journal: correct file end position detection

This fixes an issue where the journal file could grow over the set
limit if:
1. Last entry on previous rollover did not fill the file fully
2. The information about the difference between the last entry and the
file size is lost

In such case, the end of file is detected later, leading to
increase of the file size by the size of the first evicted entries.

src/knot/server/journal.c

@@ -483,10 +483,10 @@ int journal_write_in(journal_t *j, journal_node_t **rn, uint64_t id, size_t len)
 	dbg_journal("journal: will write id=%llu, node=%u, size=%zu, fsize=%zu\n",
 	            (unsigned long long)id, j->qtail, len, j->fsize);
 
-	/* Calculate remaining bytes to reach file size limit. */
-	size_t fs_remaining = 0;
-	if (j->fsize < j->fslimit) {
-		fs_remaining = j->fslimit - j->fsize;
+	/* Calculate file end position (with imposed limits). */
+	size_t file_end = j->fsize;
+	if (file_end > j->fslimit) {
+		file_end = j->fslimit;
 	}
 
 	int seek_ret = 0;
@@ -495,12 +495,12 @@ int journal_write_in(journal_t *j, journal_node_t **rn, uint64_t id, size_t len)
 	dbg_journal("journal: free.pos = %u free.len = %u\n",
 	            j->free.pos, j->free.len);
 	journal_node_t *n = j->nodes + j->qtail;
-	if (j->free.pos + j->free.len == j->fsize) {
+	if (j->free.pos + len >= file_end) {
 
 		dbg_journal_verb("journal: * is last node\n");
 
 		/* Grow journal file until the size limit. */
-		if(j->free.len < len && len <= fs_remaining) {
+		if(j->free.pos + len < j->fslimit) {
 			size_t diff = len - j->free.len;
 			dbg_journal("journal: * growing by +%zu, pos=%u, "
 			            "new fsize=%zu\n",
@@ -509,10 +509,8 @@ int journal_write_in(journal_t *j, journal_node_t **rn, uint64_t id, size_t len)
 			j->fsize += diff; /* Appending increases file size. */
 			j->free.len += diff;
 
-		}
-
-		/*  Rewind if resize is needed, but the limit is reached. */
-		if(j->free.len < len && len > fs_remaining) {
+		} else {
+			/*  Rewind if resize is needed, but the limit is reached. */
 			journal_node_t *head = j->nodes + j->qhead;
 			j->fsize = j->free.pos;
 			j->free.pos = head->pos;