types: member m_fd in BeesFileRange must be protected against data races

We had an unfortunate pattern of:

	const BeesFileRange bfr;
	shared_ptr<BeesContext> ctx;
	// ...
	BEESNOTE("foo " << bfr);
	bfr.fd(ctx);
	BEESNOTE("foo after opening: " << bfr);

If dump_status started running after the first BEESNOTE, but before
the second, then bfr.fd() might expose a single Fd object's shared_ptr
member to two threads at the same time (the thread running dump_status
and the thread running BEESNOTE) without protection by a lock.  One of
the threads would see a partially-initialized Fd object, and the other
thread would crash on an assertion failure, e.g.

	#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
	#1  0x00007f4c4fde5537 in __GI_abort () at abort.c:79
	#2  0x00007f4c4fde540f in __assert_fail_base (fmt=0x7f4c4ff4e128 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=0x5557605629dd "!m_destroyed", file=0x5557605627c0 "../include/crucible/namedptr.h", line=77, function=<optimized out>) at assert.c:92
	#3  0x00007f4c4fdf4662 in __GI___assert_fail (assertion=assertion@entry=0x5557605629dd "!m_destroyed", file=file@entry=0x5557605627c0 "../include/crucible/namedptr.h", line=line@entry=77,
	    function=function@entry=0x555760562970 "crucible::NamedPtr<Return, Arguments>::Value::~Value() [with Return = crucible::IOHandle; Arguments = {int}]") at assert.c:101
	#4  0x00005557605306f6 in crucible::NamedPtr<crucible::IOHandle, int>::Value::~Value (this=0x7f4a3c2ff0d0, __in_chrg=<optimized out>) at ../include/crucible/namedptr.h:77
	#5  0x00005557605137da in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x7f4a3c2ff0c0) at /usr/include/c++/10/bits/shared_ptr_base.h:151
	#6  std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x7f4a3c2ff0c0) at /usr/include/c++/10/bits/shared_ptr_base.h:151
	#7  std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=0x7f4c4c5b5f28, __in_chrg=<optimized out>) at /usr/include/c++/10/bits/shared_ptr_base.h:733
	#8  std::__shared_ptr<crucible::IOHandle, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=0x7f4c4c5b5f20, __in_chrg=<optimized out>) at /usr/include/c++/10/bits/shared_ptr_base.h:1183
	#9  std::shared_ptr<crucible::IOHandle>::~shared_ptr (this=0x7f4c4c5b5f20, __in_chrg=<optimized out>) at /usr/include/c++/10/bits/shared_ptr.h:121
	#10 crucible::Fd::~Fd (this=0x7f4c4c5b5f20, __in_chrg=<optimized out>) at ../include/crucible/fd.h:46
	#11 BeesFileRange::file_size (this=0x7f4c4e5ba4a0) at bees-types.cc:156
	#12 0x0000555760513950 in operator<< (os=..., bfr=...) at bees-types.cc:80
	#13 0x000055576050d662 in std::function<void (std::ostream&)>::operator()(std::ostream&) const (__args#0=..., this=0x7f4c4e5b9f60) at /usr/include/c++/10/bits/std_function.h:622
	#14 BeesNote::get_status[abi:cxx11]() () at bees-trace.cc:165
	#15 0x00005557604c9676 in BeesContext::dump_status (this=0x5557611c4de0) at bees-context.cc:89
	#16 0x00005557605206fb in std::function<void ()>::operator()() const (this=this@entry=0x7f4c4c5b65f0) at /usr/include/c++/10/bits/std_function.h:622
	#17 crucible::catch_all(std::function<void ()> const&, std::function<void (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)> const&) (f=..., explainer=...) at error.cc:55
	#18 0x000055576050aaa7 in operator() (__closure=0x5557611c52c8) at bees-thread.cc:22
	#19 0x00007f4c501beed0 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
	#20 0x00007f4c502c8ea7 in start_thread (arg=<optimized out>) at pthread_create.c:477
	#21 0x00007f4c4febddef in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Fix by making BeesFileRange::m_fd really const (not just mutable),
then fix all the broken code referencing it.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-types.cc

@@ -287,7 +287,7 @@ BeesFileRange::fd() const
 }
 
 Fd
-BeesFileRange::fd(const shared_ptr<BeesContext> &ctx) const
+BeesFileRange::fd(const shared_ptr<BeesContext> &ctx)
 {
 	// If we don't have a fid we can't do much here
 	if (m_fid) {