units: symlink well-known Varlink services into /run/varlink/registry/

So far we didn't provide any concept to enumerate local Varlink
services. Let's change that.

Let's define very light-weight scheme for this: provide a well-known dir
/run/varlink/registry/ where services that implement public interfaces
can link their sockets into. When enumerating services it's thus
sufficient to enumerate inodes in that directory.

The usecase for this is twofold:

1. It's simply very useful to be able to see which public services are
   bound on the local system, for debugging/admin/development purposes.

2. At Amutable we'd like to optionally provide a HTTP-to-Varlink bridge
   on individual nodes, that allows remote peers (after authentication)
   to access local Varlink services. For that it's essential we know the
   list of services and their entrypoints to expose, it would be
   security-wise highly problematic for clients to provide AF_UNIX
   entrypoint paths when connecting. hence: let's instead just have a
   dir with the public stuff, and let's ensure the HTTP-to-Varlink
   bridge simply exposes that stuff, and nothing else.

Non-public interfaces (such as the oomd interfaces between PID 1 and
oomd), and interfaces with multiple implementors (such as the resolved
hook interface, or the metrics collection stuff) should not be linked
in.

This is inspired by the Varlink.org "registry" concept, briefly
explained here:

https://varlink.org/FAQ#how-do-i-find-the-service-which-implements-a-local-interface

Note however that the described Varlink interface is not actually
implemented here, the directory is introduced however in a fashion that
conceptually matches the registry defined there, and would allow us to
implement the registry interface on top of it. (One of the reason the
registry Varlink API is not implemented right now is that the URI format
it relies on is entirely unspecified in the Varlink docs right now. Some
research needs to be done to extract what's implemented in the reference
implementation and to determine how it maps to the Varlink entrypoint
address format systemd's own tooling currently uses)

This primarily installs the symlinks via Symlinks= in unit files and via
a new tmpfiles.d/ drop-in. But since we touch all .socket units relating
to Varlink this also sets the FileDescriptorName= to varlink for each,
just to minimize diffrences and make things work more alike (the
services in questin don't care about the name, so this doesn't change).
In one case we replace a pair of separate sockets for two closely
related varlink services by a socket and a symlink, so that we can
safely use Symlinks= to also install the registry symlinks.

tmpfiles.d/20-systemd-varlink.conf

@@ -0,0 +1,18 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# See tmpfiles.d(5) for details.
+
+# Varlink AF_UNIX entrypoint socket inodes of relevant system services may be
+# linked into this directory, to make them "well-known". Typically, tools such
+# as the Varlink HTTP bridge expose services linked in here as public
+# interfaces. The symlink should be named after the Varlink interface to
+# expose. Interfaces that may be implemented by multiple services (such as
+# the generic "io.systemd.service") should not be symlinked here.
+d /run/varlink/registry/ 0755 root root
+
+# Socket activated services should use Symlinks= in the .socket unit file to
+# create these symlinks. If that's not applicable, consider creating the
+# symlinks via a tmpfiles.d/ snippet, like we do here, so that registration can
+# be influenced by the administrator.
+L /run/varlink/registry/io.systemd.Unit - - - - ../../systemd/io.systemd.Manager
+L /run/varlink/registry/io.systemd.Manager - - - - ../../systemd/io.systemd.Manager
+L /run/varlink/registry/io.systemd.Journal - - - - ../../systemd/journal/io.systemd.journal

tmpfiles.d/meson.build

@@ -13,6 +13,7 @@ files = [['README'                                  ],
          ['systemd-nspawn.conf',  'ENABLE_MACHINED' ],
          ['systemd-pstore.conf',  'ENABLE_PSTORE'   ],
          ['systemd-resolve.conf', 'ENABLE_RESOLVE'  ],
+         ['20-systemd-varlink.conf'                 ],
          ['systemd-tmp.conf'                        ],
          ['tmp.conf'                                ],
          ['x11.conf'                                ],
@@ -59,3 +60,5 @@ endforeach
 if install_sysconfdir
         install_emptydir(sysconfdir / 'tmpfiles.d')
 endif
+
+subdir('user')

