open block_log in read only mode

When index is invalid/missing then it is reconstructed in the same location as block_log. If it is not possible to write in this location there is message:

1298930ms block_log.cpp:623             construct_index      ] head_block_num: 5000000 
1298930ms block_log.cpp:639             construct_index      ] Reconstructing Block Log Index from block 10001 to 5000000
1298930ms block_log.cpp:732             construct_index      ] 0 exception: unspecified
Error opening temporary new index file input/block_log.index.new: Permission denied
    {"filename":"input/block_log.index.new","error":"Permission denied"}
    block_log.cpp:649 construct_index
1298931ms compress_block_log.cpp:338    fill_pending_queue   ] 0 exception: unspecified
Error opening temporary new index file input/block_log.index.new: Permission denied
    {"filename":"input/block_log.index.new","error":"Permission denied"}
    block_log.cpp:649 construct_index
rethrow
    {}
    block_log.cpp:732 construct_index
terminate called after throwing an instance of 'fc::exception'
[1]    3372035 abort (core dumped)  ./compress_block_log --input-block-log=input --output-block-log=output

or

2682656ms compress_block_log.cpp:338    fill_pending_queue   ] 0 exception: unspecified
Error opening block index file input/block_log.index: No such file or directory
    {"filename":"input/block_log.index","error":"No such file or directory"}
    block_log.cpp:221 open