auto updates

Add support to auto-update containers running in systemd units as
generated with `podman generate systemd --new`.

`podman auto-update` looks up containers with a specified
"io.containers.autoupdate" label (i.e., the auto-update policy).

If the label is present and set to "image", Podman reaches out to the
corresponding registry to check if the image has been updated.  We
consider an image to be updated if the digest in the local storage is
different than the one of the remote image.  If an image must be
updated, Podman pulls it down and restarts the container.  Note that the
restarting sequence relies on systemd.

At container-creation time, Podman looks up the "PODMAN_SYSTEMD_UNIT"
environment variables and stores it verbatim in the container's label.
This variable is now set by all systemd units generated by
`podman-generate-systemd` and is set to `%n` (i.e., the name of systemd
unit starting the container).  This data is then being used in the
auto-update sequence to instruct systemd (via DBUS) to restart the unit
and hence to restart the container.

Note that this implementation of auto-updates relies on systemd and
requires a fully-qualified image reference to be used to create the
container.  This enforcement is necessary to know which image to
actually check and pull.  If we used an image ID, we would not know
which image to check/pull anymore.

Fixes: #3575
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

2 files changed, 16 insertions, 0 deletions

diff --git a/pkg/systemd/generate/systemdgen.go b/pkg/systemd/generate/systemdgen.go
index 4cd7745c0..eb15d4927 100644
--- a/pkg/systemd/generate/systemdgen.go
+++ b/pkg/systemd/generate/systemdgen.go
@@ -16,6 +16,10 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
+// EnvVariable "PODMAN_SYSTEMD_UNIT" is set in all generated systemd units and
+// is set to the unit's (unique) name.
+const EnvVariable = "PODMAN_SYSTEMD_UNIT"
+
 // ContainerInfo contains data required for generating a container's systemd
 // unit file.
 type ContainerInfo struct {
@@ -57,6 +61,8 @@ type ContainerInfo struct {
 	// RunCommand is a post-processed variant of CreateCommand and used for
 	// the ExecStart field in generic unit files.
 	RunCommand string
+	// EnvVariable is generate.EnvVariable and must not be set.
+	EnvVariable string
 }
 
 var restartPolicies = []string{"no", "on-success", "on-failure", "on-abnormal", "on-watchdog", "on-abort", "always"}
@@ -94,6 +100,7 @@ Before={{- range $index, $value := .RequiredServices -}}{{if $index}} {{end}}{{
 {{- end}}
 
 [Service]
+Environment={{.EnvVariable}}=%n
 Restart={{.RestartPolicy}}
 {{- if .New}}
 ExecStartPre=/usr/bin/rm -f %t/%n-pid %t/%n-cid
@@ -138,6 +145,8 @@ func CreateContainerSystemdUnit(info *ContainerInfo, opts Options) (string, erro
 		info.Executable = executable
 	}
 
+	info.EnvVariable = EnvVariable
+
 	// Assemble the ExecStart command when creating a new container.
 	//
 	// Note that we cannot catch all corner cases here such that users
diff --git a/pkg/systemd/generate/systemdgen_test.go b/pkg/systemd/generate/systemdgen_test.go
index bbdccdcf8..3269405a6 100644
--- a/pkg/systemd/generate/systemdgen_test.go
+++ b/pkg/systemd/generate/systemdgen_test.go
@@ -44,6 +44,7 @@ Wants=network.target
 After=network-online.target
 
 [Service]
+Environment=PODMAN_SYSTEMD_UNIT=%n
 Restart=always
 ExecStart=/usr/bin/podman start 639c53578af4d84b8800b4635fa4e680ee80fd67e0e6a2d4eea48d1e3230f401
 ExecStop=/usr/bin/podman stop -t 10 639c53578af4d84b8800b4635fa4e680ee80fd67e0e6a2d4eea48d1e3230f401
@@ -64,6 +65,7 @@ Wants=network.target
 After=network-online.target
 
 [Service]
+Environment=PODMAN_SYSTEMD_UNIT=%n
 Restart=always
 ExecStart=/usr/bin/podman start foobar
 ExecStop=/usr/bin/podman stop -t 10 foobar
@@ -88,6 +90,7 @@ BindsTo=a.service b.service c.service pod.service
 After=a.service b.service c.service pod.service
 
 [Service]
+Environment=PODMAN_SYSTEMD_UNIT=%n
 Restart=always
 ExecStart=/usr/bin/podman start foobar
 ExecStop=/usr/bin/podman stop -t 10 foobar
@@ -110,6 +113,7 @@ Requires=container-1.service container-2.service
 Before=container-1.service container-2.service
 
 [Service]
+Environment=PODMAN_SYSTEMD_UNIT=%n
 Restart=always
 ExecStart=/usr/bin/podman start jadda-jadda-infra
 ExecStop=/usr/bin/podman stop -t 10 jadda-jadda-infra
@@ -130,6 +134,7 @@ Wants=network.target
 After=network-online.target
 
 [Service]
+Environment=PODMAN_SYSTEMD_UNIT=%n
 Restart=always
 ExecStartPre=/usr/bin/rm -f %t/%n-pid %t/%n-cid
 ExecStart=/usr/bin/podman run --conmon-pidfile %t/%n-pid --cidfile %t/%n-cid --cgroups=no-conmon -d --name jadda-jadda --hostname hello-world awesome-image:latest command arg1 ... argN
@@ -152,6 +157,7 @@ Wants=network.target
 After=network-online.target
 
 [Service]
+Environment=PODMAN_SYSTEMD_UNIT=%n
 Restart=always
 ExecStartPre=/usr/bin/rm -f %t/%n-pid %t/%n-cid
 ExecStart=/usr/bin/podman run --conmon-pidfile %t/%n-pid --cidfile %t/%n-cid --cgroups=no-conmon --detach --name jadda-jadda --hostname hello-world awesome-image:latest command arg1 ... argN
@@ -174,6 +180,7 @@ Wants=network.target
 After=network-online.target
 
 [Service]
+Environment=PODMAN_SYSTEMD_UNIT=%n
 Restart=always
 ExecStartPre=/usr/bin/rm -f %t/%n-pid %t/%n-cid
 ExecStart=/usr/bin/podman run --conmon-pidfile %t/%n-pid --cidfile %t/%n-cid --cgroups=no-conmon -d awesome-image:latest