[Bug build/17104] New: CFLAGS="-Wall -Wextra" gdb/confgure --with-babeltrace fails

["dilyan.palauzov at aegee dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=17104

            Bug ID: 17104
           Summary: CFLAGS="-Wall -Wextra" gdb/confgure --with-babeltrace
                    fails
           Product: gdb
           Version: 7.7
            Status: NEW
          Severity: normal
          Priority: P2
         Component: build
          Assignee: unassigned at sourceware dot org
          Reporter: dilyan.palauzov at aegee dot org

My config.site contains

export CFLAGS='-pipe -O3 -flto -Wl,-S -Wl,--hash-style=gnu -Wl,-O1 -Wl,-z,relro
-Wall -Wextra'

so when I run gdb-7.7.1/configure the CFLAGS are filled accordingly.  When I do
gdb-7.7.1/gdb/configure --with-babeltrace, the effectively executed code from
gdb/configure.ac is (as in gdb-7.7.1 and currently on master):

2401 if test "x$with_babeltrace" = "xno"; then
2402   AC_MSG_WARN([babletrace support disabled; GDB is unable to read CTF
data.])
2403 else
2404   # Append -Werror to CFLAGS so that configure can catch the warning
2405   # "assignment from incompatible pointer type", which is related to
2406   # the babeltrace change from 1.0.3 to 1.1.0.  Babeltrace 1.1.0 works
2407   # in GDB, while babeltrace 1.0.3 is broken.
2408   # AC_LIB_HAVE_LINKFLAGS may modify CPPFLAGS in it, so it should be
2409   # safe to save and restore CFLAGS here.
2410   saved_CFLAGS=$CFLAGS
2411   CFLAGS="$CFLAGS -Werror"
2412   AC_LIB_HAVE_LINKFLAGS([babeltrace], [babeltrace-ctf],
2413                         [#include <babeltrace/babeltrace.h>
2414                          #include <babeltrace/ctf/events.h>
2415                          #include <babeltrace/ctf/iterator.h>],
2416                         [struct bt_iter_pos *pos = bt_iter_get_pos
(bt_ctf_get_iter (NULL));
2417                         struct bt_ctf_event *event = NULL;
2418                         const struct bt_definition *scope;
2419
2420                         scope = bt_ctf_get_top_level_scope (event,
2421                                                           
BT_STREAM_EVENT_HEADER);
2422                         bt_ctf_get_uint64 (bt_ctf_get_field (event, scope,
"id"));
2423                         ])
2424   CFLAGS=$saved_CFLAGS

and config.log contains accordingly:

configure:14369: checking whether to use babeltrace
configure:14371: result: yes
configure:14796: checking for libbabeltrace
configure:14824: gcc -o conftest -pipe -O3 -flto -Wl,-S -Wl,--hash-style=gnu
-Wl,-O1 -Wl,-z,relro -Wall -Wextra -Werror   -L/usr/lib64 -L/lib64 conftest.c
-ldl -ldl -lncurses -lz -lm    /usr/lib64/libbabeltrace.so
/usr/lib64/libbabeltrace-ctf.so /usr/lib64/libpopt.so
/usr/lib64/libgmodule-2.0.so /lib64/libdl.so /usr/lib64/libglib-2.0.so
/usr/lib64/libiconv.so /usr/lib64/libpcre.so /lib64/libpthread.so
/usr/lib64/libuuid.so -Wl,-rpath -Wl,/usr/lib64 -Wl,-rpath -Wl,/lib64 >&5
conftest.c: In function 'main':
conftest.c:191:21: error: unused variable 'pos' [-Werror=unused-variable]
 struct bt_iter_pos *pos = bt_iter_get_pos (bt_ctf_get_iter (NULL));
                     ^
cc1: all warnings being treated as errors
configure:14824: $? = 1
configure: failed program was:


What I want to say is that CFLAGS at the end has -Wall -Wextra -Werror and
check fails, because the pos variable is unused, which is normally a warning,
but in this case also an error (-Werror).

Please ensure that running ./configure with CFLAGS='-Wall -Wextra' and gcc483
does not cause "error: unused variable 'pos' ", so that babeltrace is detected
correctly.

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