lib/util: Prefer backtrace_symbols() for internal backtraces

Backtraces when Samba is in PANIC state are better with
backtrace_symbols() than with libunwind on Ubuntu 20.04 x86_64
so move libunwind to a off-by-default option, prompted for
if backtrace_symbols() is not available.

Based on a request by Fco Javier Felix <ffelix@inode64.com>

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>

lib/util/fault.c

@@ -222,9 +222,13 @@ _PUBLIC_ void smb_panic(const char *why)
 void log_stack_trace(void)
 {
 #ifdef HAVE_LIBUNWIND
-	/* Try to use libunwind before any other technique since on ia64
-	 * libunwind correctly walks the stack in more circumstances than
-	 * backtrace.
+	/*
+	 * --with-libunwind is required to use libunwind, the
+	 * backtrace_symbols() code below is the default.
+	 *
+	 * This code is available because a previous version of this
+	 * comment asserted that on ia64 libunwind correctly walks the
+	 * stack in more circumstances than backtrace.
 	 */
 	unw_cursor_t cursor;
 	unw_context_t uc;

lib/util/wscript

@@ -2,6 +2,15 @@ def options(opt):
     ''' This is a bit strange, but disable is the flag, not enable. '''
     opt.add_option('--disable-fault-handling', action='store_true', dest='disable_fault_handling', help=('disable the fault handlers'), default=False)
 
+    # We do not want libunwind by default (backtrace_symbols() in
+    # glibc is better) but allow (eg) IA-64 to build with it where it
+    # might be better (per old comment in fault.c)
+    opt.samba_add_onoff_option('libunwind',
+                               default=None,
+                               help='''Use libunwind instead of the default backtrace_symbols()
+                               from libc, for example on IA-64 where it might give a better
+                               backtrace.''')
+
     opt.add_option('--with-systemd',
                    help=("Enable systemd integration"),
                    action='store_true', dest='enable_systemd')

lib/util/wscript_configure

@@ -1,23 +1,35 @@
 #!/usr/bin/env python
-from waflib import Logs, Options
+from waflib import Logs, Options, Errors
 
 import os, sys
 
 if Options.options.disable_fault_handling:
     conf.DEFINE('HAVE_DISABLE_FAULT_HANDLING',1)
 
-# backtrace could be in libexecinfo or in libc
+# backtrace could be in libexecinfo or in libc.
+# This is our preferred backtrace handler (more useful output than libunwind as at Ubuntu 20.04 x86_64)
 conf.CHECK_FUNCS_IN('backtrace backtrace_symbols', 'execinfo', checklibc=True, headers='execinfo.h')
 conf.CHECK_HEADERS('execinfo.h')
 
 conf.SET_TARGET_TYPE('LIBUNWIND', 'EMPTY')
-if conf.check_cfg(package='libunwind-generic',
-                  args='--cflags --libs',
-                  msg='Checking for libunwind',
-                  uselib_store='LIBUNWIND',
-                  mandatory=False):
-    if conf.CHECK_HEADERS('libunwind.h'):
-        conf.SET_TARGET_TYPE('LIBUNWIND', 'SYSLIB')
+if Options.options.with_libunwind:
+    if conf.check_cfg(package='libunwind-generic',
+                      args='--cflags --libs',
+                      msg='Checking for libunwind',
+                      uselib_store='LIBUNWIND',
+                      mandatory=False):
+        if conf.CHECK_HEADERS('libunwind.h'):
+            conf.SET_TARGET_TYPE('LIBUNWIND', 'SYSLIB')
+    else:
+        raise Errors.WafError('--with-libunwind specified but libunwind not found')
+elif Options.options.with_libunwind == None:
+    if not conf.CONFIG_SET('HAVE_BACKTRACE_SYMBOLS') \
+       and not Options.options.disable_fault_handling:
+        raise Errors.WafError(
+'''backtrace_symbols() not found but
+--with-libunwind not specified.
+Use --without-libunwind to build without internal backtrace support or
+--disable-fault-handling to totally defer fault handling to the OS.''')
 
 conf.CHECK_STRUCTURE_MEMBER('struct statvfs', 'f_frsize', define='HAVE_FRSIZE', headers='sys/statvfs.h')
 

script/autobuild.py

@@ -480,10 +480,11 @@ tasks = {
     # MIT Kerberos from the current system.  Runtime behaviour is
     # confirmed via the ktest (static ccache and keytab) environment
 
+    # This environment also used to confirm we can still build with --with-libunwind
     "samba-ktest-mit": {
         "sequence": [
             ("random-sleep", random_sleep(300, 900)),
-            ("configure", "./configure.developer --without-ad-dc --with-system-mitkrb5 " + samba_configure_params),
+            ("configure", "./configure.developer --without-ad-dc --with-libunwind --with-system-mitkrb5 " + samba_configure_params),
             ("make", "make -j"),
             ("test", make_test(include_envs=[
             "ktest", # ktest is also tested in fileserver, samba and