sysupdate: don't double-prefix definitions with --root=

Definitions enumerated under --root= are already rooted. Passing those
paths to the config parsers with the same root prefixes the root again,
so feature and transfer files are parsed from the wrong path.

Repro: create root/etc/sysupdate.d/rootfeat.feature and
01-root.transfer, then run:
build/systemd-sysupdate --root="$root" --verify=no --offline features rootfeat

Before: parsing failed at line 1 with a bogus Source Type= error.

Fixes #42783.
Follow-up for: e1384cfb09

src/basic/conf-files.c

@@ -102,6 +102,9 @@ static int conf_file_prefix_root(ConfFile *c, const char *root, ConfFilesFlags f
 
         assert(c);
 
+        if (FLAGS_SET(flags, CONF_FILES_DONT_PREFIX_ROOT))
+                return 0;
+
         int log_level = conf_files_log_level(flags);
 
         r = chaseat_prefix_root(c->result, root, &p);

src/sysupdate/sysupdate.c

@@ -255,7 +255,7 @@ static int read_features(
         r = conf_files_list_strv_full(
                         ".feature",
                         c->root,
-                        CONF_FILES_REGULAR|CONF_FILES_FILTER_MASKED|CONF_FILES_WARN,
+                        CONF_FILES_REGULAR|CONF_FILES_FILTER_MASKED|CONF_FILES_WARN|CONF_FILES_DONT_PREFIX_ROOT,
                         dirs,
                         &files,
                         &n_files);
@@ -303,7 +303,7 @@ static int read_transfers(
         assert(suffix);
 
         r = conf_files_list_strv_full(suffix, c->root,
-                                      CONF_FILES_REGULAR|CONF_FILES_FILTER_MASKED|CONF_FILES_WARN,
+                                      CONF_FILES_REGULAR|CONF_FILES_FILTER_MASKED|CONF_FILES_WARN|CONF_FILES_DONT_PREFIX_ROOT,
                                       dirs, &files, &n_files);
         if (r < 0)
                 return log_error_errno(r, "Failed to enumerate sysupdate.d/*%s definitions: %m", suffix);

test/units/TEST-72-SYSUPDATE.sh

@@ -698,6 +698,39 @@ cmp "$WORKDIR/source/tiny-v1.bin" "$WORKDIR/blobs/tiny-v1.bin"
 "$SYSUPDATE" features |& grep "No features." >/dev/null
 [[ $(varlinkctl call "$VARLINK_SOCKET" io.systemd.SysUpdate.ListFeatures '{"target":{"class":"host"}}' | jq -r '.features') == "[]" ]]
 
+# Test that definitions found through --root= are not prefixed twice.
+rm -rf "$WORKDIR/root-definitions"
+mkdir -p "$WORKDIR/root-definitions/etc/sysupdate.d" \
+         "$WORKDIR/root-definitions/etc" \
+         "$WORKDIR/root-definitions/source" \
+         "$WORKDIR/root-definitions/target"
+printf '%s\n' 'ID=repro' 'VERSION_ID=1' >"$WORKDIR/root-definitions/etc/os-release"
+cat >"$WORKDIR/root-definitions/etc/sysupdate.d/rootfeat.feature" <<EOF
+[Feature]
+Description=Root Feature
+EOF
+cat >"$WORKDIR/root-definitions/etc/sysupdate.d/01-root.transfer" <<EOF
+[Transfer]
+
+[Source]
+Type=regular-file
+Path=/source
+MatchPattern=root-@v.bin
+
+[Target]
+Type=regular-file
+Path=/target
+MatchPattern=root-@v.bin
+InstancesMax=2
+EOF
+"$SYSUPDATE" \
+    --root="$WORKDIR/root-definitions" \
+    --verify=no \
+    --offline \
+    features rootfeat \
+    | grep -F "Root Feature" >/dev/null
+rm -rf "$WORKDIR/root-definitions"
+
 # Cleanup
 rm "$CONFIGDIR/01-tiny-url.transfer"
 rm "$WORKDIR/source/tiny-v1.bin"