[RFC][PATCH] gdb/debuginfod: Support on-demand downloading of debuginfo

[Tom Tromey]

>>>>> "Aaron" == Aaron Merey via Gdb-patches <gdb-patches@sourceware.org> writes:

Aaron> This patch helps address the issue by adding support for on-demand
Aaron> downloading and reading of debuginfo.  The basic approach it takes is
Aaron> to use debuginfod to download just the .gdb_index of a debuginfo file
Aaron> as soon as the corresponding library is linked.  GDB then relies on the
Aaron> information in the index for as long as possible.  When the index isn't
Aaron> enough then debuginfo is downloaded and read. This helps avoid unnecessary
Aaron> downloads.

Makes sense to me.

Aaron> Although this patch specifically uses .gdb_index, other indexes such
Aaron> as .debug_names could be supported in much the same way.

I mentioned this in bugzilla, but .debug_names will be worse here
because it requires .debug_str and .debug_aranges to be downloaded as
well, and the former in particular will contain a lot of data that's not
immediately needed.

Aaron> This patch adds a command 'set debuginfod enabled lazy' which enables
Aaron> on-demand debuginfo downloading/reading.  If this 'lazy' mode is enabled
Aaron> and a solib's debuginfo cannot be found locally, the new function
Aaron> dwarf2_has_separate_index is called in elf_symfile_read. This function
Aaron> queries debuginfod servers for the .gdb_index matching the build-id
Aaron> of the solib.  If it's found, a new objfile is created to hold the .gdb_index
Aaron> information.  The new objfile flag OBJF_INDEX_READLATER is used to indicate
Aaron> that the objfile contains quick_symbols_functions for an index has deferred
Aaron> debuginfo reading.

It seems to me that OBJF_INDEX_READLATER should not be needed.

Instead, what I'd propose is creating the separate debuginfo objfile and
providing it with an instance of dwarf2_gdb_index -- but one that
arranges to download the remaining data when necessary.

It's fine to add new virtual methods to dwarf2_base_index_functions or
wherever if this helps.  E.g, dw2_instantiate_symtab could be a virtual
method in the base class, and then the subclass could override it to
first download the remaining sections.

You'd have to consider how to handle errors in this situation, but
probably just printing a warning (once per file) and carrying on would
be good enough.  The idea behind warn-and-carry-on is that the user
asked for this lazy behavior, if their network goes down then the kind
of asked for it; and anyway maybe they can rescue the situation with
"noshared" or something.

Maybe you tried something like this already and ran into problems?  I'm
handwaving a lot, maybe there are hidden difficulties.

TBH the current gdb design where separate debug files are their own
objfile seems like a mistake to me.  I haven't ever looked at changing
this though.  But, maybe it doesn't have to really be changed,
dwarf2/read.c could just provide one of these subclasses attached to the
parent objfile.

Aaron> Also with this patch, GDB will not build with the latest release of
Aaron> debuginfod, hence the RFC. The debuginfod ELF/DWARF section query

Is it possible to detect the non-existence of debuginfod_find_section at
compile time so that we can avoid bumping the minimum requirement?

Tom