Ports/gdb: Implement wait and mourn_inferior overrides for our target

While troubleshooting why gdb wasn't working when attempting to debug
serenity programs I noticed two things:

 - The contract of serenity's `waitpid(..)` appears to be slightly
   different than the generic ptrace target expects. We need to make
   sure we pass `WSTOPPED`, and it can return different errno values
   that we would want to re-try on.

-  The contract of serenity's `ptrace(..)` implementation appears to
   diverge as well, as we are expected to call `PT_ATTACH` before we
   call `PT_CONTINUE`, otherwise `ptrace(..)` will just error out.

These two patches fix the behavior of wait and mourn_inferior so that
they work as expected on serenity and allow us to attach and then wait
for a process to exit while running under gdb.

Ports/gdb/patches/0005-serenity-Implement-custom-wait-override-for-the-sere.patch

@@ -0,0 +1,114 @@
+From a80fc99e8f2e1c5ac1620a8c6c26d65daa98204e Mon Sep 17 00:00:00 2001
+From: Brian Gianforcaro <b.gianfo@gmail.com>
+Date: Sun, 20 Feb 2022 00:32:51 -0800
+Subject: [PATCH 5/6] serenity: Implement custom wait override for the
+ serenity_nat_target
+
+While troubleshooting why gdb wasn't working when attempting to debug
+serenity programs I noticed two things:
+
+ - The contract of serenity's `waitpid(..)` appears to be slightly
+   different than the generic ptrace target expects. We need to make
+   sure we pass `WSTOPPED`, and it can return different errno values
+   that we would want to re-try on.
+
+-  The contract of serenity's `ptrace(..)` implementation appears to
+   diverge as well, as we are expected to call `PT_ATTACH` before we
+   call `PT_CONTINUE`, otherwise `ptrace(..)` will just error out.
+
+Allow gdb to understand these differences, I've overloaded the
+serenity_nat_target::wait(..) method and added the logic there.
+---
+ gdb/serenity-nat.c | 62 +++++++++++++++++++++++++++++++++++++++++++++-
+ gdb/serenity-nat.h |  4 +++
+ 2 files changed, 65 insertions(+), 1 deletion(-)
+
+diff --git a/gdb/serenity-nat.c b/gdb/serenity-nat.c
+index ff740d4..b1d39d4 100644
+--- a/gdb/serenity-nat.c
++++ b/gdb/serenity-nat.c
+@@ -10,4 +10,64 @@
+ #include "gdbsupport/gdb_wait.h"
+ 
+ #include "inf-child.h"
+-#include "serenity-nat.h"
+\ No newline at end of file
++#include "serenity-nat.h"
++
++ptid_t serenity_nat_target::wait (ptid_t ptid, struct target_waitstatus* ourstatus, target_wait_flags)
++{
++    int pid;
++    int status, save_errno;
++
++    do
++    {
++        set_sigint_trap ();
++
++        do
++        {
++            errno = 0;
++            pid = waitpid (ptid.pid (), &status, WSTOPPED);
++            if (errno != 0)
++            {
++                save_errno = errno;
++                perror_with_name (("waitpid"));
++            }
++        }
++        while (pid == -1 && (save_errno == EINTR || save_errno == EAGAIN));
++
++        clear_sigint_trap ();
++
++        if (pid == -1)
++        {
++            fprintf_unfiltered (gdb_stderr,
++                    _("Child process unexpectedly missing: %s.\n"),
++                    safe_strerror (save_errno));
++
++            /* Claim it exited with unknown signal.  */
++            ourstatus->kind = TARGET_WAITKIND_SIGNALLED;
++            ourstatus->value.sig = GDB_SIGNAL_UNKNOWN;
++            return inferior_ptid;
++        }
++
++        /* Ignore terminated detached child processes.  */
++        if (!WIFSTOPPED (status) && find_inferior_pid (this, pid) == nullptr)
++            pid = -1;
++    }
++    while (pid == -1);
++
++    store_waitstatus (ourstatus, status);
++
++    /* Serenity requires us to PT_ATTACH before we PT_CONTINUE, however GDB doesn't
++     * provide a hook for us to do that before we issue the PT_CONTINUE, so we are
++     * forced to do it here.
++     */
++    if (!m_attach_before_continue_called) {
++        errno = 0;
++        ptrace (PT_ATTACH, pid, (PTRACE_TYPE_ARG3)0, 0);
++        if (errno != 0) {
++            save_errno = errno;
++            printf_unfiltered (_("PT_ATTACH failed: %d - %s \n"), save_errno, safe_strerror (save_errno));
++        }
++        m_attach_before_continue_called = true;
++    }
++
++    return ptid_t (pid);
++}
+diff --git a/gdb/serenity-nat.h b/gdb/serenity-nat.h
+index dcd24ce..9ae3b87 100644
+--- a/gdb/serenity-nat.h
++++ b/gdb/serenity-nat.h
+@@ -11,6 +11,10 @@
+ 
+ class serenity_nat_target : public inf_ptrace_target
+ {
++  ptid_t wait (ptid_t, struct target_waitstatus *, target_wait_flags) override;
++
++private:
++  bool m_attach_before_continue_called { false };
+ };
+ 
+ #endif /* serenity-nat.h */
+-- 
+2.32.0
+