[Bug symtab/17547] over-eager debuginfo reading

["amerey at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=17547

--- Comment #11 from Aaron Merey <amerey at redhat dot com> ---
(In reply to Tom Tromey from comment #10)
> My current work for bug #29942 involves trying to move some gdb work
> into the background, so that when many libraries must be read at
> once, gdb isn't blocking while processing each one.
> 
> The corresponding idea with debuginfod would be to read the .gdb_index
> remotely -- but not even wait for the download to complete before
> setting up the "quick_symbol_functions" object.  Any code trying to
> actually use the index would have to wait for the download to complete,
> but this seems fine to me (assuming the user opts in, like in your patch).
> 
> Perhaps another thing that could be done here is to also trigger a
> background download of the corresponding full files, so they are
> more likely to be available when needed.

These are nice enhancements. I'll try prototyping some basic support
for background downloading and keep an eye out for your PR29942 patch. 

> One funny thing here is that while I've been wanting gdb to move
> towards the DWARF 5 index (see bug #24820), .gdb_index is
> actually more self-contained -- the DWARF 5 index refers to
> .debug_str, and gdb also needs .debug_aranges to work.  So, .gdb_index
> is a little easier for this use.

debuginfod supports the downloading of any ELF sections from a
given file, so we could implement deferred downloads using 
.debug_names and related sections in addition to .gdb_index or
instead of it.

Even if we keep using .gdb_index for this we still may end up
downloading other sections. One issue with deferred downloads that
I'd like to address: commands that require searching debuginfos
for a source file name trigger a mass download of all deferred
debuginfo, since .gdb_index doesn't contain file name information.
Maybe gdb could get this information from downloading
.debug_line/.debug_line_str in addition to .gdb_index.

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