PR 20569, segv in follow_exec

The following testcases make GDB crash whenever an invalid sysroot is
provided, when GDB is unable to find a valid path to the symbol file:

 gdb.base/catch-syscall.exp
 gdb.base/execl-update-breakpoints.exp
 gdb.base/foll-exec-mode.exp
 gdb.base/foll-exec.exp
 gdb.base/foll-vfork.exp
 gdb.base/pie-execl.exp
 gdb.multi/bkpt-multi-exec.exp
 gdb.python/py-finish-breakpoint.exp
 gdb.threads/execl.exp
 gdb.threads/non-ldr-exc-1.exp
 gdb.threads/non-ldr-exc-2.exp
 gdb.threads/non-ldr-exc-3.exp
 gdb.threads/non-ldr-exc-4.exp
 gdb.threads/thread-execl.exp

The immediate cause of the segv is that follow_exec is passing a NULL
argument (the result of exec_file_find) to strlen.

However, the problem is deeper than that: follow_exec simply isn't
prepared for the case where sysroot translation fails to locate the
new executable.  Actually all callers of exec_file_find have bugs due
to confusion between host and target pathnames.  This commit attempts
to fix all that.

In terms of the testcases that were formerly segv'ing, GDB now prints
a warning but continues execution of the new program, so that the
tests now mostly FAIL instead.  You could argue the FAILs are due to a
legitimate problem with the test environment setting up the sysroot
translation incorrectly.

A new representative test is added which exercises the ne wwarning
code path even with native testing.

Tested on x86_64 Fedora 23, native and gdbserver.

gdb/ChangeLog:
2016-10-25  Sandra Loosemore  <sandra@codesourcery.com>
	    Luis Machado  <lgustavo@codesourcery.com>
	    Pedro Alves  <palves@redhat.com>

	PR gdb/20569
	* exceptions.c (exception_print_same): Moved here from exec.c.
	* exceptions.h (exception_print_same): Declare.
	* exec.h: Include "symfile-add-flags.h".
	(try_open_exec_file): New declaration.
	* exec.c (exception_print_same): Moved to exceptions.c.
	(try_open_exec_file): New function.
	(exec_file_locate_attach): Rename exec_file and full_exec_path
	variables to avoid confusion between target and host pathnames.
	Move pathname processing logic to exec_file_find.  Do not return
	early if pathname lookup fails; Call try_open_exec_file.
	* infrun.c (follow_exec): Split and rename execd_pathname variable
	to avoid confusion between target and host pathnames.  Warn if
	pathname lookup fails.  Pass target pathname to
	target_follow_exec, not hostpathname.  Call try_open_exec_file.
	* main.c (symbol_file_add_main_adapter): New function.
	(captured_main_1): Use it.
	* solib-svr4.c (open_symbol_file_object): Adjust to pass
	symfile_add_flags to symbol_file_add_main.
	* solib.c (exec_file_find): Incorporate fallback logic for relative
	pathnames formerly in exec_file_locate_attach.
	* symfile.c (symbol_file_add_main, symbol_file_add_main_1):
	Replace 'from_tty' parameter with a symfile_add_file.
	(symbol_file_command): Adjust to pass symfile_add_flags to
	symbol_file_add_main.
	* symfile.h (symbol_file_add_main): Replace 'from_tty' parameter
	with a symfile_add_file.

gdb/testsuite/ChangeLog:
2016-10-25  Luis Machado  <lgustavo@codesourcery.com>

	* gdb.base/exec-invalid-sysroot.exp: New file.

gdb/exceptions.c

@@ -256,3 +256,21 @@ catch_errors (catch_errors_ftype *func, void *func_args, char *errstring,
     return 0;
   return val;
 }
+
+/* See exceptions.h.  */
+
+int
+exception_print_same (struct gdb_exception e1, struct gdb_exception e2)
+{
+  const char *msg1 = e1.message;
+  const char *msg2 = e2.message;
+
+  if (msg1 == NULL)
+    msg1 = "";
+  if (msg2 == NULL)
+    msg2 = "";
+
+  return (e1.reason == e2.reason
+	  && e1.error == e2.error
+	  && strcmp (msg1, msg2) == 0);
+}