flatpak-integrator: use more sophisticated code to pass around fds

the previous approach didn't pass stdout/err properly causing the
host->browser communication to fail while browser->host was working
fine.

BUG: 500000

flatpak-integrator/flatpak-host-wrapper

@@ -7,16 +7,25 @@
 
 set -eu
 
-args=$(printf \'%s\' "$1"
-shift
-printf ", '%s'" "$@"
-printf '\n')
+# This packs $1 and $2 into a single string for gdbus of the form "'$1', '$2'\n"
+args=$(printf "'%s', '%s'\n" "$1" "$2")
 
+# The descriptor madness needs some explaining:
+# We expect the forked gdbus to pick up three descriptors from us: 3, 4, and 5, respectively presenting our
+# stdin, stdout, and stderr. This is because we need our io set to be distinct from gdbus' io set so gdbus doesn't write
+# invalid data on the pipes and breaks stuff (e.g. it'd write the return value of the call but that is invalid nonesense to firefox).
+#
+# 3<&0: dup our input stdin (i.e. 0) to gdbus' fd 3
+# 4>&1: dup our output stdout (i.e. 1) to gdbus' fd 4
+# 5>&2: dup our output stderr (i.e. 2) to gdbus' fd 5
+# 1>/dev/null: redirect gdbus' stdout to /dev/null because we don't care about it
+#
+# gdbus will then send 3,4,5 into dbus, effectively forwarding our descriptors. We meanwhile ignore their stdout.
 gdbus call \
     --session \
     --dest org.kde.plasma.browser.integration \
     --object-path /org/kde/plasma/browser/integration \
     --method org.kde.plasma.browser.integration.FlatpakIntegrator.Link \
-    ["$args"] 0 1 2 > /dev/stderr
+    ["$args"] 3 4 5 3<&0 4>&1 5>&2 1>/dev/null
 
 sleep infinity