It occured to me that `intrusive_list<solib>`, as returned by
`solib_ops::current_sos`, for instance, is not very safe. The
current_sos method returns the ownership of the solib objects
(heap-allocated) to its caller, but the `intrusive_list<solib>` type
does not convey it. If a function is building an
`intrusive_list<solib>` and something throws, the solibs won't
automatically be deleted. Introduce owning_intrusive_list to fill this
gap.
Interface
---------
The interface of owning_intrusive_list is mostly equivalent to
intrusive_list, with the following differences:
- When destroyed, owning_intrusive_list deletes all element objects.
The clear method does so as well.
- The erase method destroys the removed object.
- The push_front, push_back and insert methods accept a `unique_ptr<T>`
(compared to `T &` for intrusive_list), taking ownership of the
object.
- owning_intrusive_list has emplace_front, emplace_back and emplace
methods, allowing to allocate and construct an object directly in the
list. This is really just a shorthand over std::make_unique and
insert (or push_back / push_front if you don't care about the return
value), but I think it is nicer to read:
list.emplace (pos, "hello", 2);
rather than
list.insert (pos, std::make_unique<Foo> ("hello", 2));
These methods are not `noexcept`, since the allocation or the
constructor could throw.
- owning_intrusive_list has a release method, allowing to remove an
element without destroying it. The release method returns a
pair-like struct with an iterator to the next element in the list
(like the erase method) and a unique pointer transferring the
ownership of the released element to the caller.
- owning_intrusive_list does not have a clear_and_dispose method, since
that is typically used to manually free items.
Implementation
--------------
owning_intrusive_list privately inherits from intrusive_list, in order
to re-use the linked list machinery. It adds ownership semantics around
it.
Testing
-------
Because of the subtle differences in the behavior in behavior and what
we want to test for each type of intrusive list, I didn't see how to
share the tests for the two implementations. I chose to copy the
intrusive_list tests and adjust them for owning_intrusive_list.
The verify_items function was made common though, and it tries to
dereference the items in the list, to make sure they have not been
deleted by mistake (which would be caught by Valgrind / ASan).
Change-Id: Idbde09c1417b79992a0a9534d6907433e706f760
Co-Authored-By: Pedro Alves <pedro@palves.net>
Reviewed-by: Keith Seitz <keiths@redhat.com>
Some methods of intrusive_list are marked noexcept. But really,
everything in that file could be noexcept. Add it everywhere.
The only one I had a doubt about is clear_and_dispose: what if the
disposer throws? The boost equivalent [1] is noexcept and requires the
disposer not to throw. The rationale is probably the same as for
destructors. What if the disposer throws for an element in the middle
of the list? Do you skip the remaining elements? Do you swallow the
exception and keep calling the disposer for the remaining elements?
It's simpler to say no exceptions allowed.
[1] https://www.boost.org/doc/libs/1_79_0/doc/html/boost/intrusive/list.html#idm33710-bb
Change-Id: I402646cb12c6b7906f4bdc2ad85203d8c8cdf2cc
Reviewed-by: Keith Seitz <keiths@redhat.com>
This commit is the result of the following actions:
- Running gdb/copyright.py to update all of the copyright headers to
include 2024,
- Manually updating a few files the copyright.py script told me to
update, these files had copyright headers embedded within the
file,
- Regenerating gdbsupport/Makefile.in to refresh it's copyright
date,
- Using grep to find other files that still mentioned 2023. If
these files were updated last year from 2022 to 2023 then I've
updated them this year to 2024.
I'm sure I've probably missed some dates. Feel free to fix them up as
you spot them.
It seems to me like the code should used the defined type aliases, for
consistency.
Change-Id: Ib52493ff18ad29464405275bc10a0c6704ed39e9
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>
This commit is the result of running the gdb/copyright.py script,
which automated the update of the copyright year range for all
source files managed by the GDB project to be updated to include
year 2023.
Tromey noticed that intrusive_list_node leaves its data members
public, which seems sub-optimal.
This commit makes intrusive_list_node's data fields private.
intrusive_list_iterator, intrusive_list_reverse_iterator, and
intrusive_list do need to access the fields, so they are made friends.
Change-Id: Ia8b306b40344cc218d423c8dfb8355207a612ac5
This commit brings all the changes made by running gdb/copyright.py
as per GDB's Start of New Year Procedure.
For the avoidance of doubt, all changes in this commits were
performed by the script.
The threads that need a step-over are currently linked using an
hand-written intrusive doubly-linked list, so that seems a very good
candidate for intrusive_list, convert it.
For this, we have a use case of appending a list to another one (in
start_step_over). Based on the std::list and Boost APIs, add a splice
method. However, only support splicing the other list at the end of the
`this` list, since that's all we need.
Add explicit default assignment operators to
reference_to_pointer_iterator, which are otherwise implicitly deleted.
This is needed because to define thread_step_over_list_safe_iterator, we
wrap reference_to_pointer_iterator inside a basic_safe_iterator, and
basic_safe_iterator needs to be able to copy-assign the wrapped
iterator. The move-assignment operator is therefore not needed, only
the copy-assignment operator is. But for completeness, add both.
