store_avro_kafka: add naive defaults to avro schemas

In order to support avro schema evolution, add naive default
values to each field (except the timestamp, which ldms metric
sets will always contain). Technically for the default form of
schema evolution in kafka, we only need to have defaults for
any _new_ fields that we add when evolving a schema. However,
we are not tracking the various iterations of the schema over
time, so just setting defaults for all fields allows us
greater flexibility.

ldms/src/ldmsd/ldmsd_decomp.c

@@ -797,6 +797,41 @@ static const char *col_type_str(enum ldms_value_type type)
 	return type_str[type];
 }
 
+static const char *col_default(enum ldms_value_type type)
+{
+	static char *default_str[] = {
+	    [LDMS_V_CHAR] = "\" \"",
+	    [LDMS_V_U8] = "0",
+	    [LDMS_V_S8] = "0",
+	    [LDMS_V_U16] = "0",
+	    [LDMS_V_S16] = "0",
+	    [LDMS_V_U32] = "0",
+	    [LDMS_V_S32] = "0",
+	    [LDMS_V_U64] = "0",
+	    [LDMS_V_S64] = "0",
+	    [LDMS_V_F32] = "0.0",
+	    [LDMS_V_D64] = "0.0",
+	    [LDMS_V_CHAR_ARRAY] = "\"\"",
+	    [LDMS_V_U8_ARRAY] = "[]",
+	    [LDMS_V_S8_ARRAY] = "[]",
+	    [LDMS_V_U16_ARRAY] = "[]",
+	    [LDMS_V_S16_ARRAY] = "[]",
+	    [LDMS_V_U32_ARRAY] = "[]",
+	    [LDMS_V_S32_ARRAY] = "[]",
+	    [LDMS_V_U64_ARRAY] = "[]",
+	    [LDMS_V_S64_ARRAY] = "[]",
+	    [LDMS_V_F32_ARRAY] = "[]",
+	    [LDMS_V_D64_ARRAY] = "[]",
+	    [LDMS_V_LIST] = "nosup",
+	    [LDMS_V_LIST_ENTRY] = "nosup",
+	    [LDMS_V_RECORD_TYPE] = "nosup",
+	    [LDMS_V_RECORD_INST] = "nosup",
+	    [LDMS_V_RECORD_ARRAY] = "nosup",
+	    [LDMS_V_TIMESTAMP] = "0"
+	};
+	return default_str[type];
+}
+
 static char get_avro_char(char c)
 {
 	if (isalnum(c))
@@ -844,6 +879,12 @@ int ldmsd_row_to_json_avro_schema(ldmsd_row_t row, char **str, size_t *len)
 		}
 		switch (col->type) {
 		case LDMS_V_TIMESTAMP:
+			rc = strbuf_printf(&h,
+                                           "{\"name\":\"%s\",\"type\":%s}",
+					   avro_name, col_type_str(col->type));
+			if (rc)
+				goto err_0;
+			break;
 		case LDMS_V_CHAR:
 		case LDMS_V_U8:
 		case LDMS_V_S8:
@@ -856,8 +897,11 @@ int ldmsd_row_to_json_avro_schema(ldmsd_row_t row, char **str, size_t *len)
 		case LDMS_V_F32:
 		case LDMS_V_D64:
 		case LDMS_V_CHAR_ARRAY:
-			rc = strbuf_printf(&h, "{\"name\":\"%s\",\"type\":%s}",
-					   avro_name, col_type_str(col->type));
+			rc = strbuf_printf(&h,
+                                           "{\"name\":\"%s\",\"type\":%s,"
+                                           "\"default\":%s}",
+					   avro_name, col_type_str(col->type),
+                                           col_default(col->type));
 			if (rc)
 				goto err_0;
 			break;
@@ -873,8 +917,10 @@ int ldmsd_row_to_json_avro_schema(ldmsd_row_t row, char **str, size_t *len)
 		case LDMS_V_D64_ARRAY:
 			rc = strbuf_printf(&h,
 					   "{\"name\":\"%s\","
-					   "\"type\":{ \"type\" : \"array\", \"items\": %s }}",
-					   avro_name, col_type_str(col->type));
+					   "\"type\":{ \"type\" : \"array\", \"items\": %s },"
+                                           "\"default\":%s}",
+					   avro_name, col_type_str(col->type),
+                                           col_default(col->type));
 			if (rc)
 				goto err_0;
 			break;