tmpfiles.d/user/20-systemd-varlink.conf

@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# See tmpfiles.d(5) for details.
+
+d %t/varlink/registry/ 0755
+
+L %t/varlink/registry/io.systemd.Unit - - - - ../../systemd/io.systemd.Manager
+L %t/varlink/registry/io.systemd.Manager - - - - ../../systemd/io.systemd.Manager

tmpfiles.d/user/meson.build

@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+files = [ '20-systemd-varlink.conf' ]
+
+foreach f : files
+        install_data(f, install_dir : usertmpfilesdir)
+endforeach
+
+if install_sysconfdir
+        install_emptydir(sysconfdir / 'user-tmpfiles.d')
+endif

units/systemd-ask-password.socket

@@ -15,6 +15,7 @@ Before=sockets.target
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.AskPassword
+Symlinks=/run/varlink/registry/io.systemd.AskPassword
 FileDescriptorName=varlink
 SocketMode=0666
 Accept=yes

units/systemd-bootctl.socket

@@ -16,6 +16,7 @@ Before=sockets.target
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.BootControl
+Symlinks=/run/varlink/registry/io.systemd.BootControl
 FileDescriptorName=varlink
 SocketMode=0600
 Accept=yes

units/systemd-creds.socket

@@ -15,6 +15,7 @@ Before=sockets.target
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.Credentials
+Symlinks=/run/varlink/registry/io.systemd.Credentials
 FileDescriptorName=varlink
 SocketMode=0666
 Accept=yes

units/systemd-factory-reset.socket

@@ -15,6 +15,7 @@ Before=sockets.target
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.FactoryReset
+Symlinks=/run/varlink/registry/io.systemd.FactoryReset
 FileDescriptorName=varlink
 SocketMode=0666
 Accept=yes

units/systemd-hostnamed.socket

@@ -15,5 +15,6 @@ Documentation=man:machine-info(5)
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.Hostname
+Symlinks=/run/varlink/registry/io.systemd.Hostname
 FileDescriptorName=varlink
 SocketMode=0666

units/systemd-importd.socket

@@ -20,5 +20,6 @@ Before=shutdown.target
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.Import
+Symlinks=/run/varlink/registry/io.systemd.Import
 FileDescriptorName=varlink
 SocketMode=0666

units/systemd-logind-varlink.socket

@@ -13,6 +13,7 @@ Documentation=man:systemd-logind.service(8)
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.Login
+Symlinks=/run/varlink/registry/io.systemd.Login
 FileDescriptorName=varlink
 SocketMode=0666
 Service=systemd-logind.service

units/systemd-machined.socket

@@ -13,6 +13,6 @@ Documentation=man:systemd-machined.service(8)
 
 [Socket]
 ListenStream=/run/systemd/machine/io.systemd.Machine
-ListenStream=/run/systemd/machine/io.systemd.MachineImage
+Symlinks=/run/systemd/machine/io.systemd.MachineImage /run/varlink/registry/io.systemd.Machine /run/varlink/registry/io.systemd.MachineImage
 FileDescriptorName=varlink
 SocketMode=0666

units/systemd-mountfsd.socket

@@ -16,6 +16,8 @@ Before=sockets.target shutdown.target
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.MountFileSystem
+Symlinks=/run/varlink/registry/io.systemd.MountFileSystem
+FileDescriptorName=varlink
 SocketMode=0666
 
 [Install]

units/systemd-mute-console.socket

@@ -17,6 +17,7 @@ Before=shutdown.target
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.MuteConsole
+Symlinks=/run/varlink/registry/io.systemd.MuteConsole
 FileDescriptorName=varlink
 SocketMode=0600
 Accept=yes

units/systemd-networkd-varlink.socket

@@ -17,6 +17,7 @@ Conflicts=shutdown.target
 
 [Socket]
 ListenStream=/run/systemd/netif/io.systemd.Network
+Symlinks=/run/varlink/registry/io.systemd.Network
 FileDescriptorName=varlink
 SocketMode=0666
 Service=systemd-networkd.service

units/systemd-nsresourced.socket

@@ -16,7 +16,8 @@ Before=sockets.target shutdown.target
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.NamespaceResource
-Symlinks=/run/systemd/userdb/io.systemd.NamespaceResource
+Symlinks=/run/systemd/userdb/io.systemd.NamespaceResource /run/varlink/registry/io.systemd.NamespaceResource
+FileDescriptorName=varlink
 SocketMode=0666
 
