pkg/promtail: remove journal target forced path (#1298)

* pkg/promtail: remove journal target forced path

The forced default path for the journal target (/var/log/journal) did
not apply to all Linux systems: systemd actually defaults to reading
from _two_ default paths: /var/log/journal and /run/log/journal. Not
setting an explicit default will enable both of those paths to be
searched for journal logs.

The comment mentioning the local machine ID was technically accurate,
but is normally dealt with by bind mounting /etc/machine-id. That's the
standard approach for running fluent-bit in a container, and should be
the standard approach for Promtail as well.

Fixes #1286.

docs/clients/promtail/configuration.md

@@ -576,7 +576,7 @@ labels:
   [ <labelname>: <labelvalue> ... ]
 
 # Path to a directory to read entries from. Defaults to system
-# path when empty.
+# paths (/var/log/journal and /run/log/journal) when empty.
 [path: <string>]
 ```
 
@@ -964,7 +964,6 @@ scrape_configs:
   - job_name: journal
     journal:
       max_age: 12h
-      path: /var/log/journal
       labels:
         job: systemd-journal
     relabel_configs:

docs/clients/promtail/scraping.md

@@ -77,8 +77,8 @@ See [Relabeling](#relabeling) for more information.
 
 ## Journal Scraping (Linux Only)
 
-On systems with `systemd`, Promtail also supports reading from the journal. Unlike 
-file scraping which is defined in the `static_configs` stanza, journal scraping is 
+On systems with `systemd`, Promtail also supports reading from the journal. Unlike
+file scraping which is defined in the `static_configs` stanza, journal scraping is
 defined in a `journal` stanza:
 
 ```yaml
@@ -94,16 +94,32 @@ scrape_configs:
         target_label: 'unit'
 ```
 
-All fields defined in the `journal` section are optional, and are just provided 
-here for reference. The `max_age` field ensures that no older entry than the 
+All fields defined in the `journal` section are optional, and are just provided
+here for reference. The `max_age` field ensures that no older entry than the
 time specified will be sent to Loki; this circumvents "entry too old" errors.
-The `path` field tells Promtail where to read journal entries from. The labels 
+The `path` field tells Promtail where to read journal entries from. The labels
 map defines a constant list of labels to add to every journal entry that Promtail
-reads. 
+reads.
+
+By default, Promtail reads from the journal by looking in the `/var/log/journal`
+and `/run/log/journal` paths. If running Promtail inside of a Docker container,
+the path appropriate to your distribution should be bind mounted inside of
+Promtail along with binding `/etc/machine-id`. Bind mounting `/etc/machine-id`
+to the path of the same name is required for the journal reader to know which
+specific journal to read from. For example:
+
+```bash
+docker run \
+  -v /var/log/journal/:/var/log/journal/ \
+  -v /run/log/journal/:/run/log/journal/ \
+  -v /etc/machine-id:/etc/machine-id \
+  grafana/promtail:latest \
+  -config.file=/path/to/config/file.yaml
+```
 
-When Promtail reads from the journal, it brings in all fields prefixed with 
-`__journal_` as internal labels. Like in the example above, the `_SYSTEMD_UNIT` 
-field from the journal was transformed into a label called `unit` through 
+When Promtail reads from the journal, it brings in all fields prefixed with
+`__journal_` as internal labels. Like in the example above, the `_SYSTEMD_UNIT`
+field from the journal was transformed into a label called `unit` through
 `relabel_configs`. See [Relabeling](#relabeling) for more information.
 
 ## Relabeling

pkg/promtail/targets/journaltarget.go

@@ -154,18 +154,8 @@ func journalTargetWithReader(
 		return nil, errors.Wrap(err, "parsing journal reader 'max_age' config value")
 	}
 
-	// Default to system path if not defined. Passing an empty string to
-	// sdjournal is valid but forces reads from the journal to be from
-	// the local machine id only, which contradicts the default behavior
-	// of when a path is specified. To standardize, we manually default the
-	// path here.
-	journalPath := targetConfig.Path
-	if journalPath == "" {
-		journalPath = "/var/log/journal"
-	}
-
 	cfg := t.generateJournalConfig(journalConfigBuilder{
-		JournalPath: journalPath,
+		JournalPath: targetConfig.Path,
 		Position:    position,
 		MaxAge:      maxAge,
 		EntryFunc:   entryFunc,