Re: warnings in unit tests

[Bruno Haible <bruno@clisp.org>]

Paul Eggert wrote:
> For what it's worth I'm more with Bruno on this. For the tests, the cost 
> of these warnings outweighs the benefit.
> 
> It'd be OK with me to disable the troublesome warnings globally for the 
> tests subdirectory, using -Wno-missing-prototypes or whatever.

Thanks for your backing, Paul.

It's not the actual warnings that I want to disable. It's more the
expectation that the Gnulib tests "should be warning-free" that is
to be corrected. Implemented through this:

1) Add a '-Wno-error' option that disables the '-Werror' from [1]
   that has no raison d'être.

2) A message that indicates that warnings can be ignored in this
   directory.

[1] https://git.savannah.gnu.org/gitweb/?p=grep.git;a=blob;f=gnulib-tests/Makefile.am;h=3085f6357b7ece8e68c93650bf409a7609079d19;hb=HEAD


2021-06-10  Bruno Haible  <bruno@clisp.org>

	Clarify that compiler warnings in the Gnulib tests can be ignored.
	* gnulib-tool (func_emit_tests_Makefile_am): Emit overrides for CFLAGS
	and CXXFLAGS. Emit a dependency of 'all' on 'all-notice' that prints a
	notice.
	(func_emit_initmacro_start): Add a second argument. If it is true, emit
	code to require gl_CC_ALLOW_WARNINGS and gl_CXX_ALLOW_WARNINGS.
	(func_import, func_create_testdir): All callers updated.
	* m4/gnulib-common.m4 (gl_CC_ALLOW_WARNINGS, gl_CXX_ALLOW_WARNINGS): New
	macros.

diff --git a/gnulib-tool b/gnulib-tool
index 237693a..d41d58c 100755
--- a/gnulib-tool
+++ b/gnulib-tool
@@ -4201,6 +4201,25 @@ func_emit_tests_Makefile_am ()
     fi
   done
   echo
+  # Insert a '-Wno-error' option in the compilation commands emitted by
+  # Automake, between $(AM_CPPFLAGS) and before the reference to @CFLAGS@.
+  # Why?
+  # 1) Because parts of the Gnulib tests exercise corner cases (invalid
+  #    arguments, endless recursions, etc.) that a compiler may warn about,
+  #    even with just the normal '-Wall' option.
+  # 2) Because every package maintainer has their preferred set of warnings
+  #    that they may want to enforce in the main source code of their package.
+  #    But Gnulib tests are maintained in Gnulib and don't end up in binaries
+  #    that that package installs; therefore it does not make sense for
+  #    package maintainers to enforce the absence of warnings on these tests.
+  # Why before @CFLAGS@?
+  # - Because "the user is always right": If a user adds '-Werror' to their
+  #   CFLAGS, they have asked for errors, they will get errors. But they have
+  #   no right to complain about these errors, because Gnulib does not support
+  #   '-Werror'.
+  echo "CFLAGS = @GL_CFLAG_ALLOW_WARNINGS@ @CFLAGS@"
+  echo "CXXFLAGS = @GL_CXXFLAG_ALLOW_WARNINGS@ @CXXFLAGS@"
+  echo
   echo "AM_CPPFLAGS = \\"
   if $for_test; then
     echo "  -DGNULIB_STRICT_CHECKING=1 \\"
@@ -4248,6 +4267,14 @@ func_emit_tests_Makefile_am ()
   echo
   cat "$tmp"/main_snippets "$tmp"/longrunning_snippets \
     | sed -e 's|\$(top_srcdir)/build-aux/|$(top_srcdir)/'"$auxdir"'/|g'
+  # Arrange to print a message before compiling the files in this directory.
+  echo "all: all-notice"
+  echo "all-notice:"
+  echo "	@echo '## ---------------------------------------------------- ##'"
+  echo "	@echo '## ------------------- Gnulib tests ------------------- ##'"
+  echo "	@echo '## You can ignore compiler warnings in this directory.  ##'"
+  echo "	@echo '## ---------------------------------------------------- ##'"
+  echo
   echo "# Clean up after Solaris cc."
   echo "clean-local:"
   echo "	rm -rf SunWS_cache"
@@ -4262,9 +4289,11 @@ func_emit_tests_Makefile_am ()
   rm -f "$tmp"/main_snippets "$tmp"/longrunning_snippets
 }
 
-# func_emit_initmacro_start macro_prefix
+# func_emit_initmacro_start macro_prefix gentests
 # emits the first few statements of the gl_INIT macro to standard output.
 # - macro_prefix             prefix of gl_EARLY, gl_INIT macros to use
+# - gentests                 true if a tests Makefile.am is being generated,
+#                            false otherwise
 # - module_indicator_prefix  prefix of GNULIB_<modulename> variables to use
 func_emit_initmacro_start ()
 {
@@ -4298,6 +4327,10 @@ func_emit_initmacro_start ()
   # Scope the GNULIB_<modulename> variables.
   echo "  m4_pushdef([GL_MODULE_INDICATOR_PREFIX], [${module_indicator_prefix}])"
   echo "  gl_COMMON"
+  if "$2"; then
+    echo "  AC_REQUIRE([gl_CC_ALLOW_WARNINGS])"
+    echo "  AC_REQUIRE([gl_CXX_ALLOW_WARNINGS])"
+  fi
 }
 
 # func_emit_initmacro_end macro_prefix
@@ -5778,7 +5811,7 @@ s,//*$,/,'
       sed_replace_build_aux="$sed_noop"
     fi
     echo "  gl_m4_base='$m4base'"
-    func_emit_initmacro_start $macro_prefix
+    func_emit_initmacro_start $macro_prefix false
     echo "  gl_source_base='$sourcebase'"
     if test -n "$witness_c_macro"; then
       echo "  m4_pushdef([gl_MODULE_INDICATOR_CONDITION], [$witness_c_macro])"
@@ -5791,7 +5824,7 @@ s,//*$,/,'
     func_emit_initmacro_end $macro_prefix
     echo "  gltests_libdeps="
     echo "  gltests_ltlibdeps="
-    func_emit_initmacro_start ${macro_prefix}tests
+    func_emit_initmacro_start ${macro_prefix}tests $gentests
     echo "  gl_source_base='$testsbase'"
     # Define a tests witness macro that depends on the package.
     # PACKAGE is defined by AM_INIT_AUTOMAKE, PACKAGE_TARNAME is defined by AC_INIT.
@@ -6436,7 +6469,7 @@ func_create_testdir ()
            ba
          }'
        echo "gl_m4_base='../$m4base'"
-       func_emit_initmacro_start $macro_prefix
+       func_emit_initmacro_start $macro_prefix true
        # We don't have explicit ordering constraints between the various
        # autoconf snippets. It's cleanest to put those of the library before
        # those of the tests.
@@ -6552,7 +6585,7 @@ func_create_testdir ()
      sed_replace_build_aux="$sed_noop"
    fi
    echo "gl_m4_base='$m4base'"
-   func_emit_initmacro_start $macro_prefix
+   func_emit_initmacro_start $macro_prefix false
    echo "gl_source_base='$sourcebase'"
    if $single_configure; then
      func_emit_autoconf_snippets "$main_modules" "$main_modules" func_verify_module true false false
@@ -6563,7 +6596,7 @@ func_create_testdir ()
    if $single_configure; then
      echo "  gltests_libdeps="
      echo "  gltests_ltlibdeps="
-     func_emit_initmacro_start ${macro_prefix}tests
+     func_emit_initmacro_start ${macro_prefix}tests true
      echo "  gl_source_base='$testsbase'"
      # Define a tests witness macro.
      echo "  ${macro_prefix}tests_WITNESS=IN_GNULIB_TESTS"
diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
index 7db4be1..bfa1645 100644
--- a/m4/gnulib-common.m4
+++ b/m4/gnulib-common.m4
@@ -1,4 +1,4 @@
-# gnulib-common.m4 serial 65
+# gnulib-common.m4 serial 66
 dnl Copyright (C) 2007-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -666,6 +666,72 @@ AC_DEFUN([gl_CACHE_VAL_SILENT],
   ])
 ])
 
+# gl_CC_ALLOW_WARNINGS
+# sets and substitutes a variable GL_CFLAG_ALLOW_WARNINGS, to a $(CC) option
+# that reverts a preceding '-Werror' option, if available.
+# This is expected to be '-Wno-error' on gcc, clang (except clang/MSVC), xlclang
+# and empty otherwise.
+AC_DEFUN([gl_CC_ALLOW_WARNINGS],
+[
+  AC_REQUIRE([AC_PROG_CC])
+  AC_CACHE_CHECK([for C compiler option to allow warnings],
+    [gl_cv_cc_wallow],
+    [rm -f conftest*
+     echo 'int dummy;' > conftest.c
+     AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS -c conftest.c 2>conftest1.err]) >/dev/null
+     AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS -Wno-error -c conftest.c 2>conftest2.err]) >/dev/null
+     dnl Test the number of error output lines, because AIX xlc accepts the
+     dnl option '-Wno-error', just to produce a warning
+     dnl "Option -Wno-error was incorrectly specified. The option will be ignored."
+     dnl afterwards.
+     if test $? = 0 && test `wc -l < conftest1.err` = `wc -l < conftest2.err`; then
+       gl_cv_cc_wallow='-Wno-error'
+     else
+       gl_cv_cc_wallow=none
+     fi
+     rm -f conftest*
+    ])
+  case "$gl_cv_cc_wallow" in
+    none) GL_CFLAG_ALLOW_WARNINGS='' ;;
+    *)    GL_CFLAG_ALLOW_WARNINGS="$gl_cv_cc_wallow" ;;
+  esac
+  AC_SUBST([GL_CFLAG_ALLOW_WARNINGS])
+])
+
+# gl_CXX_ALLOW_WARNINGS
+# sets and substitutes a variable GL_CXXFLAG_ALLOW_WARNINGS, to a $(CC) option
+# that reverts a preceding '-Werror' option, if available.
+AC_DEFUN([gl_CXX_ALLOW_WARNINGS],
+[
+  dnl Requires AC_PROG_CXX or gl_PROG_ANSI_CXX.
+  if test -n "$CXX" && test "$CXX" != no; then
+    AC_CACHE_CHECK([for C++ compiler option to allow warnings],
+      [gl_cv_cxx_wallow],
+      [rm -f conftest*
+       echo 'int dummy;' > conftest.cc
+       AC_TRY_COMMAND([${CXX-c++} $CXXFLAGS $CPPFLAGS -c conftest.cc 2>conftest1.err]) >/dev/null
+       AC_TRY_COMMAND([${CXX-c++} $CXXFLAGS $CPPFLAGS -Wno-error -c conftest.cc 2>conftest2.err]) >/dev/null
+       dnl Test the number of error output lines, because AIX xlC accepts the
+       dnl option '-Wno-error', just to produce a warning
+       dnl "Option -Wno-error was incorrectly specified. The option will be ignored."
+       dnl afterwards.
+       if test $? = 0 && test `wc -l < conftest1.err` = `wc -l < conftest2.err`; then
+         gl_cv_cxx_wallow='-Wno-error'
+       else
+         gl_cv_cxx_wallow=none
+       fi
+       rm -f conftest*
+      ])
+    case "$gl_cv_cxx_wallow" in
+      none) GL_CXXFLAG_ALLOW_WARNINGS='' ;;
+      *)    GL_CXXFLAG_ALLOW_WARNINGS="$gl_cv_cxx_wallow" ;;
+    esac
+  else
+    GL_CXXFLAG_ALLOW_WARNINGS=''
+  fi
+  AC_SUBST([GL_CXXFLAG_ALLOW_WARNINGS])
+])
+
 dnl Expands to some code for use in .c programs that, on native Windows, defines
 dnl the Microsoft deprecated alias function names to the underscore-prefixed
 dnl actual function names. With this macro, these function names are available