From 9bef473d17ec01efe760462271791dfaaea280c0 Mon Sep 17 00:00:00 2001
From: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Date: Wed, 29 Jul 2015 22:35:19 +0200
Subject: [PATCH 1/3] configure: refactor backtrace() detection

The current code assumes that if __GLIBC__ is not defined, backtrace
support is not available, and defines BTRFS_DISABLE_BACKTRACE.

However, this macro is already defined by the configure.ac script when
--disable-backtrace is passed. This means that if you are using a C
library like musl which does not define __GLIBC__, and you pass
--disable-backtrace, you get a macro redefinition.

Instead of relying on __GLIBC__, this commit implements a proper
configure.ac based detection of backtrace support:

 * If the user passes --enable-backtrace, we check if the backtrace()
   function is available. If not, we abort the configure process with
   an error. Otherwise we enable backtrace support by defining
   HAVE_BACKTRACE.

 * If the user passes --disable-backtrace, then we don't enable
   backtrace support.

 * If the user passes nothing special, we auto-detect: if backtrace()
   is available, we use it, otherwise we simply warn at configure time
   but not fail.

Upstream-status: pending
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Brendan Heading <brendanheading@gmail.com>
---
 configure.ac | 32 ++++++++++++++++++++++----------
 kerncompat.h |  9 ++++-----
 2 files changed, 26 insertions(+), 15 deletions(-)

diff --git a/configure.ac b/configure.ac
index c3a22d1..e936a10 100644
--- a/configure.ac
+++ b/configure.ac
@@ -63,22 +63,34 @@ AC_DEFUN([PKG_STATIC], [
   fi
 ])
 
-
+# Provide a --{enable,disable}-backtrace option. If not passed, set
+# enable_backtrace to 'auto' so that we try to enable backtrace
+# support if available, but if not available, we gracefully fallback
+# without backtrace support.
 AC_ARG_ENABLE([backtrace],
   AS_HELP_STRING([--disable-backtrace], [disable btrfs backtrace]),
-  [], [enable_backtrace=yes]
+  [enable_backtrace=${enableval}], [enable_backtrace=auto]
 )
 
-AS_IF([test "x$enable_backtrace" = xno], [
-  AC_DEFINE([BTRFS_DISABLE_BACKTRACE], [1], [disable backtrace stuff in kerncompat.h ])
-])
-
-if test "x$enable_backtrace" = xyes; then
-	AC_CHECK_HEADERS([execinfo.h])
-	AC_CHECK_FUNCS([backtrace backtrace_symbols_fd], [],
-	      AC_MSG_ERROR([standard library does not have backtrace support]))
+# Backtrace support requested (enable_backtrace is either 'yes' or
+# 'auto'), so check for needed headers and functions.
+if test "x$enable_backtrace" != xno; then
+   AC_CHECK_HEADERS([execinfo.h])
+   AC_CHECK_FUNCS([backtrace backtrace_symbols_fd])
 fi
 
+if test "x$ac_cv_func_backtrace" = xno; then
+    # If backtrace support was requested but not available, we fail if
+    # --enable-backtrace was passed, or we simply warn if we're
+    # auto-detecting.
+    if test "x$enable_backtrace" = xyes ; then
+	AC_MSG_ERROR([standard library does not have backtrace support])
+    elif test "x$enable_backtrace" = xauto ; then
+        AC_MSG_WARN([standard library does not have backtrace support, disabled])
+    fi
+else
+    AC_DEFINE([HAVE_BACKTRACE], [1], [Enable backtrace support])
+fi
 
 AC_ARG_ENABLE([documentation],
 	      AS_HELP_STRING([--disable-documentation], [do not build domumentation]),
diff --git a/kerncompat.h b/kerncompat.h
index 5d92856..8318665 100644
--- a/kerncompat.h
+++ b/kerncompat.h
@@ -33,11 +33,10 @@
 #include <features.h>
 
 #ifndef __GLIBC__
-#define BTRFS_DISABLE_BACKTRACE
 #define __always_inline __inline __attribute__ ((__always_inline__))
 #endif
 
-#ifndef BTRFS_DISABLE_BACKTRACE
+#ifdef HAVE_BACKTRACE
 #include <execinfo.h>
 #endif
 
@@ -65,7 +64,7 @@
 #define ULONG_MAX       (~0UL)
 #endif
 
-#ifndef BTRFS_DISABLE_BACKTRACE
+#ifdef HAVE_BACKTRACE
 #define MAX_BACKTRACE	16
 static inline void print_trace(void)
 {
@@ -285,7 +284,7 @@ static inline long IS_ERR(const void *ptr)
 #define vmalloc(x) malloc(x)
 #define vfree(x) free(x)
 
-#ifndef BTRFS_DISABLE_BACKTRACE
+#ifdef HAVE_BACKTRACE
 #define BUG_ON(c) assert_trace(#c, __FILE__, __func__, __LINE__, !(c))
 #else
 #define BUG_ON(c) assert(!(c))
@@ -293,7 +292,7 @@ static inline long IS_ERR(const void *ptr)
 
 #define WARN_ON(c) BUG_ON(c)
 
-#ifndef BTRFS_DISABLE_BACKTRACE
+#ifdef HAVE_BACKTRACE
 #define	ASSERT(c) assert_trace(#c, __FILE__, __func__, __LINE__, (c))
 #else
 #define ASSERT(c) assert(c)
-- 
2.4.3