configure: checks for headers and -fno-rtti

BUILD

@@ -36,18 +36,34 @@ this file by hand, before proceeding with the build. Note that the the generated
 a symbolic link to a system-specific default config file. It is not necessary to edit the file; you
 can override variables on the "make" command line if you prefer.
 
+You can also create and edit the "mconfig" file completely by hand (or start by copying one for a
+particular OS from the "configs" directory) to choose appropriate values for the configuration
+variables defined within.
+
+
+"configure" script
+=-=-=-=-=-=-=-=-=-
+
 An alternative to "make mconfig" is to use the provided "configure" script. It will try to
 generate a suitable "mconfig" file, based on sensible defaults and options provided on the command
-line when the script is run.
+line when the script is run. It also has checks for the current system environment to
+enable/disable certain features based on the availability of requirements (such as headers).
+
+The "configure" also checks for usability of the "-fno-rtti" compiler flag. This is achieved by
+testing the flag itself and running a test source code to see if it works correctly. However,
+because of the nature of cross-compiling, it's not possible to compile and run the test source
+code on the building machine, and instead, "configure" disables "-fno-rtti" in this case and prints
+out this warning:
+
+    Cross-compilation detected. Disabling -fno-rtti to make sure exceptions will always work.
+
+See "Special note for run-time type information", below, for more information about the "-fno-rtti"
+flag.
 
 For more information on available options from the configure script, run:
 
     ./configure --help
 
-You can also create and edit the "mconfig" file completely by hand (or start by copying one for a
-particular OS from the "configs" directory) to choose appropriate values for the configuration
-variables defined within.
-
 
 Main build variables
 =-=-=-=-=-=-=-=-=-=-
@@ -140,9 +156,8 @@ Dinit should generally build fine with no additional options, other than:
 
 Recommended options, supported by at least GCC and Clang, are:
  -Os       : optimise for size
- -fno-rtti : disable RTTI (run-time type information), it is not required by Dinit. However, on
-             some platforms such as Mac OS (and historically FreeBSD, IIRC), this prevents
-             exceptions working correctly.
+ -fno-rtti : see "Special note for run-time type information", below. May cause Dinit to crash on
+             errors when compiled with Clang+libcxxrt on some platforms like FreeBSD.
  -fno-plt  : enables better code generation for non-static builds, but may cause unit test
              failures on some older versions of FreeBSD (eg 11.2-RELEASE-p4 with clang++ 6.0.0).
  -flto     : perform link-time optimisation (option required at compile and link).
@@ -292,3 +307,26 @@ upgrading GCC. If you have libstdc++ corresponding to GCC 5.x or 6.x, you *must*
 old ABI, but Dinit will be broken if you upgrade to GCC 7. If you have libstdc++ from GCC 7, you
 *must* build with the new ABI. If the wrong ABI is used, Dinit may still run successfully but any
 attempt to load a non-existing service, for example, will cause Dinit to crash.
+
+
+Special note for run-time type information
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+
+RTTI, or run-time type information, is a feature of C++ that exposes information about an object's
+data type at runtime. This can be used for "dynamic_cast<>" and to manipulate type information at
+runtime using the "typeid" operator. Exception handling also uses the same info.
+
+GCC and Clang provide a compiler flag ("-fno-rtti") which would disable RTTI generation. However
+GCC would still generate RTTI for exception handling and therefore "-fno-rtti" is a viable option
+for saving output binary size.
+
+Clang on some platforms (such as FreeBSD) would break handling of certain exceptions when using
+"-fno-rtti" and this will result in Dinit crashing when an error occurs (such as when loading a
+non-existing service).
+
+Details regarding the issue with Clang can be found here:
+
+    https://github.com/llvm/llvm-project/issues/66117
+
+To avoid Dinit crashes when using Clang+libcxxrt it's recommended to not use the "-fno-rtti" flag.
+Clang+libcxxrt is the default compiler+runtime-library on platforms like FreeBSD and macOS.

