Fix k8s-file log corruption during log rotation

The k8s-file log driver was corrupting log entries when the log file
reached max_size and needed rotation. This manifested as garbled output
with repeated timestamps and broken log entries.

The root cause was in writev_buffer_flush() which incorrectly handled
partial writes. When writev() returned a partial write, the function
would modify the original iovec base pointers, corrupting the buffer
state. During log rotation, this corrupted state would carry over to
the new file descriptor, causing subsequent log entries to be malformed.

Changes:
- Simplify writev_buffer_flush() to use individual write() calls instead
  of complex writev() partial write handling
- Always reset buffer state after log rotation to ensure clean state
  with new file descriptor
- Remove conditional buffer reset that could leave corrupted state

This fixes the issue where long-running containers (like GitLab CE)
would produce corrupted logs after reaching the configured max_size.

Fixes: Log corruption with --log-driver k8s-file --log-opt max_size=20mb

Signed-off-by: Jindrich Novy <jnovy@redhat.com>

src/ctr_logging.c

@@ -444,12 +444,13 @@ static int write_k8s_log(stdpipe_t pipe, const char *buf, ssize_t buflen)
 		if ((log_size_max > 0) && (k8s_bytes_written + bytes_to_be_written) > log_size_max) {
 			if (writev_buffer_flush(k8s_log_fd, &bufv) < 0) {
 				nwarn("failed to flush buffer to log");
-				/*
-				 * We are going to reopen the file anyway, in case of
-				 * errors discard all we have in the buffer.
-				 */
-				bufv.iovcnt = 0;
 			}
+			/*
+			 * Always reset the buffer after rotation to ensure clean state
+			 * with the new file descriptor. Any unflushed data is lost, but
+			 * this prevents corruption of subsequent log entries.
+			 */
+			bufv.iovcnt = 0;
 			reopen_k8s_file();
 		}
 
@@ -520,35 +521,28 @@ static bool get_line_len(ptrdiff_t *line_len, const char *buf, ssize_t buflen)
 static ssize_t writev_buffer_flush(int fd, writev_buffer_t *buf)
 {
 	size_t count = 0;
-	int iovcnt = buf->iovcnt;
-	struct iovec *iov = buf->iov;
 
-	while (iovcnt > 0) {
-		ssize_t res;
-		do {
-			res = writev(fd, iov, iovcnt);
-		} while (res == -1 && errno == EINTR);
+	for (int i = 0; i < buf->iovcnt; i++) {
+		const char *ptr = buf->iov[i].iov_base;
+		size_t remaining = buf->iov[i].iov_len;
 
-		if (res <= 0)
-			return -1;
-
-		count += res;
-
-		while (res > 0) {
-			size_t from_this = MIN((size_t)res, iov->iov_len);
-			iov->iov_len -= from_this;
-			iov->iov_base += from_this;
-			res -= from_this;
-
-			if (iov->iov_len == 0) {
-				iov++;
-				iovcnt--;
+		while (remaining > 0) {
+			ssize_t written = write(fd, ptr, remaining);
+			if (written < 0) {
+				if (errno == EINTR)
+					continue;
+				return -1;
 			}
+			if (written == 0)
+				return -1;
+
+			ptr += written;
+			remaining -= written;
+			count += written;
 		}
 	}
 
 	buf->iovcnt = 0;
-
 	return count;
 }