[Bug remote/28489] Allow caching of remote objfiles

["tromey at sourceware dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28489

--- Comment #12 from Tom Tromey <tromey at sourceware dot org> ---
(In reply to Tom Tromey from comment #7)
> It occurred to me that to make this work, we'll need a new protocol
> extension (no big deal) and gdbserver will have to know how to
> extract the build id (a bigger deal).

I was thinking about this again and I did a little experiment.

I instrumented BFD to print how much data it reads from the underlying file.
Then I wrote a small program to open an ELF and extract its build id.
When I coalesce all the reads I end up with it reading 5388 bytes
in 3 different parts of the file.

This doesn't seem so bad -- gdb could just do the reads using the
existing protocol and then switch to a local file (or start one) if the
build-id matches (or is new).

Furthermore, it seems like there isn't really a need to read the
entire file eagerly.  gdb won't ever use most of it, I think.
The idea here would be to have gdb cache pages from the file locally,
and keep a bitmap (in the same file) indicating which pages are valid.
Then when BFD requests some new page, go fetch it from the remote.

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