ansi-color: fix stack overflow with debug level and invalid SYSTEMD_COLORS env var

When SYSTEMD_COLORS is invalid, parse_systemd_colors() logs about it.
Logging helpers then call into parse_systemd_colors() to pretty-print
the log message, which then fails, so it logs about the failure,
rinse and repeat until segfault.

Follow-up for c8210d98a4b64af6fadb1cb765c0451758af1303

mkosi/mkosi.conf

@@ -14,7 +14,11 @@ PassEnvironment=
         NO_BUILD
         WIPE
         SANITIZERS
+        CC
+        CXX
         CFLAGS
+        CPPFLAGS
+        CXXFLAGS
         LDFLAGS
         LLVM
         MESON_VERBOSE

mkosi/mkosi.conf.d/opensuse/mkosi.conf

@@ -57,6 +57,7 @@ Packages=
         knot
         libapparmor1
         libcap-progs
+        libtss2-tcti-device0
         multipath-tools
         ncat
         open-iscsi

mkosi/mkosi.images/build/mkosi.conf.d/arch/mkosi.build.chroot

@@ -36,10 +36,10 @@ fi
 # instead. The rootfs is overlaid with a writable tmpfs during the build script so these changes don't end up
 # in the image itself.
 tee --append /etc/makepkg.conf >/dev/null <<EOF
-export CC="$( ((LLVM)) && echo clang || echo gcc)"
-export CXX="$( ((LLVM)) && echo clang++ || echo g++)"
-export CC_LD="$( ((LLVM)) && echo lld)"
-export CXX_LD="$( ((LLVM)) && echo lld)"
+export CC="$( ((CC)) && echo "$CC" || ((LLVM)) && echo clang || echo gcc)"
+export CXX="$( ((CXX)) && echo "$CXX" || ((LLVM)) && echo clang++ || echo g++)"
+export CC_LD="$( ((LD)) && echo "$LD" || ((LLVM)) && echo lld)"
+export CXX_LD="$( ((LD)) && echo "$LD" || ((LLVM)) && echo lld)"
 export CFLAGS="\$CFLAGS $MKOSI_CFLAGS $CFLAGS"
 export CXXFLAGS="\$CXXFLAGS $MKOSI_CFLAGS $CFLAGS"
 export LDFLAGS="\$LDFLAGS $MKOSI_LDFLAGS $LDFLAGS"

mkosi/mkosi.images/build/mkosi.conf.d/debian-ubuntu/mkosi.build.chroot

@@ -67,12 +67,14 @@ DEB_BUILD_PROFILES="$(awk '$1=$1' <<<"\
 ")"
 
 # TODO: Drop GENSYMBOLS_LEVEL once https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=986746 is fixed.
+# Note that CXXFLAGS is for the C++ compiler, and CPPFLAGS is for the C/C++ preprocessor, they are not the same:
+# https://en.wikipedia.org/wiki/CFLAGS
 build() {
     env \
-    CC="$( ((LLVM)) && echo clang || echo gcc)" \
-    CXX="$( ((LLVM)) && echo clang++ || echo g++)" \
-    CC_LD="$( ((LLVM)) && echo lld)" \
-    CXX_LD="$( ((LLVM)) && echo lld)" \
+    CC="$( ((CC)) && echo "$CC" || ((LLVM)) && echo clang || echo gcc)" \
+    CXX="$( ((CXX)) && echo "$CXX" || ((LLVM)) && echo clang++ || echo g++)" \
+    CC_LD="$( ((LD)) && echo "$LD" || ((LLVM)) && echo lld)" \
+    CXX_LD="$( ((LD)) && echo "$LD" || ((LLVM)) && echo lld)" \
     DEB_BUILD_OPTIONS="$(awk '$1=$1' <<<"\
         $( ((WITH_TESTS)) || echo nocheck) \
         $( ((WITH_DOCS)) || echo nodoc) \
@@ -84,6 +86,7 @@ build() {
     DEB_BUILD_PROFILES="$DEB_BUILD_PROFILES" \
     DEB_CFLAGS_APPEND="$MKOSI_CFLAGS $CFLAGS" \
     DEB_CXXFLAGS_APPEND="$MKOSI_CFLAGS $CFLAGS" \
+    DEB_CPPFLAGS_APPEND="$CPPFLAGS" \
     DEB_LDFLAGS_APPEND="$MKOSI_LDFLAGS $LDFLAGS" \
     DPKG_FORCE="unsafe-io" \
     DPKG_DEB_COMPRESSOR_TYPE="none" \

mkosi/mkosi.images/build/mkosi.conf.d/opensuse/mkosi.build.chroot

@@ -86,10 +86,10 @@ build() {
     --unset CFLAGS \
     --unset CXXFLAGS \
     --unset LDFLAGS \
-    CC="$( ((LLVM)) && echo clang || echo gcc)" \
-    CXX="$( ((LLVM)) && echo clang++ || echo g++)" \
-    CC_LD="$( ((LLVM)) && echo lld)" \
-    CXX_LD="$( ((LLVM)) && echo lld)" \
+    CC="$( ((CC)) && echo "$CC" || ((LLVM)) && echo clang || echo gcc)" \
+    CXX="$( ((CXX)) && echo "$CXX" || ((LLVM)) && echo clang++ || echo g++)" \
+    CC_LD="$( ((LD)) && echo "$LD" || ((LLVM)) && echo lld)" \
+    CXX_LD="$( ((LD)) && echo "$LD" || ((LLVM)) && echo lld)" \
     rpmbuild \
         -bb \
         --noprep \

src/basic/ansi-color.c

@@ -36,18 +36,20 @@ void reset_ansi_feature_caches(void) {
 ColorMode parse_systemd_colors(void) {
         const char *e;
 
+        /* Note: do not log in this function, to avoid infinite recursion issues, as the log functions call
+         * this when deciding whether to color the output. */
+
         e = getenv("SYSTEMD_COLORS");
         if (!e)
                 return _COLOR_MODE_INVALID;
 
-        ColorMode m = color_mode_from_string(e);
-        if (m < 0)
-                return log_debug_errno(m, "Failed to parse $SYSTEMD_COLORS value '%s', ignoring: %m", e);
-
-        return m;
+        return color_mode_from_string(e);
 }
 
 static ColorMode get_color_mode_impl(void) {
+        /* Note: do not log in this function, to avoid infinite recursion issues, as the log functions call
+         * this when deciding whether to color the output. */
+
         /* Returns the mode used to choose output colors. The possible modes are COLOR_OFF for no colors,
          * COLOR_16 for only the base 16 ANSI colors, COLOR_256 for more colors, and COLOR_24BIT for
          * unrestricted color output. */