Fixes PR 25475: ensure exec-file-mismatch "ask" always asks in case of mismatch.

As explained in https://sourceware.org/bugzilla/show_bug.cgi?id=25475,
when the currently loaded file has no debug symbol,
symbol_file_add_with_addrs does not ask a confirmation to the user
before loading the new symbol file.  The behaviour is not consistent
when symbol_file_add_with_addrs is called due to exec-file-mismatch "ask"
setting.

The PR discusses several solutions/approaches.
The preferred approach (suggested by Joel) is to ensure that GDB always asks
a confirmation when it loads a new symbol file due to exec-file-mismatch,
using a new SYMFILE add-flag.

I tested this manually.  If OK, we can remove the bypass introduced by Tom
in 6b9374f1, in order to always answer to the 'load' question.

gdb/ChangeLog
2020-06-24  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* symfile-add-flags.h: New flag SYMFILE_ALWAYS_CONFIRM.
	* exec.c (validate_exec_file): If from_tty, set both
	SYMFILE_VERBOSE (== from_tty) and SYMFILE_ALWAYS_CONFIRM.
	* symfile.c (symbol_file_add_with_addrs): if always_confirm
	and from_tty, unconditionally ask a confirmation.

1 files changed, 6 insertions, 0 deletions

diff --git a/gdb/symfile-add-flags.h b/gdb/symfile-add-flags.h
index 740357bc125..2b2c2e68f27 100644
--- a/gdb/symfile-add-flags.h
+++ b/gdb/symfile-add-flags.h
@@ -44,6 +44,12 @@ enum symfile_add_flag : unsigned
 
     /* The new objfile should be marked OBJF_NOT_FILENAME.  */
     SYMFILE_NOT_FILENAME = 1 << 5,
+
+    /* If SYMFILE_VERBOSE (interpreted as from_tty) and SYMFILE_ALWAYS_CONFIRM,
+       always ask user to confirm loading the symbol file.
+       Without this flag, symbol_file_add_with_addrs asks a confirmation only
+       for a main symbol file replacing a file having symbols.  */
+    SYMFILE_ALWAYS_CONFIRM = 1 << 6,
  };
 
 DEF_ENUM_FLAGS_TYPE (enum symfile_add_flag, symfile_add_flags);