Re: GDB now takes 4 minutes to start up with remote gdbserver target

[Gary Benson <gbenson at redhat dot com>]

Gary Benson wrote:
> Sandra Loosemore wrote:
> > On 07/24/2015 10:05 AM, Pedro Alves wrote:
> > > On 07/24/2015 04:27 PM, Paul_Koning@Dell.com wrote:
> > > > But having sysroot default to target is also a bad idea for lots
> > > > of other people.  Consider embedded systems: you presumably have
> > > > stripped images there, but unstripped ones on your build host.
> > > 
> > > But in that scenario, with the old default sysroot, how was gdb
> > > finding the binaries on the build host?  The binaries on the
> > > equilalent locations on the host's root will certainly not match
> > > the embedded/target system's.  In that scenario, you must have
> > > been pointing the "set sysroot" somewhere local?  And if you do
> > > that, nothing changes in 7.10, gdb will still access the files on
> > > the local filesystem.
> > > 
> > > From the discussion so far, it seems that the only case that ends
> > > up regressing is the case where the host and target share both the
> > > filesystem, and the host/target paths match.  I don't know off
> > > hand how to make gdb aware of that automatically.
> > 
> > There's also the case where the host and target sysroot locations do
> > not match at all.  As I said, this used to work reasonably well for
> > application debugging, where the user isn't interested in debugging
> > shared libraries and doesn't care if the shared library symbol
> > information isn't available to GDB.  It used to print a helpful
> > message suggesting using "set sysroot" if the user wants the shared
> > library information, instead of hanging on startup with no
> > indication of what the trouble is or how to fix it.  I can't see the
> > new default behavior as an improvement over the old.
> > 
> > > That seems like enough of a special case that could well be
> > > handled by an explicit "set sysroot /" in e.g., the toolchain's
> > > system-gdbinit, or by building gdb with "--with-sysroot=/".
> > 
> > There are a bunch of possible workarounds for this, but why can't we
> > make GDB "just work" by default, as it used to, instead of requiring
> > users to build GDB differently or install a workaround or issue
> > extra commands manually that they didn't used to need at all?
> 
> I have an idea for a solution to this.  I should know in a few hours
> if it can work.  Tomorrow morning at the very latest.

Ok, here goes...

 * From a user's perspective GDB is magically prefixing *some*
   executable and shared library filenames with "target:".

 * From a developer's perspective this magic prefixing is implemented
   by having the string "target:" as the default sysroot.

My proposal is to make the default sysroot be "" again, and add the
prefix in solib_find_1 if certain conditions are met, specifically:

 * Executable filenames get prefixed with "target:" iff:
     Automatic "target:" prefixing is enabled
     AND gdb_sysroot is ""
     AND the filesystem is nonlocal

 * Shared library filenames get prefixed with "target:" iff:
     Automatic "target:" prefixing is enabled
     AND gdb_sysroot is ""
     AND the filesystem is nonlocal
     AND exec_filename starts with "target:"

There's a new boolean here, "set auto-target-prefix on|off", which
is enabled by default.

So:

 gdb; target remote :9999 -> "target:" prefix applied
 gdb FILE; target remote :9999 -> no "target:" prefix

The latter is how Sandra is invoking GDB.  Also:
 
 gdb -n PID, and
 gdb; attach PID -> "target:" prefix applied iff FS is nonlocal

Aside from the fact that this should fix Sandra's use case without
breaking any I care about, I like that users doing non-remote, non-
container debugging will not see "target:" prefixes onscreen unless
they're actually necessary.

Comments?

Cheers,
Gary

-- 
http://gbenson.net/