build: gate htssafe pointer-dest regressions with -Werror=attribute-warning

The strcpybuff/strcatbuff/strncatbuff family in htssafe.h silently degrades
to an unchecked strcpy/strcat/strncat when the destination is a bare char*
(capacity unknown); the pointer-destination stubs carry a 'warning' function
attribute to flag every such call. The migration that converted those 241
sites is complete, so any new char* destination is a regression.

Promote that attribute to a hard error in our own build via
-Werror=attribute-warning (gcc) / -Werror=user-defined-warnings (clang),
probed with AX_CHECK_COMPILE_FLAG so each compiler picks up only the spelling
it accepts. htssafe.h is unchanged, so downstream consumers of the installed
header still see a plain warning rather than a build break.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

configure

@@ -15541,6 +15541,91 @@ else case e in #(
 esac
 fi
 
+# Make htssafe.h's pointer-dest 'warning' attribute a hard error in our build
+# (migration is at zero; a new char* dest is a regression). gcc/clang each take
+# only their own spelling; downstream keeps the plain warning, not a build break.
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether C compiler accepts -Werror=attribute-warning" >&5
+printf %s "checking whether C compiler accepts -Werror=attribute-warning... " >&6; }
+if test ${ax_cv_check_cflags___Werror_attribute_warning+y}
+then :
+  printf %s "(cached) " >&6
+else case e in #(
+  e)
+  ax_check_save_flags=$CFLAGS
+  CFLAGS="$CFLAGS  -Werror=attribute-warning"
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main (void)
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"
+then :
+  ax_cv_check_cflags___Werror_attribute_warning=yes
+else case e in #(
+  e) ax_cv_check_cflags___Werror_attribute_warning=no ;;
+esac
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.beam conftest.$ac_ext
+  CFLAGS=$ax_check_save_flags ;;
+esac
+fi
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $ax_cv_check_cflags___Werror_attribute_warning" >&5
+printf "%s\n" "$ax_cv_check_cflags___Werror_attribute_warning" >&6; }
+if test "x$ax_cv_check_cflags___Werror_attribute_warning" = xyes
+then :
+  DEFAULT_CFLAGS="$DEFAULT_CFLAGS -Werror=attribute-warning"
+else case e in #(
+  e) : ;;
+esac
+fi
+
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether C compiler accepts -Werror=user-defined-warnings" >&5
+printf %s "checking whether C compiler accepts -Werror=user-defined-warnings... " >&6; }
+if test ${ax_cv_check_cflags___Werror_user_defined_warnings+y}
+then :
+  printf %s "(cached) " >&6
+else case e in #(
+  e)
+  ax_check_save_flags=$CFLAGS
+  CFLAGS="$CFLAGS  -Werror=user-defined-warnings"
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main (void)
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"
+then :
+  ax_cv_check_cflags___Werror_user_defined_warnings=yes
+else case e in #(
+  e) ax_cv_check_cflags___Werror_user_defined_warnings=no ;;
+esac
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.beam conftest.$ac_ext
+  CFLAGS=$ax_check_save_flags ;;
+esac
+fi
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $ax_cv_check_cflags___Werror_user_defined_warnings" >&5
+printf "%s\n" "$ax_cv_check_cflags___Werror_user_defined_warnings" >&6; }
+if test "x$ax_cv_check_cflags___Werror_user_defined_warnings" = xyes
+then :
+  DEFAULT_CFLAGS="$DEFAULT_CFLAGS -Werror=user-defined-warnings"
+else case e in #(
+  e) : ;;
+esac
+fi
+
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether C compiler accepts -fstrict-aliasing -Wstrict-aliasing" >&5
 printf %s "checking whether C compiler accepts -fstrict-aliasing -Wstrict-aliasing... " >&6; }
 if test ${ax_cv_check_cflags___fstrict_aliasing__Wstrict_aliasing+y}

configure.ac

@@ -84,6 +84,11 @@ AX_CHECK_COMPILE_FLAG([-Wformat-nonliteral], [DEFAULT_CFLAGS="$DEFAULT_CFLAGS -W
 AX_CHECK_COMPILE_FLAG([-Wmissing-parameter-type], [DEFAULT_CFLAGS="$DEFAULT_CFLAGS -Wmissing-parameter-type"])
 AX_CHECK_COMPILE_FLAG([-Wold-style-definition], [DEFAULT_CFLAGS="$DEFAULT_CFLAGS -Wold-style-definition"])
 AX_CHECK_COMPILE_FLAG([-Wignored-qualifiers], [DEFAULT_CFLAGS="$DEFAULT_CFLAGS -Wignored-qualifiers"])
+# Make htssafe.h's pointer-dest 'warning' attribute a hard error in our build
+# (migration is at zero; a new char* dest is a regression). gcc/clang each take
+# only their own spelling; downstream keeps the plain warning, not a build break.
+AX_CHECK_COMPILE_FLAG([-Werror=attribute-warning], [DEFAULT_CFLAGS="$DEFAULT_CFLAGS -Werror=attribute-warning"])
+AX_CHECK_COMPILE_FLAG([-Werror=user-defined-warnings], [DEFAULT_CFLAGS="$DEFAULT_CFLAGS -Werror=user-defined-warnings"])
 AX_CHECK_COMPILE_FLAG([-fstrict-aliasing -Wstrict-aliasing], [DEFAULT_CFLAGS="$DEFAULT_CFLAGS -fstrict-aliasing -Wstrict-aliasing"])
 AX_CHECK_COMPILE_FLAG([-fstack-protector], [DEFAULT_CFLAGS="$DEFAULT_CFLAGS -fstack-protector"])
 AX_CHECK_COMPILE_FLAG([-fstack-clash-protection], [DEFAULT_CFLAGS="$DEFAULT_CFLAGS -fstack-clash-protection"])

src/htsserver.c

@@ -1783,7 +1783,7 @@ static int LANG_LIST(const char *path, char *buffer, size_t buffer_size) {
   buffer[0] = '\0';
   do {
     QLANG_T(i);
-    strlcpybuff(lang_str, "LANGUAGE_NAME", buffer_size);
+    strlcpybuff(lang_str, "LANGUAGE_NAME", sizeof(lang_str));
     htslang_load(lang_str, sizeof(lang_str), path);
     if (strlen(lang_str) > 0) {
       if (buffer[0])