Allow for env substitution in backup filename (#39)

README.md

@@ -122,6 +122,16 @@ You can populate below template according to your requirements and use it as you
 
 # BACKUP_FILENAME="backup-%Y-%m-%dT%H-%M-%S.tar.gz"
 
+# Setting BACKUP_FILENAME_EXPAND to true allows for environment variable
+# placeholders in BACKUP_FILENAME and in BACKUP_LATEST_SYMLINK that will get
+# expanded at runtime, e.g. `backup-$HOSTNAME-%Y-%m-%dT%H-%M-%S.tar.gz`.
+# Expansion happens before interpolating strftime tokens.
+# It is disabled by default.
+# Please note that you will need to escape the `$` when providing the value
+# in a docker-compose.yml file, i.e. using $$VAR instead of $VAR.
+
+# BACKUP_FILENAME_TEMPLATE="true"
+
 # When storing local backups, a symlink to the latest backup can be created
 # in case a value is given for this key. This has no effect on remote backups.
 

cmd/backup/main.go

@@ -103,6 +103,7 @@ type script struct {
 type config struct {
 	BackupSources              string        `split_words:"true" default:"/backup"`
 	BackupFilename             string        `split_words:"true" default:"backup-%Y-%m-%dT%H-%M-%S.tar.gz"`
+	BackupFilenameExpand       bool          `split_words:"true"`
 	BackupLatestSymlink        string        `split_words:"true"`
 	BackupArchive              string        `split_words:"true" default:"/archive"`
 	BackupRetentionDays        int32         `split_words:"true" default:"-1"`
@@ -153,6 +154,11 @@ func newScript() (*script, error) {
 	}
 
 	s.file = path.Join("/tmp", s.c.BackupFilename)
+	if s.c.BackupFilenameExpand {
+		s.file = os.ExpandEnv(s.file)
+		s.c.BackupLatestSymlink = os.ExpandEnv(s.c.BackupLatestSymlink)
+	}
+	s.file = timeutil.Strftime(&s.start, s.file)
 
 	_, err := os.Stat("/var/run/docker.sock")
 	if !os.IsNotExist(err) {
@@ -413,7 +419,6 @@ func (s *script) stopContainers() (func() error, error) {
 // takeBackup creates a tar archive of the configured backup location and
 // saves it to disk.
 func (s *script) takeBackup() error {
-	s.file = timeutil.Strftime(&s.start, s.file)
 	backupSources := s.c.BackupSources
 
 	if s.c.BackupFromSnapshot {

test/compose/docker-compose.yml

@@ -14,6 +14,7 @@ services:
 
   backup: &default_backup_service
     image: offen/docker-volume-backup:${TEST_VERSION}
+    hostname: hostnametoken
     depends_on:
       - minio
     restart: always
@@ -23,8 +24,9 @@ services:
       AWS_ENDPOINT: minio:9000
       AWS_ENDPOINT_PROTO: http
       AWS_S3_BUCKET_NAME: backup
-      BACKUP_FILENAME: test.tar.gz
-      BACKUP_LATEST_SYMLINK: test.latest.tar.gz.gpg
+      BACKUP_FILENAME_EXPAND: 'true'
+      BACKUP_FILENAME: test-$$HOSTNAME.tar.gz
+      BACKUP_LATEST_SYMLINK: test-$$HOSTNAME.latest.tar.gz.gpg
       BACKUP_CRON_EXPRESSION: 0 0 5 31 2 ?
       BACKUP_RETENTION_DAYS: ${BACKUP_RETENTION_DAYS:-7}
       BACKUP_PRUNING_LEEWAY: 5s

test/compose/run.sh

@@ -14,12 +14,12 @@ docker-compose exec backup backup
 
 docker run --rm -it \
   -v compose_backup_data:/data alpine \
-  ash -c 'apk add gnupg && echo 1234secret | gpg -d --pinentry-mode loopback --passphrase-fd 0 --yes /data/backup/test.tar.gz.gpg > /tmp/test.tar.gz && tar -xf /tmp/test.tar.gz -C /tmp && test -f /tmp/backup/app_data/offen.db'
+  ash -c 'apk add gnupg && echo 1234secret | gpg -d --pinentry-mode loopback --passphrase-fd 0 --yes /data/backup/test-hostnametoken.tar.gz.gpg > /tmp/test-hostnametoken.tar.gz && tar -xf /tmp/test-hostnametoken.tar.gz -C /tmp && test -f /tmp/backup/app_data/offen.db'
 
 echo "[TEST:PASS] Found relevant files in untared remote backup."
 
-test -L ./local/test.latest.tar.gz.gpg
-echo 1234secret | gpg -d --yes --passphrase-fd 0 ./local/test.tar.gz.gpg > ./local/decrypted.tar.gz
+test -L ./local/test-hostnametoken.latest.tar.gz.gpg
+echo 1234secret | gpg -d --yes --passphrase-fd 0 ./local/test-hostnametoken.tar.gz.gpg > ./local/decrypted.tar.gz
 tar -xf ./local/decrypted.tar.gz -C /tmp && test -f /tmp/backup/app_data/offen.db
 rm ./local/decrypted.tar.gz
 test -L /tmp/backup/app_data/db.link