in_tail: detect rotation on symlink targets

Kiichiro Okano reported that Fluent Bit keeps reading from the old file
when docker rotates a log file (in issue 1712).

This bug occurs because Fluent Bit only checks the identify of the source
file (if it is a symlink), and never check if the target file is the same
as before. For this reason, Fluent Bit would keep the file handler to the
old target file even after the symlink switched to the new file.

Change in_tail to check the identities of both the source file and the
target file, and thus makes it aware of both rotation patterns.

Signed-off-by: Fujimoto Seiji <[email protected]>

Author: fujimotos

plugins/in_tail/tail_file.c

@@ -966,12 +966,9 @@ int flb_tail_file_is_rotated(struct flb_tail_config *ctx,
         }
         else {
             /* The file name is there, check if the same that we have */
-            if (st.st_ino == file->link_inode) {
-                return FLB_FALSE;
+            if (st.st_ino != file->link_inode) {
+                return FLB_TRUE;
             }
-
-            /* no rotation */
-            return FLB_TRUE;
         }
     }