add dockervolume:// snapshotting

Author: joonas-fi

README.md

@@ -54,13 +54,24 @@ Docker host, configure the Docker integration via µbackup config)
 `$ docker run -e "BACKUP_COMMAND=..."` for example) that contains the command used to take
 a backup of the important state inside the container.
 
-If you need to backup a single file inside a container, use: `BACKUP_COMMAND=cat /yourfile.db`
+For different use cases, for `BACKUP_COMMAND` specify:
 
-For PostgreSQL, you could use: `BACKUP_COMMAND=pg_dump -U postgres nameOfYourDatabase`
+- If you need to backup a single file inside a container
+  * Use: `BACKUP_COMMAND=cat /yourfile.db`
 
-For a directory, you could use: `BACKUP_COMMAND=tar -cC /yourdirectory -f - .` (`-` means
-`$ tar` will write the archive to `stdout`, `.` just means to process all files in the
-selected directory)
+- For databases etc. that support atomic dumping, e.g. PostgreSQL
+  * Use: `BACKUP_COMMAND=pg_dump -U postgres nameOfYourDatabase`
+
+- For a directory
+  * Use: `BACKUP_COMMAND=tar -cC /yourdirectory -f - .`
+  * `-` means `$ tar` will write the archive to `stdout`, `.` just means to process all files in the
+    selected directory).
+
+- If there's no tooling inside the container (think `FROM scratch`), if you have a Docker
+  volume you want to archive in its entirety
+  * Use: `BACKUP_COMMAND=dockervolume://` and µbackup will find the volume source data via
+    Docker and use `tar` to dump the whole directory tree.
+  * For this option, you have to run µbackup with `$ docker run -v /var/lib/docker/volumes:/var/lib/docker/volumes`
 
 This simple approach is surprisingly flexible and its streaming approach is more efficient
 than having to write temporary files.

cmd/ubackup/dockerdiscovery.go

@@ -7,6 +7,7 @@ import (
 	"github.com/function61/gokit/ezhttp"
 	"github.com/function61/gokit/udocker"
 	"github.com/function61/ubackup/pkg/ubtypes"
+	"log"
 	"net/http"
 	"strings"
 )
@@ -77,11 +78,34 @@ func dockerDiscoverBackupTargets(ctx context.Context, dockerEndpoint string) ([]
 	return targets, nil
 }
 
+// "dockervolume://" => docker volume snapshotter
 // "cat /data/example.db" => ["docker", "exec", "cat", "/data/example.db"]
 func createSnapshotter(
 	backupCommand string,
 	container udocker.Container,
 ) ubtypes.Snapshotter {
+	if backupCommand == "dockervolume://" {
+		volumeMounts := []udocker.Mount{}
+		for _, mount := range container.Mounts {
+			if mount.Type == "volume" {
+				volumeMounts = append(volumeMounts, mount)
+			}
+		}
+
+		if len(volumeMounts) != 1 {
+			log.Printf(
+				"disqualifying container %s with dockervolume:// because len(volumeMounts) != 1; got %d",
+				container.Name,
+				len(volumeMounts))
+
+			return nil
+		}
+
+		return newCommandOutputSnapshotter(
+			[]string{"tar", "--create", "."},
+			volumeMounts[0].Source)
+	}
+
 	// FIXME: this doesn't support spaces..
 	backupCommandParts := strings.Split(backupCommand, " ")