build: Select whether to include funopen() in the build system

This makes sure we include it when expected, alongside the man pages,
and the test cases, and do not accidentally break the ABI if the system
starts providing such interface.

configure.ac

@@ -297,6 +297,7 @@ AC_CHECK_FUNCS([\
   flock \
   fopencookie \
   __fpurge \
+  funopen \
   getauxval \
   getentropy \
   getexecname \
@@ -305,8 +306,31 @@ AC_CHECK_FUNCS([\
   pstat_getproc \
   sysconf \
 ])
+
+need_funopen=yes
+AS_CASE([$host_os],
+  [*-musl*], [
+    # On musl >= 1.1.19, fopencookie() got implemented, and because we were
+    # checking for its presence to decide whether to build funopen(), it got
+    # included in builds even when previously it had not been included, which
+    # is partially an ABI issue, but given that disabling it now would be
+    # worse, we'll ignore this as this is only a problem with downgrades. And
+    # enable it explicitly
+    need_funopen=yes
+  ],
+  [darwin*], [
+    # On macOS we do not have fopencookie(), and cannot implement it.
+    need_funopen=no
+  ],
+)
+
 AM_CONDITIONAL([HAVE_GETENTROPY], [test "x$ac_cv_func_getentropy" = "xyes"])
-AM_CONDITIONAL([HAVE_FOPENCOOKIE], [test "x$ac_cv_func_fopencookie" = "xyes"])
+AM_CONDITIONAL([NEED_FUNOPEN], [test "x$need_funopen" = "xyes"])
+AS_IF([test "x$need_funopen" = "xno" && \
+       test "x$ac_cv_func_funopen" != "xyes" && \
+       test "x$ac_cv_func_fopencookie" = "xyes"], [
+  AC_MSG_WARN([[can implement funopen() now based on newly added fopencooke(), report upstream]])
+])
 
 AC_SUBST([MD5_LIBS])
 AC_SUBST([LIBBSD_LIBS])

man/Makefile.am

@@ -178,7 +178,6 @@ dist_man_MANS = \
 	fmtcheck.3bsd \
 	fparseln.3bsd \
 	fpurge.3bsd \
-	funopen.3bsd \
 	getbsize.3bsd \
 	getmode.3bsd \
 	getpeereid.3bsd \
@@ -252,3 +251,9 @@ dist_man_MANS = \
 	wcslcat.3bsd \
 	wcslcpy.3bsd \
 	# EOL
+
+if NEED_FUNOPEN
+dist_man_MANS += \
+	funopen.3bsd \
+	# EOL
+endif

src/Makefile.am

@@ -95,7 +95,6 @@ libbsd_la_SOURCES = \
 	fmtcheck.c \
 	fparseln.c \
 	fpurge.c \
-	funopen.c \
 	getbsize.c \
 	getpeereid.c \
 	heapsort.c \
@@ -137,6 +136,12 @@ libbsd_la_SOURCES += \
 	# EOL
 endif
 
+if NEED_FUNOPEN
+libbsd_la_SOURCES += \
+	funopen.c \
+	# EOL
+endif
+
 if NEED_TRANSPARENT_LIBMD
 CLEANFILES += \
 	format.ld \

src/funopen.c

@@ -138,5 +138,5 @@ funopen(const void *cookie,
 	return fopencookie(cookiewrap, mode, funcswrap);
 }
 #else
-#warning "Function funopen() is not provided on this platform."
+#error "Function funopen() needs to be ported."
 #endif

test/Makefile.am

@@ -55,7 +55,7 @@ check_PROGRAMS = \
 	vis-openbsd \
 	# EOL
 
-if HAVE_FOPENCOOKIE
+if NEED_FUNOPEN
 check_PROGRAMS += funopen
 endif