main/postmarketos-usb-moded: use a generator to configure UID/GID for umtprd

MTP transfers write files to the home folder of the user.
If no GID and UID are specified, uMTP-responder will use the GID/UID
under which it is running at the time of the file transfer.
This causes users to not be able to access the files on the device after transfering.

This patch adds a systemd generator that:

1) reads /etc/default_user, and appends config to
   /etc/umtprd/umtprd.conf to set the MTP default user/group. If this
   file doesn't exist, 10000:10000 is the fallback uid:gid.
2) installs this config to /run/umtprd/umptrd.conf
3) adds a unit override to /run for umtprd.service that runs the daemon
   with this modified config

The generator is used instead of hardcoding a uid and gid in umtprd.conf
so that this can support immutable installs or situations where the
default UID/GID is not 10000.

Co-authored-by: Dylan Van Assche <dylan@postmarketos.org>
Signed-off-by: Clayton Craft <craftyguy@postmarketos.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7658>

.ci/shellcheck.sh

@@ -21,6 +21,7 @@ sh_files="
 	./main/postmarketos-initramfs/init_functions.sh
 	./main/postmarketos-mkinitfs-hook-netboot/netboot.sh
 	./main/postmarketos-ui-os-installer/rootfs-usr-bin-pmos_setup.sh
+	./main/postmarketos-usb-moded/rootfs-usr-lib-systemd-system-generators-umtprd-config-generator
 	./main/ttyescape/*.post-install
 	./main/unl0kr/unlock.sh
 	./device/community/soc-qcom/call_audio_idle_suspend_workaround.sh

main/postmarketos-usb-moded/APKBUILD

@@ -1,6 +1,6 @@
 # Maintainer: Clayton Craft <clayton@craftyguy.net>
 pkgname=postmarketos-usb-moded
-pkgver=1
+pkgver=2
 pkgrel=0
 pkgdesc="Meta package for configuring usb-moded on postmarketOS"
 url="https://postmarketos.org"
@@ -31,6 +31,7 @@ _source644="
 _source755="
 	usr/bin/usb-moded-developer-mode
 	usr/bin/usb-moded-tethering-mode
+	usr/lib/systemd/system-generators/umtprd-config-generator
 "
 
 # Avoid filename based checksum conflicts by including the whole path.
@@ -99,4 +100,5 @@ af41dc4ceb7255dbb88273916306c6e7e66c879bb67523c9571f9e812c9b36ab00ea666ade49369c
 79dc304cc5d455135f929a0ba60487f03b1b66d09245d972f685ee5ddabc8414f74159e47bbb13bff862163fa07dd9497a30ab7e32074bff95f2189685d851a2  rootfs-usr-lib-systemd-system-usb-moded.service.d-override.conf
 9aab1a91a817e2337320ee0fa1bf6133e65e9269631560c1860c3b777adb336d036cebbc2510d1df35a82bf4396d553ff9e2edd0004027c7a3130168f7f50d90  rootfs-usr-bin-usb-moded-developer-mode
 5cc40b1a0c86904d824f8f03e3f44900aaf407c5c462902f4f58cd58c603be1c5919651b91320983e21da42f5891e29df504bf4f1bddef64fedc68c2accdccbc  rootfs-usr-bin-usb-moded-tethering-mode
+ca4b82b0cccbbb498b8898c4e360142136c46687e97bce85553dc1793bf8ae2e0f509c2a1acdf96b08dc7c6b69f098b56181f9033ad669cabbf5a159ad8efbaa  rootfs-usr-lib-systemd-system-generators-umtprd-config-generator
 "

main/postmarketos-usb-moded/rootfs-usr-lib-systemd-system-generators-umtprd-config-generator

@@ -0,0 +1,39 @@
+#!/bin/sh
+# systemd generator for uMTP-Responder configuration
+# Generates config with user UID/GID in /run
+
+set -e
+
+late_dir="$3"
+
+# fallback UID/GID
+uid=10000
+gid=10000
+
+user_file="/etc/default_user"
+if [ -f "$user_file" ]; then
+    user=$(cat "$user_file")
+    if [ -n "$user" ]; then
+        # Get UID/GID, fall back to defaults on failure
+        uid=$(id -u "$user" 2>/dev/null || echo 10000)
+        gid=$(id -g "$user" 2>/dev/null || echo 10000)
+    fi
+fi
+
+mkdir -p /run/umtprd
+
+cat /etc/umtprd/umtprd.conf > /run/umtprd/umtprd.conf
+cat << EOF >> /run/umtprd/umtprd.conf
+
+# GID/UID for file operations (generated)
+default_uid $uid
+default_gid $gid
+EOF
+
+# unit override to use generated config
+mkdir -p "${late_dir}/umtprd.service.d"
+cat << EOF > "${late_dir}/umtprd.service.d/10-config-path.conf"
+[Service]
+ExecStart=
+ExecStart=/usr/bin/umtprd -conf /run/umtprd/umtprd.conf
+EOF