configure

@@ -83,6 +83,34 @@ cxx_works()
     fi
 }
 
+# Test if the compiler successfully compiles the specified source file. If the optional compilation
+# argument is specified ($3) then pass it to the compiler along with the already-established
+# arguments (this can be used to test if the compiler accepts the argument).
+#   $1 - the name of the source file to compile
+#   $2 - the name of the result object file
+#   $3 - the argument to test (optional)
+#   CXX - the name of the compiler
+#   CXXFLAGS, CXXFLAGS_EXTRA - any predetermined flags
+try_compile_source()
+{
+    eval $CXX $CXXFLAGS ${3:-} $CXXFLAGS_EXTRA '"$configtmpdir"/"$1"' \
+            -c -o '"$configtmpdir"/"$2"' > /dev/null 2>&1
+}
+
+# Test if the specified object file can be linked into an executable. If the optional link argument
+# is specified ($3) then pass it to the compiler driver along with the already-established
+# arguments (this can be used to test if the linker accepts the argument).
+#   $1 - the name of the object file to link
+#   $2 - the name of the result executable file
+#   $3 - the argument to test (optional)
+#   CXX - the name of the compiler
+#   LDFLAGS, LDFLAGS_EXTRA - any predetermined flags
+try_link_object()
+{
+    eval $CXX $LDFLAGS $LDFLAGS_EXTRA ${3:-} '"$configtmpdir"/"$1"' \
+            -o '"$configtmpdir"/"$2"' > /dev/null 2>&1
+}
+
 # Test if the compiler accepts an argument (for compilation only); if so add it to named variable.
 # Note: this function will return failure if the flag is unsupported (use try_optional_cxx_argument
 # instead, to avoid errorring out).
@@ -93,8 +121,7 @@ cxx_works()
 try_cxx_argument()
 {
     info Checking whether "$2" is accepted for compilation...
-    if eval $CXX $CXXFLAGS $2 $CXXFLAGS_EXTRA $LDFLAGS $LDFLAGS_EXTRA '"$configtmpdir"/testfile.cc' \
-            -c -o '"$configtmpdir"/testfile.o' > /dev/null 2>&1; then
+    if try_compile_source testfile.cc testfile.o "$2"; then
         rm "$configtmpdir"/testfile.o
         sub_info Yes.
         eval "$1=\"\${$1} \$2\""
@@ -127,14 +154,12 @@ try_ld_argument()
     info Checking whether "$2" is accepted as a link-time option...
     if [ ! -e "$configtmpdir"/testfile.o ]; then
         # If the object file doesn't exist yet, need to compile
-        if ! eval $CXX $CXXFLAGS $CXXFLAGS_EXTRA '"$configtmpdir"/testfile.cc' \
-                -c -o '"$configtmpdir"/testfile.o' > /dev/null 2>&1; then
+        if ! try_compile_source testfile.cc testfile.o; then
             sub_info "No (compilation failed)."
             return
         fi
     fi
-    if eval $CXX $LDFLAGS $LDFLAGS_EXTRA $2 '"$configtmpdir"/testfile.o' \
-            -o '"$configtmpdir"/testfile' > /dev/null 2>&1; then
+    if try_link_object testfile.o testfile "$2"; then
         sub_info Yes.
         rm "$configtmpdir"/testfile
         eval "$1=\"\${$1} \$2\""
@@ -145,27 +170,71 @@ try_ld_argument()
 }
 
 # Test if an argument is supported during both compiling and linking.
-#   $1 - the name of the compiler-flags variable to potentially add the argument to
-#   $2 - the name of the link-flags variable to potentially add the argument to
+#   $1 - the name of the compiler-flags variable to potentially add the argument to (or specify the
+#        dash ("-") to skip adding to the variable)
+#   $2 - the name of the link-flags variable to potentially add the argument to (or specify the
+#        dash ("-") to skip adding to the variable)
 #   $3 - the argument to test/add
 #   CXX - the name of the compiler
 #   CXXFLAGS, CXXFLAGS_EXTRA, LDFLAGS, LDFLAGS_EXTRA - any predetermined flags
 try_both_argument()
 {
     info Checking whether "$3" is accepted for compiling and linking...
-    if eval $CXX $CXXFLAGS $CXXFLAGS_EXTRA $LDFLAGS $LDFLAGS_EXTRA $3 "$configtmpdir"/testfile.cc \
-            -o "$configtmpdir"/testfile > /dev/null 2>&1; then
+    if try_compile_source testfile.cc testfile.o "$3" && try_link_object testfile.o testfile "$3"; then
         sub_info Yes.
+        rm "$configtmpdir"/testfile.o
         rm "$configtmpdir"/testfile
-        eval "$1=\"\${$1} \$3\""
-        eval "$1=\${$1# }"
-        eval "$2=\"\${$2} \$3\""
-        eval "$2=\${$2# }"
+        if [ "$1" != "-" ]; then
+            eval "$1=\"\${$1} \$3\""
+            eval "$1=\${$1# }"
+        fi
+        if [ "$2" != "-" ]; then
+            eval "$2=\"\${$2} \$3\""
+            eval "$2=\${$2# }"
+        fi
     else
         sub_info No.
     fi
 }
 
+# Test if the specified header is usable.
+#   $1 - the name of the variable to set to 1 or 0 based on header availability
+#   $2 - the header path
+check_header()
+{
+    info Checking whether "$2" header is available...
+    echo "#include <$2>" > "$configtmpdir/testheader.cc"
+    if try_compile_source testheader.cc testheader.o; then
+        sub_info "Yes."
+        rm "$configtmpdir"/testheader.o
+        eval "$1=1"
+    else
+        sub_info "No."
+        eval "$1=0"
+    fi
+}
+
+# Write the test code for testing exception handling with -fno-rtti
+write_no_rtti_check()
+{
+    cat << _EOF > "$configtmpdir"/no-rtti-check.cc
+#include <string>
+#include <stdexcept>
+
+int main()
+{
+    try {
+        std::stoull("invalid for stoull");
+    }
+    catch (std::invalid_argument &exc) {
+        return 0;
+    }
+
+    return -1;
+}
+_EOF
+}
+
 
 
 # Quote a string to preserve its original form after shell evaluation (except that an empty
@@ -399,7 +468,7 @@ if [ "$PLATFORM" = "Linux" ]; then
     : "${BUILD_SHUTDOWN:="yes"}"
     : "${SUPPORT_CGROUPS:="1"}"
     : "${SUPPORT_CAPABILITIES:="AUTO"}"
-    : "${SUPPORT_IOPRIO:="0"}"    # the required header <linux/ioprio.h> may not be available
+    : "${SUPPORT_IOPRIO:="AUTO"}"
     : "${SUPPORT_OOM_ADJ:="1"}"
     : "${SYSCONTROLSOCKET:="/run/dinitctl"}"
 else
@@ -501,20 +570,37 @@ if [ "$AUTO_CXXFLAGS" = "true" ]; then
     do
         try_optional_cxx_argument CXXFLAGS $argument
     done
-    if [ "$PLATFORM" != "Darwin" ]; then
-        try_optional_cxx_argument CXXFLAGS -fno-rtti
+
+    # -fno-rtti is a special case. Clang+libcxxrt is known for generating broken code
+    # when disabling run-time type information with code which uses exceptions.
+    info Checking whether -fno-rtti is accepted for compilation...
+    if [ -n "${CXX_FOR_BUILD:-}" ]; then
+        # May not be able to execute the check when cross-compiling.
+        sub_info Cross-compilation detected. Disabling -fno-rtti to make sure exceptions will always work.
+    else
+        write_no_rtti_check
+        if try_compile_source no-rtti-check.cc no-rtti-check.o -fno-rtti; then
+            sub_info "Yes."
+            info Checking whether -fno-rtti breaks exceptions...
+            if try_link_object no-rtti-check.o no-rtti-check && "$configtmpdir"/no-rtti-check; then
+                sub_info "No."
+                CXXFLAGS="$CXXFLAGS -fno-rtti"
+            else
+                sub_info "Yes. Disabling -fno-rtti"
+            fi
+        else
+            sub_info "No."
+        fi
     fi
 fi
 if [ "$AUTO_LDFLAGS" = true ] && [ "$AUTO_CXXFLAGS" = true ]; then
-    DUMMY_LDFLAGS=""
     # -flto must work for both compiling and linking, but we don't want to add it to LDFLAGS as,
     # if LTO is used, CXXFLAGS will always be used alongside LDFLAGS.
-    if try_both_argument CXXFLAGS DUMMY_LDFLAGS -flto; then
+    if try_both_argument CXXFLAGS - -flto; then
         HAS_LTO="true"
     else
         HAS_LTO="false"
     fi
-    unset DUMMY_LDFLAGS
 fi
 if [ "$AUTO_LDFLAGS_BASE" = true ] && [ "$PLATFORM" = FreeBSD ]; then
     try_ld_argument LDFLAGS_BASE -lrt
@@ -537,6 +623,10 @@ else
     LDFLAGS_LIBCAP=""
 fi
 
+if [ "$SUPPORT_IOPRIO" = "AUTO" ]; then
+    check_header SUPPORT_IOPRIO "linux/ioprio.h"
+fi
+
 # "XXX_MK" variables are for values already quoted for use in makefiles. The following will now be
 # set: TEST_LDFLAGS_BASE_MK, LDFLAGS_MK, TEST_LDFLAGS_MK, TEST_CXXFLAGS_MK. For other build
 # variables, we'll just do the quoting when we write the value to mconfig.
@@ -588,18 +678,16 @@ fi
 
 # Check whether sanitizers can be used for tests
 if [ "$AUTO_TEST_LDFLAGS" = "true" ] && [ "$AUTO_TEST_CXXFLAGS" = "true" ]; then
-    DUMMY_LDFLAGS=""
     if [ "$HAS_LTO" = true ]; then
         # Avoid doubling-up sanitizer options
-        LDFLAGS_VAR=DUMMY_LDFLAGS
+        LDFLAGS_VAR=-
     else
         LDFLAGS_VAR=TEST_LDFLAGS_MK
     fi
     # (Note that we append directly to TEST_CXXFLAGS_MK/TEST_LDFLAGS_MK here)
     CXXFLAGS="$established_TEST_CXXFLAGS" CXXFLAGS_EXTRA="$TEST_CXXFLAGS_EXTRA" \
             LDFLAGS="$established_TEST_LDFLAGS" LDFLAGS_EXTRA="$TEST_LDFLAGS_EXTRA" \
             try_both_argument TEST_CXXFLAGS_MK $LDFLAGS_VAR -fsanitize=address,undefined
-    unset DUMMY_LDFLAGS
 fi
 
 ## Create mconfig