Change-Id: I31b2ff67c7b78251314646b31887ef1dfebe510c
GDB currently has several objects that are put in a singly linked list,
by having the object's type have a "next" pointer directly. For
example, struct thread_info and struct inferior. Because these are
simply-linked lists, and we don't keep track of a "tail" pointer, when
we want to append a new element on the list, we need to walk the whole
list to find the current tail. It would be nice to get rid of that
walk. Removing elements from such lists also requires a walk, to find
the "previous" position relative to the element being removed. To
eliminate the need for that walk, we could make those lists
doubly-linked, by adding a "prev" pointer alongside "next". It would be
nice to avoid the boilerplate associated with maintaining such a list
manually, though. That is what the new intrusive_list type addresses.
With an intrusive list, it's also possible to move items out of the
list without destroying them, which is interesting in our case for
example for threads, when we exit them, but can't destroy them
immediately. We currently keep exited threads on the thread list, but
we could change that which would simplify some things.
Note that with std::list, element removal is O(N). I.e., with
std::list, we need to walk the list to find the iterator pointing to
the position to remove. However, we could store a list iterator
inside the object as soon as we put the object in the list, to address
it, because std::list iterators are not invalidated when other
elements are added/removed. However, if you need to put the same
object in more than one list, then std::list<object> doesn't work.
You need to instead use std::list<object *>, which is less efficient
for requiring extra memory allocations. For an example of an object
in multiple lists, see the step_over_next/step_over_prev fields in
thread_info:
/* Step-over chain. A thread is in the step-over queue if these are
non-NULL. If only a single thread is in the chain, then these
fields point to self. */
struct thread_info *step_over_prev = NULL;
struct thread_info *step_over_next = NULL;
The new intrusive_list type gives us the advantages of an intrusive
linked list, while avoiding the boilerplate associated with manually
maintaining it.
intrusive_list's API follows the standard container interface, and thus
std::list's interface. It is based the API of Boost's intrusive list,
here:
https://www.boost.org/doc/libs/1_73_0/doc/html/boost/intrusive/list.html
Our implementation is relatively simple, while Boost's is complicated
and intertwined due to a lot of customization options, which our version
doesn't have.
The easiest way to use an intrusive_list is to make the list's element
type inherit from intrusive_node. This adds a prev/next pointers to
the element type. However, to support putting the same object in more
than one list, intrusive_list supports putting the "node" info as a
field member, so you can have more than one such nodes, one per list.
As a first guinea pig, this patch makes the per-inferior thread list use
intrusive_list using the base class method.
Unlike Boost's implementation, ours is not a circular list. An earlier
version of the patch was circular: the intrusive_list type included an
intrusive_list_node "head". In this design, a node contained pointers
to the previous and next nodes, not the previous and next elements.
This wasn't great for when debugging GDB with GDB, as it was difficult
to get from a pointer to the node to a pointer to the element. With the
design proposed in this patch, nodes contain pointers to the previous
and next elements, making it easy to traverse the list by hand and
inspect each element.
The intrusive_list object contains pointers to the first and last
elements of the list. They are nullptr if the list is empty.
Each element's node contains a pointer to the previous and next
elements. The first element's previous pointer is nullptr and the last
element's next pointer is nullptr. Therefore, if there's a single
element in the list, both its previous and next pointers are nullptr.
To differentiate such an element from an element that is not linked into
a list, the previous and next pointers contain a special value (-1) when
the node is not linked. This is necessary to be able to reliably tell
if a given node is currently linked or not.
A begin() iterator points to the first item in the list. An end()
iterator contains nullptr. This makes iteration until end naturally
work, as advancing past the last element will make the iterator contain
nullptr, making it equal to the end iterator. If the list is empty,
a begin() iterator will contain nullptr from the start, and therefore be
immediately equal to the end.
Iterating on an intrusive_list yields references to objects (e.g.
`thread_info&`). The rest of GDB currently expects iterators and ranges
to yield pointers (e.g. `thread_info*`). To bridge the gap, add the
reference_to_pointer_iterator type. It is used to define
inf_threads_iterator.
Add a Python pretty-printer, to help inspecting intrusive lists when
debugging GDB with GDB. Here's an example of the output:
(top-gdb) p current_inferior_.m_obj.thread_list
$1 = intrusive list of thread_info = {0x61700002c000, 0x617000069080, 0x617000069400, 0x61700006d680, 0x61700006eb80}
It's not possible with current master, but with this patch [1] that I
hope will be merged eventually, it's possible to index the list and
access the pretty-printed value's children:
(top-gdb) p current_inferior_.m_obj.thread_list[1]
$2 = (thread_info *) 0x617000069080
(top-gdb) p current_inferior_.m_obj.thread_list[1].ptid
$3 = {
m_pid = 406499,
m_lwp = 406503,
m_tid = 0
}
Even though iterating the list in C++ yields references, the Python
pretty-printer yields pointers. The reason for this is that the output
of printing the thread list above would be unreadable, IMO, if each
thread_info object was printed in-line, since they contain so much
information. I think it's more useful to print pointers, and let the
user drill down as needed.
[1] https://sourceware.org/pipermail/gdb-patches/2021-April/178050.html
Co-Authored-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I3412a14dc77f25876d742dab8f44e0ba7c7586c0
