Fix find_end_of_wal busy loop.

It hanged if file size is less than of a normal segment. Normally that doesn't
happen, but it might in case of crash during segment init. We're going to fix
that half initialized segment by durably renaming it after cooking, so this fix
won't be needed, but better avoid busy loop anyway.

fixes https://github.com/neondatabase/neon/issues/6401

libs/postgres_ffi/src/xlog_utils.rs

@@ -207,10 +207,16 @@ pub fn find_end_of_wal(
                 let seg_offs = curr_lsn.segment_offset(wal_seg_size);
                 segment.seek(SeekFrom::Start(seg_offs as u64))?;
                 // loop inside segment
-                loop {
+                while curr_lsn.segment_number(wal_seg_size) == segno {
                     let bytes_read = segment.read(&mut buf)?;
                     if bytes_read == 0 {
-                        break; // EOF
+                        debug!(
+                            "find_end_of_wal reached end at {:?}, EOF in segment {:?} at offset {}",
+                            result,
+                            seg_file_path,
+                            curr_lsn.segment_offset(wal_seg_size)
+                        );
+                        return Ok(result);
                     }
                     curr_lsn += bytes_read as u64;
                     decoder.feed_bytes(&buf[0..bytes_read]);