in_systemd: allow a parser to be specified as part of the systemd unit

Similar to how we use kubernetes annotations to determine a parser, this
uses custom fields in systemd units to configure a parser per systemd
unit.

In the unit file this is configured as:

```
[Service]
...
LogExtraFields=FLUENT_BIT_PARSER=logfmt
```

Author: seveas

plugins/in_systemd/systemd.c

@@ -21,6 +21,8 @@
 #include <fluent-bit/flb_input_plugin.h>
 #include <fluent-bit/flb_config.h>
 #include <fluent-bit/flb_time.h>
+#include <fluent-bit/flb_parser.h>
+#include <fluent-bit/flb_log_event_decoder.h>
 
 #include "systemd_config.h"
 #include "systemd_db.h"
@@ -70,6 +72,59 @@ static int tag_compose(const char *tag, const char *unit_name,
     return 0;
 }
 
+static int flb_systemd_repack_map(struct flb_log_event_encoder *encoder,
+                               char *data,
+                               size_t data_size)
+{
+    msgpack_unpacked source_map;
+    size_t           offset;
+    int              result;
+    size_t           index;
+    msgpack_object   value;
+    msgpack_object   key;
+
+    result = FLB_EVENT_ENCODER_SUCCESS;
+
+    if (data_size > 0) {
+        msgpack_unpacked_init(&source_map);
+
+        offset = 0;
+        result = msgpack_unpack_next(&source_map,
+                                     data,
+                                     data_size,
+                                     &offset);
+
+        if (result == MSGPACK_UNPACK_SUCCESS) {
+            result = FLB_EVENT_ENCODER_SUCCESS;
+        }
+        else {
+            result = FLB_EVENT_DECODER_ERROR_DESERIALIZATION_FAILURE;
+        }
+
+        for (index = 0;
+             index < source_map.data.via.map.size &&
+             result == FLB_EVENT_ENCODER_SUCCESS;
+             index++) {
+            key   = source_map.data.via.map.ptr[index].key;
+            value = source_map.data.via.map.ptr[index].val;
+
+            result = flb_log_event_encoder_append_body_msgpack_object(
+                        encoder,
+                        &key);
+
+            if (result == FLB_EVENT_ENCODER_SUCCESS) {
+                result = flb_log_event_encoder_append_body_msgpack_object(
+                            encoder,
+                            &value);
+            }
+        }
+
+        msgpack_unpacked_destroy(&source_map);
+    }
+
+    return result;
+}
+
 static int in_systemd_collect(struct flb_input_instance *ins,
                               struct flb_config *config, void *in_context)
 {
@@ -84,11 +139,14 @@ static int in_systemd_collect(struct flb_input_instance *ins,
     long nsec;
     uint64_t usec;
     size_t length;
+    size_t plength;
     size_t threshold;
+    char *name;
     const char *sep;
     const char *key;
     const char *val;
     char *buf = NULL;
+    void *pbuf = NULL;
 #ifdef FLB_HAVE_SQLDB
     char *cursor = NULL;
 #endif
@@ -100,6 +158,7 @@ static int in_systemd_collect(struct flb_input_instance *ins,
     const void *data;
     struct flb_systemd_config *ctx = in_context;
     struct flb_time tm;
+    struct flb_parser *parser;
 
     /* Restricted by mem_buf_limit */
     if (flb_input_buf_paused(ins) == FLB_TRUE) {
@@ -154,6 +213,17 @@ static int in_systemd_collect(struct flb_input_instance *ins,
             tag_len = ctx->ins->tag_len;
         }
 
+        /* Find the parser, if specified */
+        ret = sd_journal_get_data(ctx->j, "FLUENT_BIT_PARSER", &data, &length);
+        if (ret == 0) {
+             name = flb_strndup((const char *)(data+18), length);
+             parser = flb_parser_get(name, config);
+             if (!parser) {
+                 flb_plg_error(ctx->ins, "no such parser: '%s'", sep+1);
+             }
+             free(name);
+        }
+
         if (last_tag_len == 0) {
             strncpy(last_tag, tag, tag_len);
             last_tag_len = tag_len;
@@ -219,6 +289,26 @@ static int in_systemd_collect(struct flb_input_instance *ins,
 
             len = (sep - key);
 
+            if (strncmp(key, "FLUENT_BIT_PARSER", len) == 0) {
+                continue;
+            }
+
+            /* If this is the message, apply the parser if any is specified */
+            if (parser && strncmp(key, "MESSAGE", len) == 0) {
+                val = sep + 1;
+                len = length - (sep - key) - 1;
+                ret = flb_parser_do(parser, val, len, &pbuf, &plength, &tm);
+                if (ret != -1) {
+                    ret = flb_systemd_repack_map(ctx->log_encoder, pbuf, plength);
+                    continue;
+                }
+                /* 
+                 * If the parser failed, reset the return code
+                 * to append the unparsed message as normal
+                 */
+                ret = FLB_EVENT_ENCODER_SUCCESS;
+            }
+
             if (ret == FLB_EVENT_ENCODER_SUCCESS) {
                 ret = flb_log_event_encoder_append_body_string_length(
                         ctx->log_encoder, len);