Make exec-file-mismatch compare build IDs

Make exec-file-mismatch compare build IDs

The patch makes GDB do exec-file-mismatch validation by comparing
build IDs instead of the current method of comparing filenames.

Currently, the exec-file-mismatch feature simply compares filenames to
decide whether the exec file loaded in gdb and the exec file the
target reports is running match.  This causes false positives when
remote debugging, because it'll often be the case that the paths in
the host and the target won't match.  And of course misses the case of
the files having the same name but being actually different files
(e.g., different builds).

This also broke many testcases when running against gdbserver, causing
tests to be skipped like (here native-extended-gdbserver):

  (gdb) run
  Starting program: /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/argv0-symlink/argv0-symlink-filelink
  warning: Mismatch between current exec-file /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/argv0-symlink/argv0-symlink-filelink
  and automatically determined exec-file /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/argv0-symlink/argv0-symlink
  exec-file-mismatch handling is currently "ask"
  Load new symbol table from "/home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/argv0-symlink/argv0-symlink"? (y or n) UNTESTED: gdb.base/argv0-symlink.exp: could not run to main

or to fail like (here native-gdbserver):

 (gdb) spawn /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/../../gdbserver/gdbserver --once localhost:2346 /home/pedro/gdb/binutils-gdb/build/gdb/te
 stsuite/outputs/gdb.btrace/buffer-size/skip_btrace_tests-19968.x
 Process /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.btrace/buffer-size/skip_btrace_tests-19968.x created; pid = 20040
 Listening on port 2346
 target remote localhost:2346
 Remote debugging using localhost:2346
 warning: Mismatch between current exec-file /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/temp/19968/skip_btrace_tests-19968.x
 and automatically determined exec-file /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.btrace/buffer-size/skip_btrace_tests-19968.x
 exec-file-mismatch handling is currently "ask"
 Load new symbol table from "/home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.btrace/buffer-size/skip_btrace_tests-19968.x"? (y or n) Quit
 (gdb) UNSUPPORTED: gdb.btrace/buffer-size.exp: target does not support record-btrace

The former case is about GDB not realizing the two files are the same,
because one of the them is a symlink to the other.  The latter case is
about GDB realizing that one file is a copy of the other.

Over the years, the toolchain has settled on build ID matching being
the canonical method to match core dumps to executables, and
executables with no debug info to their debug info.

This patch makes us use build IDs to match the running image of a
binary with its version loaded in gdb, which may or may not have debug
info.  This is very much like the core dump/executable matching.

The change to gdb_bfd_open is necessary to get rid of the "transfers
from remote targets can be slow" warning when we open the remote file
to read its build ID:

 (gdb) r
 Starting program: /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/break/break
 Reading /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/argv0-symlink/argv0-symlink from remote target...
 warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 warning: Mismatch between current exec-file /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/break/break
 and automatically determined exec-file /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/argv0-symlink/argv0-symlink
 exec-file-mismatch handling is currently "ask"
 Load new symbol table from "/home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.base/argv0-symlink/argv0-symlink"? (y or n)

While trying this out, I was worried that bfd would read a lot of
stuff from the binary in order to extract the build ID, making it
potentially slow, but turns out we don't read all that much.  Maybe a
couple hundred bytes, and most of it seemingly is the read-ahead
cache.  So I'm not worried about that.  Otherwise I'd consider whether
a new qXfer:buildid:read would be better.  But I'm happy that we
seemingly don't need to worry about it.

gdb/ChangeLog:
2020-05-19  Pedro Alves  <palves@redhat.com>

	* NEWS (set exec-file-mismatch): Adjust entry.
	* exec.c: Include "build-id.h".
	(validate_exec_file): Try to match build IDs instead of filenames.
	* gdb_bfd.c (struct gdb_bfd_open_closure): New.
	(gdb_bfd_iovec_fileio_open): Adjust to use gdb_bfd_open_closure
	and pass down 'warn_if_slow'.
	(gdb_bfd_open): Add 'warn_if_slow' parameter.  Use
	gdb_bfd_open_closure to pass it down.
	* gdb_bfd.h (gdb_bfd_open): Add 'warn_if_slow' parameter.

gdb/doc/ChangeLog:
2020-05-19  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Attach): Update exec-file-mismatch description to
	mention build IDs.
	(Separate Debug Files): Add "build id" anchor.