[Bug symtab/17547] over-eager debuginfo reading

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

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

--- Comment #7 from Tom Tromey <tromey at sourceware dot org> ---
Once upon a time, I added lazy reading to gdb.  The vestiges
are still in the tree, search for OBJF_PSYMTABS_READ and SYMFILE_NO_READ.
The use case was precisely what Frank is talking about - "attach"
was very slow and read a bunch of irrelevant stuff.

However, this was mostly disabled.  While it works great for some
scenarios, it also causes long pauses at "unexpected" times in other
scenarios.  For example, it makes "backtrace" randomly slow, depending
on whether the debuginfo for a frame has been read.

(The "backtrace" problem can be solved as well, by lazier reading of
the DWARF.  This is a reasonably-sized project though.)

Another problem is that setting a breakpoint becomes very expensive,
because most breakpoints require a full DWARF scan.  This too is avoidable,
at least partly, if there were a way to isolate breakpoints to a given
.so or whatever; or by the use of an index.

When disabling this feature, I think our rationale was that it's better
for users to get the delays up front, when they expect them, rather than
randomly in response to some command.


What would be truly great for something like debuginfod is to have it
index all the libraries, and then provide a fine-grained API to gdb.
Then with some work, gdb could avoid ever downloading all the DWARF
or doing a massive scan.

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