journal: implement message catalog

The message catalog can be used to attach short help texts to log lines,
keyed by their MESSAGE_ID= fields. This is useful to help the
administrator understand the context and cause of a message, find
possible solutions and find further related documentation.

Since this is keyed off MESSAGE_ID= this will only work for native
journal messages.

The message catalog supports i18n, and is useful to augment english
language system messages with explanations in the local language.

This commit only includes short explanatory messages for a few example
message IDs, we'll add more complete documentation for the relevant
systemd messages later on.

src/shared/logs-show.c

@@ -34,6 +34,26 @@
 #define PRINT_THRESHOLD 128
 #define JSON_THRESHOLD 4096
 
+static int print_catalog(FILE *f, sd_journal *j) {
+        int r;
+        _cleanup_free_ char *t = NULL, *z = NULL;
+
+
+        r = sd_journal_get_catalog(j, &t);
+        if (r < 0)
+                return r;
+
+        z = strreplace(strstrip(t), "\n", "\n-- ");
+        if (!z)
+                return log_oom();
+
+        fputs("-- ", f);
+        fputs(z, f);
+        fputc('\n', f);
+
+        return 0;
+}
+
 static int parse_field(const void *data, size_t length, const char *field, char **target, size_t *target_size) {
         size_t fl, nl;
         void *buf;
@@ -265,6 +285,9 @@ static int output_short(
         } else
                 fputs("\n", f);
 
+        if (flags & OUTPUT_CATALOG)
+                print_catalog(f, j);
+
         return 0;
 }
 
@@ -322,6 +345,9 @@ static int output_verbose(
                         fprintf(f, "\t%.*s\n", (int) length, (const char*) data);
         }
 
+        if (flags & OUTPUT_CATALOG)
+                print_catalog(f, j);
+
         return 0;
 }
 

src/shared/logs-show.h

@@ -45,7 +45,8 @@ typedef enum OutputFlags {
         OUTPUT_FOLLOW         = 1 << 1,
         OUTPUT_WARN_CUTOFF    = 1 << 2,
         OUTPUT_FULL_WIDTH     = 1 << 3,
-        OUTPUT_COLOR          = 1 << 4
+        OUTPUT_COLOR          = 1 << 4,
+        OUTPUT_CATALOG        = 1 << 5
 } OutputFlags;
 
 int output_journal(