how=run_how@entry=RUN_NORMAL)
    at /vol/gcc-9/include/c++/9.1.0/bits/basic_string.h:263
#6  0x0000000000c85007 in run_command (args=<optimized out>, 
    from_tty=<optimized out>)
    at /vol/src/gnu/gdb/hg/master/reghunt/gdb/infcmd.c:687

Looking closer, I found that in add_thread_silent as called from
procfs.c (procfs_target::create_inferior) find_inferior_ptid returns
NULL.  The all_inferiors (targ) iterator comes up empty.

Going from there, I see that in add_thread_silent

m_target_stack = {m_top = file_stratum, m_stack = {0x20190e0 <the_dummy_target>, 0x200b8c0 <exec_ops>, 0x0, 0x0, 0x0, 0x0, 0x0}}}

i.e. the_procfs_target is missing compared to the_amd64_linux_nat_target
on Linux/x86_64.

I've managed to get a bit further with the following patch which is
intended to push the procfs target first: