Re: [PATCH v3 6/8] gdb/remote: Parse tdesc field in stop reply and threads list XML

[Simon Marchi <simark@simark.ca>]

On 2/1/23 09:32, Andrew Burgess via Gdb-patches wrote:
> Thiago Jung Bauermann via Gdb-patches <gdb-patches@sourceware.org>
> writes:
> 
>> gdbserver added the concept of target description IDs to the remote
>> protocol and uses them in the threads list XML and in the 'T AA' stop
>> reply packet.  It also allows fetching a target description with a given
>> ID.  This patch is for the GDB-side support.  The target descriptions
>> obtained this way aren't yet used but will be in the next patch.
>>
>> In the DTD for the threads list XML, add a "tdesc" attribute to the
>> <thread> node.  A tdesc_id field is added to the stop_reply and
>> thread_item structs.  An m_remote member is added to the
>> threads_listing_context struct, and to simplify its initialisation a
>> constructor is added as well.  This is to provide access to the remote
>> state in start_thread.
>>
>> Finally, the remote_state object keeps a map of the target descriptions
>> that have been received from the target, keyed by their ID.  There are
>> also methods to get a target description given its ID, and to fetch target
>> descriptions for IDs that were mentioned by gdbserver but not yet
>> retrieved by GDB.  The latter gets called after parsing the response of
>> qXfer:threads:read and of the stop reply packet.
>> ---
>>  gdb/features/threads.dtd |  1 +
>>  gdb/remote.c             | 85 +++++++++++++++++++++++++++++++++++++++-
>>  gdb/xml-tdesc.c          | 27 ++++++++++---
>>  gdb/xml-tdesc.h          |  6 +++
>>  4 files changed, 112 insertions(+), 7 deletions(-)
>>
>> diff --git a/gdb/features/threads.dtd b/gdb/features/threads.dtd
>> index 036b2ce58837..3102d1352978 100644
>> --- a/gdb/features/threads.dtd
>> +++ b/gdb/features/threads.dtd
>> @@ -11,3 +11,4 @@
>>  
>>  <!ATTLIST thread id CDATA #REQUIRED>
>>  <!ATTLIST thread core CDATA #IMPLIED>
>> +<!ATTLIST thread tdesc CDATA #IMPLIED>
>> diff --git a/gdb/remote.c b/gdb/remote.c
>> index 218bca30d047..f1d1944414c3 100644
>> --- a/gdb/remote.c
>> +++ b/gdb/remote.c
>> @@ -80,6 +80,7 @@
>>  #include <unordered_map>
>>  #include "async-event.h"
>>  #include "gdbsupport/selftest.h"
>> +#include "xml-tdesc.h"
>>  
>>  /* The remote target.  */
>>  
>> @@ -238,6 +239,16 @@ class remote_state
>>    /* Get the remote arch state for GDBARCH.  */
>>    struct remote_arch_state *get_remote_arch_state (struct gdbarch *gdbarch);
>>  
>> +  /* Add new ID to the target description list.  The corresponding XML will be
>> +     requested soon.  */
>> +  void add_tdesc_id (ULONGEST id);
> 
> I'm wondering why this function is needed?  Could we not just have
> get_tdesc take a remote_target* argument, and fetch the descriptions on
> demand?  This would also allow fetch_unknown_tdescs to be removed I
> think, as well as the remote_target* inside threads_listing_context.

Piggybacking on Andrew's review again, because he said most of what I
noticed.

I had a comment going in a similar direction.  I think the intent of
threads_listing_context to be a "parsed" version of the XML document,
that can then be used by the caller that implements the logic.  So I
would just store the target description id in the thread_item, and
that's it.  update_thread_list can then fetch the missing target
descriptions, probably as Andrew suggested.

> Who owns the objects pointed too by this data structure?  From my
> reading of the code I suspect they are owned by the remote_state, in
> which case we should possibly be deleting the objects in
> remote_state::~remote_state.
> 
> The only problem with this would be, what happens to any threads that
> reference a target description within a remote connection that is close,
> and thus the referenced target description is deleted....
> 
> ... having just run a test it appears that when we disconnect from a
> remote target, the remote target itself (and the associated
> remote_state) is deleted first, and then we delete the threads of
> inferiors running on that target.  That means that if we did delete the
> target descriptions in ~remote_state, then we would, for a time, be in a
> situation where the thread_info referenced a deleted target description.
> 
> I'm not sure how easy that would be to fix, maybe we can just add some
> code in remote_unpush_target before the call to
> inferior::pop_all_targets_at_and_above?

IIUC, the tdescs would be deleted during the
pop_all_targets_at_and_above, when the refcount of the remote_target
gets to 0 and it gets deleted.  And the threads would be removed in
generic_mourn_inferior just after.

An idea could be to call generic_mourn_inferior before
remote_unpush_target (no idea if it works).  Another one would be to
get a temporary reference to the remote_target object in
remote_unpush_target, just so that it outlives the threads.
Or maybe we should say that it's a process target's responsibility to
delete any thread it "owns" before getting deleted itself.

> Anyway, I think the first attempt should be to make the m_tdescs data
> structure be:
> 
>   std::unordered_map<ULONGEST, std::unique_ptr<const target_desc>> m_tdescs;

Note that we have target_desc_up, with a custom deleter.  It's necessary
because struct target_desc is in target-description.c, not the header.
Although I wouldn't mind if we moved the struct to the header and got
rid of the custom deleter.

>> @@ -3833,6 +3870,7 @@ const struct gdb_xml_attribute thread_attributes[] = {
>>    { "core", GDB_XML_AF_OPTIONAL, gdb_xml_parse_attr_ulongest, NULL },
>>    { "name", GDB_XML_AF_OPTIONAL, NULL, NULL },
>>    { "handle", GDB_XML_AF_OPTIONAL, NULL, NULL },
>> +  { "tdesc", GDB_XML_AF_OPTIONAL, gdb_xml_parse_attr_ulongest, NULL },
> 
> Ideally s/NULL/nullptr/ here too, but this one's clearly a bit odd as
> we're in a table surrounded by legacy code.  But I think I'd still use
> nullptr in preference.

Also, feel free to submit a patch to change NULL for nullptr in this
whole array.  Or heck, the whole file.

Simon