Add support for the readnever concept

The purpose of this concept is to turn the load of debugging
information off, either globally (via the '--readnever' option), or
objfile-specific.  The implementation proposed here is an extension of
the patch distributed with Fedora GDB; looking at the Fedora patch
itself and the history, one can see some reasons why it was never
resubmitted:

  - The patch appears to have been introduced as a workaround, at
    least initially;
  - The patch is far from perfect, as it simply shunts the load of
    DWARF debugging information, without really worrying about the
    other debug format.
  - Who really does non-symbolic debugging anyways?

One use of this feature is when a user simply wants to do the
following sequence: attach, dump core, detach.  Loading the debugging
information in this case is an unnecessary cause of delay.

This patch expands the version shipped with Fedora GDB in order to
make the feature available for all the debuginfo backends, not only
for DWARF.  It also implements a per-objfile flag which can be
activated by using the "-readnever" command when using the
'add-symbol-file' or 'symbol-file' commands.

It's also worth mentioning that this patch tests whether GDB correctly
fails to initialize if both '--readnow' and '--readnever' options are
passed.

Tested on the BuildBot.

gdb/ChangeLog:

2017-12-01  Andrew Cagney  <cagney@redhat.com>
	    Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	* NEWS (Changes since GDB 8.0: Mention new '--readnever'
	feature.
	* coffread.c (coff_symfile_read): Do not map over sections with
	'coff_locate_sections' if readnever is on.
	* dwarf2read.c (dwarf2_has_info): Return 0 if
	readnever is on.
	* elfread.c (elf_symfile_read): Do not map over sections with
	'elf_locate_sections' if readnever is on.
	* main.c (validate_readnow_readnever): New function.
	(captured_main_1): Add support for --readnever.
	(print_gdb_help): Document --readnever.
	* objfile-flags.h (enum objfile_flag) <OBJF_READNEVER>: New
	flag.
	* symfile.c (readnever_symbol_files): New global.
	(symbol_file_add_with_addrs): Set 'OBJF_READNEVER' when
	'READNEVER_SYMBOL_FILES' is set.
	(validate_readnow_readnever): New function.
	(symbol_file_command): Handle '-readnever' option.
	Call 'validate_readnow_readnever'.
	(add_symbol_file_command): Handle '-readnever' option.
	Call 'validate_readnow_readnever'.
	(_initialize_symfile): Document new '-readnever' option for
	both 'symbol-file' and 'add-symbol-file' commands.
	* top.h (readnever_symbol_files): New extern global.
	* xcoffread.c (xcoff_initial_scan): Do not read debug
	information if readnever is on.

gdb/doc/ChangeLog:

2017-12-01  Andrew Cagney  <cagney@redhat.com>
	    Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (File Options): Document --readnever.
	(Commands to Specify Files): Likewise, for 'symbol-file' and
	'add-symbol-file'.

gdb/testsuite/ChangeLog:

2017-12-01  Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb.base/readnever.c, gdb.base/readnever.exp: New files.

1 files changed, 37 insertions, 10 deletions

diff --git a/gdb/symfile.c b/gdb/symfile.c
index 4bbe0b5..36b34e6 100644
--- a/gdb/symfile.c
+++ b/gdb/symfile.c
@@ -81,6 +81,7 @@ static void clear_symtab_users_cleanup (void *ignore);
 
 /* Global variables owned by this file.  */
 int readnow_symbol_files;	/* Read full symbols immediately.  */
+int readnever_symbol_files;	/* Never read full symbols.  */
 
 /* Functions this file defines.  */
 
@@ -1131,6 +1132,12 @@ symbol_file_add_with_addrs (bfd *abfd, const char *name,
       flags |= OBJF_READNOW;
       add_flags &= ~SYMFILE_NO_READ;
     }
+  else if (readnever_symbol_files
+	   || (parent != NULL && (parent->flags & OBJF_READNEVER)))
+    {
+      flags |= OBJF_READNEVER;
+      add_flags |= SYMFILE_NO_READ;
+    }
 
