in_tail: fix last_processed_bytes calculation

Fix last_processed_bytes calculation in multiple line scenario.

Signed-off-by: zshuang0316 <163435379+zshuang0316@users.noreply.github.com>

Author: zshuang0316

plugins/in_tail/tail_file.c

@@ -648,7 +648,7 @@ static int process_content(struct flb_tail_file *file, size_t *bytes)
         processed_bytes += len + 1;
         lines++;
         file->parsed = 0;
-        file->last_processed_bytes += processed_bytes;
+        file->last_processed_bytes = processed_bytes;
     }
 
 #ifdef FLB_HAVE_UNICODE_ENCODER

tests/runtime/data/tail/parsers_multiline.conf

@@ -0,0 +1,6 @@
+[MULTILINE_PARSER]
+    name multiline-regex
+    type regex
+    flush_timeout 5000
+    rule "start_state" "/(^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}.\d{3})(.*)/" "cont"
+    rule "cont" "/^(?!\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}.\d{3}).*/" "cont"

tests/runtime/in_tail.c

@@ -1390,6 +1390,90 @@ void flb_test_offset_key()
     test_tail_ctx_destroy(ctx);
 }
 
+void flb_test_multiline_offset_key()
+{
+    struct flb_lib_out_cb cb_data;
+    struct test_tail_ctx *ctx;
+    char *file[] = {"multiline_offset.log"};
+    char *offset_key = "OffsetKey";
+    char *msg_before_tail = "[2025-06-16 20:42:22,291] INFO - aaaaaaaaaaa";
+    char *msg_before_tail2 = "[2025-06-16 20:42:22,500] Error";
+    char *msg_final = "[2025-06-16 20:45:29,234] Fatal";
+    char expected_msg[1024] = {0};
+    int ret;
+    int num;
+
+    char *expected_strs[] = {msg_final, &expected_msg[0]};
+    struct str_list expected = {
+                                .size = sizeof(expected_strs)/sizeof(char*),
+                                .lists = &expected_strs[0],
+    };
+
+    clear_output_num();
+
+    cb_data.cb = cb_check_json_str_list;
+    cb_data.data = &expected;
+
+    ret = snprintf(&expected_msg[0], sizeof(expected_msg), "\"%s\":%ld", offset_key, strlen(msg_before_tail)+strlen(NEW_LINE)+strlen(msg_before_tail2)+strlen(NEW_LINE));
+    if(!TEST_CHECK(ret >= 0)) {
+        TEST_MSG("snprintf failed");
+        exit(EXIT_FAILURE);
+    }
+
+    ctx = test_tail_ctx_create(&cb_data, &file[0], sizeof(file)/sizeof(char *), FLB_TRUE);
+    if (!TEST_CHECK(ctx != NULL)) {
+        TEST_MSG("test_ctx_create failed");
+        exit(EXIT_FAILURE);
+    }
+
+    ret = flb_service_set(ctx->flb, "Parsers_File", DPATH "/parsers_multiline.conf", NULL);
+    TEST_CHECK(ret == 0);
+
+    ret = flb_input_set(ctx->flb, ctx->o_ffd,
+                        "path", file[0],
+                        "offset_key", offset_key,
+                        "multiline.parser", "multiline-regex",
+                        NULL);
+    TEST_CHECK(ret == 0);
+
+    ret = flb_output_set(ctx->flb, ctx->o_ffd,
+                         "format", "json",
+                         NULL);
+    TEST_CHECK(ret == 0);
+
+    ret = write_msg(ctx, msg_before_tail, strlen(msg_before_tail));
+    if (!TEST_CHECK(ret > 0)) {
+        test_tail_ctx_destroy(ctx);
+        exit(EXIT_FAILURE);
+    }
+
+    ret = write_msg(ctx, msg_before_tail2, strlen(msg_before_tail2));
+    if (!TEST_CHECK(ret > 0)) {
+        test_tail_ctx_destroy(ctx);
+        exit(EXIT_FAILURE);
+    }
+
+    /* Start the engine */
+    ret = flb_start(ctx->flb);
+    TEST_CHECK(ret == 0);
+
+    ret = write_msg(ctx, msg_final, strlen(msg_final));
+    if (!TEST_CHECK(ret > 0)) {
+        test_tail_ctx_destroy(ctx);
+        exit(EXIT_FAILURE);
+    }
+
+    /* waiting to flush */
+    flb_time_msleep(500);
+
+    num = get_output_num();
+    if (!TEST_CHECK(num > 0))  {
+        TEST_MSG("no outputs");
+    }
+
+    test_tail_ctx_destroy(ctx);
+}
+
 void flb_test_skip_empty_lines()
 {
     struct flb_lib_out_cb cb_data;
@@ -2345,6 +2429,7 @@ TEST_LIST = {
     {"path_key", flb_test_path_key},
     {"exclude_path", flb_test_exclude_path},
     {"offset_key", flb_test_offset_key},
+    {"multiline_offset_key", flb_test_multiline_offset_key},
     {"skip_empty_lines", flb_test_skip_empty_lines},
     {"skip_empty_lines_crlf", flb_test_skip_empty_lines_crlf},
     {"ignore_older", flb_test_ignore_older},