 [Install]

units/systemd-pcrextend.socket

@@ -17,6 +17,7 @@ ConditionSecurity=measured-uki
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.PCRExtend
+Symlinks=/run/varlink/registry/io.systemd.PCRExtend
 FileDescriptorName=varlink
 SocketMode=0600
 Accept=yes

units/systemd-pcrlock.socket

@@ -17,6 +17,7 @@ ConditionSecurity=measured-uki
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.PCRLock
+Symlinks=/run/varlink/registry/io.systemd.PCRLock
 FileDescriptorName=varlink
 SocketMode=0600
 Accept=yes

units/systemd-repart.socket

@@ -17,6 +17,7 @@ Before=shutdown.target
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.Repart
+Symlinks=/run/varlink/registry/io.systemd.Repart
 FileDescriptorName=varlink
 SocketMode=0600
 Accept=yes

units/systemd-resolved-monitor.socket

@@ -17,6 +17,7 @@ Conflicts=shutdown.target
 [Socket]
 Service=systemd-resolved.service
 ListenStream=/run/systemd/resolve/io.systemd.Resolve.Monitor
+Symlinks=/run/varlink/registry/io.systemd.Resolve.Monitor
 FileDescriptorName=varlink-monitor
 SocketMode=0666
 

units/systemd-resolved-varlink.socket

@@ -17,6 +17,7 @@ Conflicts=shutdown.target
 [Socket]
 Service=systemd-resolved.service
 ListenStream=/run/systemd/resolve/io.systemd.Resolve
+Symlinks=/run/varlink/registry/io.systemd.Resolve
 FileDescriptorName=varlink
 SocketMode=0666
 

units/systemd-sysext.socket

@@ -17,6 +17,7 @@ ConditionCapability=CAP_SYS_ADMIN
 
 [Socket]
 ListenStream=/run/systemd/io.systemd.sysext
+Symlinks=/run/varlink/registry/io.systemd.sysext
 FileDescriptorName=varlink
 SocketMode=0666
 Accept=yes

units/systemd-udevd-varlink.socket

@@ -17,6 +17,7 @@ ConditionPathIsReadWrite=/sys
 [Socket]
 Service=systemd-udevd.service
 ListenStream=/run/udev/io.systemd.Udev
+Symlinks=/run/varlink/registry/io.systemd.Udev
 FileDescriptorName=varlink
 SocketMode=0600
 RemoveOnStop=yes

units/systemd-userdbd.socket

@@ -15,7 +15,8 @@ Before=sockets.target
 
 [Socket]
 ListenStream=/run/systemd/userdb/io.systemd.Multiplexer
-Symlinks=/run/systemd/userdb/io.systemd.NameServiceSwitch /run/systemd/userdb/io.systemd.DropIn
+Symlinks=/run/systemd/userdb/io.systemd.NameServiceSwitch /run/systemd/userdb/io.systemd.DropIn /run/varlink/registry/io.systemd.UserDatabase
+FileDescriptorName=varlink
 SocketMode=0666
 RemoveOnStop=yes
 

units/user/systemd-ask-password.socket

@@ -15,6 +15,7 @@ Before=sockets.target
 
 [Socket]
 ListenStream=%t/systemd/io.systemd.AskPassword
+Symlinks=%t/varlink/registry/io.systemd.AskPassword
 FileDescriptorName=varlink
 SocketMode=0600
 Accept=yes

units/user/systemd-importd.socket

@@ -14,5 +14,6 @@ Documentation=man:org.freedesktop.import1(5)
 
 [Socket]
 ListenStream=%t/systemd/io.systemd.Import
+Symlinks=%t/varlink/registry/io.systemd.Import
 FileDescriptorName=varlink
 SocketMode=0600

units/user/systemd-machined.socket

@@ -13,6 +13,6 @@ Documentation=man:systemd-machined.service(8)
 
 [Socket]
 ListenStream=%t/systemd/machine/io.systemd.Machine
-ListenStream=%t/systemd/machine/io.systemd.MachineImage
+Symlinks=%t/systemd/machine/io.systemd.MachineImage %t/varlink/registry/io.systemd.Machine %t/varlink/registry/io.systemd.MachineImage
 FileDescriptorName=varlink
 SocketMode=0600