   /* Give user a chance to burp if we'd be
      interactively wiping out any existing symbols.  */
@@ -1594,6 +1601,16 @@ find_separate_debug_file_by_debuglink (struct objfile *objfile)
   return debugfile;
 }
 
+/* Make sure that OBJF_{READNOW,READNEVER} are not set
+   simultaneously.  */
+
+static void
+validate_readnow_readnever (objfile_flags flags)
+{
+  if ((flags & OBJF_READNOW) && (flags & OBJF_READNEVER))
+    error (_("-readnow and -readnever cannot be used simultaneously"));
+}
+
 /* This is the symbol-file command.  Read the file, analyze its
    symbols, and add a struct symtab to a symtab list.  The syntax of
    the command is rather bizarre:
@@ -1631,17 +1648,20 @@ symbol_file_command (const char *args, int from_tty)
 	{
 	  if (strcmp (arg, "-readnow") == 0)
 	    flags |= OBJF_READNOW;
+	  else if (strcmp (arg, "-readnever") == 0)
+	    flags |= OBJF_READNEVER;
 	  else if (*arg == '-')
 	    error (_("unknown option `%s'"), arg);
 	  else
-	    {
-	      symbol_file_add_main_1 (arg, add_flags, flags);
-	      name = arg;
-	    }
+	    name = arg;
 	}
 
       if (name == NULL)
 	error (_("no symbol file name was specified"));
+
+      validate_readnow_readnever (flags);
+
+      symbol_file_add_main_1 (name, add_flags, flags);
     }
 }
 
@@ -2238,6 +2258,8 @@ add_symbol_file_command (const char *args, int from_tty)
 	    }
 	  else if (strcmp (arg, "-readnow") == 0)
 	    flags |= OBJF_READNOW;
+	  else if (strcmp (arg, "-readnever") == 0)
+	    flags |= OBJF_READNEVER;
 	  else if (strcmp (arg, "-s") == 0)
 	    {
 	      expecting_sec_name = 1;
@@ -2248,6 +2270,8 @@ add_symbol_file_command (const char *args, int from_tty)
 	}
     }
 
+  validate_readnow_readnever (flags);
+
   /* This command takes at least two arguments.  The first one is a
      filename, and the second is the address where this file has been
      loaded.  Abort now if this address hasn't been provided by the
@@ -3878,26 +3902,29 @@ _initialize_symfile (void)
 
   observer_attach_free_objfile (symfile_free_objfile);
 
-#define READNOW_HELP \
+#define READNOW_READNEVER_HELP \
   "The '-readnow' option will cause GDB to read the entire symbol file\n\
 immediately.  This makes the command slower, but may make future operations\n\
-faster."
+faster.\n\
+The '-readnever' option will prevent GDB from reading the symbol file's\n\
+symbolic debug information."
 
   c = add_cmd ("symbol-file", class_files, symbol_file_command, _("\
 Load symbol table from executable file FILE.\n\
-Usage: symbol-file [-readnow] FILE\n\
+Usage: symbol-file [-readnow | -readnever] FILE\n\
 The `file' command can also load symbol tables, as well as setting the file\n\
-to execute.\n" READNOW_HELP), &cmdlist);
+to execute.\n" READNOW_READNEVER_HELP), &cmdlist);
   set_cmd_completer (c, filename_completer);
 
   c = add_cmd ("add-symbol-file", class_files, add_symbol_file_command, _("\
 Load symbols from FILE, assuming FILE has been dynamically loaded.\n\
-Usage: add-symbol-file FILE ADDR [-readnow | -s SECT-NAME SECT-ADDR]...\n\
+Usage: add-symbol-file FILE ADDR [-readnow | -readnever | \
+-s SECT-NAME SECT-ADDR]...\n\
 ADDR is the starting address of the file's text.\n\
 Each '-s' argument provides a section name and address, and\n\
 should be specified if the data and bss segments are not contiguous\n\
 with the text.  SECT-NAME is a section name to be loaded at SECT-ADDR.\n"
-READNOW_HELP),
+READNOW_READNEVER_HELP),
 	       &cmdlist);
   set_cmd_completer (c, filename_completer);