imfile: improved rotation detection messages, slightly different mode

Rotation detection seeks backwards, what caused issues as least in one isolated
case. We try to work around this by only doing positive seeks. We also have
added diagnostic information to the warning messages rsyslog emits on
rotation detection.

see also rsyslog/rsyslog#3249

runtime/stream.c

@@ -644,13 +644,13 @@ strmHandleEOF(strm_t *const pThis)
 
 /* helper to checkTruncation */
 static rsRetVal ATTR_NONNULL()
-rereadTruncated(strm_t *const pThis, const char *const reason)
+rereadTruncated(strm_t *const pThis, const char *const reason, const long long data)
 {
 	DEFiRet;
 
 	LogMsg(errno, RS_RET_FILE_TRUNCATED, LOG_WARNING, "file '%s': truncation detected, "
-		"(%s) - re-start reading from beginning",
-		pThis->pszCurrFName, reason);
+		"(%s) - re-start reading from beginning (data %lld)",
+		pThis->pszCurrFName, reason, data);
 	DBGPRINTF("checkTruncation, file %s last buffer CHANGED\n", pThis->pszCurrFName);
 	CHKiRet(strmCloseFile(pThis));
 	CHKiRet(strmOpenFile(pThis));
@@ -675,34 +675,48 @@ checkTruncation(strm_t *const pThis)
 {
 	DEFiRet;
 	int ret;
-	off64_t backseek;
+	off64_t newpos;
 	assert(pThis->bReopenOnTruncate);
 
 	DBGPRINTF("checkTruncation, file %s, iBufPtrMax %zd\n", pThis->pszCurrFName, pThis->iBufPtrMax);
 	if(pThis->iBufPtrMax == 0) {
 		FINALIZE;
 	}
 
-	int currpos = lseek64(pThis->fd, 0, SEEK_CUR);
-	backseek = -1 * (off64_t) pThis->iBufPtrMax;
-	dbgprintf("checkTruncation in actual processing, currpos %d, backseek is %d\n", (int)currpos, (int) backseek);
-	ret = lseek64(pThis->fd, backseek, SEEK_CUR);
+	const off64_t currpos = lseek64(pThis->fd, 0, SEEK_CUR);
+	if(currpos < 0) {
+		iRet = rereadTruncated(pThis, "cannot obtain current position", 0);
+		FINALIZE;
+	}
+
+	newpos = currpos - (off64_t) pThis->iBufPtrMax;
+	dbgprintf("checkTruncation in actual processing, currpos %lld, newpos is %lld\n",
+		(long long) currpos, (long long) newpos);
+	if(newpos < 0) {
+		LogError(0, RS_RET_INTERNAL_ERROR, "program bug: file '%s': newpos < 0 --> %lld, "
+			"currpos %lld, iBufPtrMax %lld",
+			pThis->pszCurrFName, (long long) newpos, (long long) currpos,
+			(long long) pThis->iBufPtrMax);
+		FINALIZE;
+	}
+
+	ret = lseek64(pThis->fd, newpos, SEEK_SET);
 	if(ret < 0) {
-		iRet = rereadTruncated(pThis, "cannot seek backward to begin of last block");
+		iRet = rereadTruncated(pThis, "cannot seek backward to begin of last block", newpos);
 		FINALIZE;
 	}
 
 	const ssize_t lenRead = read(pThis->fd, pThis->pIOBuf_truncation, pThis->iBufPtrMax);
 	dbgprintf("checkTruncation proof-read: %d bytes\n", (int) lenRead);
 	if(lenRead < 0) {
-		iRet = rereadTruncated(pThis, "last block could not be re-read");
+		iRet = rereadTruncated(pThis, "last block could not be re-read", pThis->iBufPtrMax);
 		FINALIZE;
 	}
 
 	if(!memcmp(pThis->pIOBuf_truncation, pThis->pIOBuf, pThis->iBufPtrMax)) {
 		DBGPRINTF("checkTruncation, file %s last buffer unchanged\n", pThis->pszCurrFName);
 	} else {
-		iRet = rereadTruncated(pThis, "last block data different");
+		iRet = rereadTruncated(pThis, "last block data different", 0);
 	}
 
 finalize_it: