TLDR:

• partition table's size field now carries a mask
• one bit of the mask makes the partition read-only
• the bit is set when a partition is attached by soft linking it
• inserts on read-only partitions are ignored and logged (to offer continuous service on ILP)
• updates on read-only partitions result in an error
• directories and soft links to directories work the same way across the codebase, any folder can in reality be a soft link to anywhere in the file system, including other volumes
• revisited Path class to improve usability

The partition table segment is a block within the _txn file that contains 4 longs per attached partition:

1. partition timestamp
2. partition size
3. partition txn (transaction number/version)
4. partition column version (data version)

Each group of 4 longs is addressed by partitionIndex. These values are updated on commit. When a partition is detached/droppped, its associated 4 longs in the partition segment are removed.

The highest value for a partition's size is 0xFFFFFFFFFFFL (15 Tera Rows), leaving the highest 0xFFFFF of the 64 bit long available to use as a mask. This PR introduces a change in the partition segment size long to be structured in this way:

when read-only bit is set, the partition is read only.

To check a partition's size we obtain maskedSize from the partition table (offset + PARTITION_MASKED_SIZE_OFFSET) and we conduct an & with PARTITION_SIZE_MASK.

To update a partition's size we obtain maskedSize from the partition table and update it like this:

maskedSize = (maskedSize & PARTITION_MASK_MASK) | (partitionSize & PARTITION_SIZE_MASK)

where PARTITION_MASK_MASK is 0x7FFFF00000000000L.

To check that a partition is read only we obtain maskedSize and test the 62nd bit.

The read-only flag is set by the attach partition operation, for partitions attached from soft link. There are no other code paths to set it (although in testing I have hacked a way).

When read-only is set, these operations FAIL (ReadOnlyViolationException "skipping RW operation on RO partition"):

• any update

When read-only is set, these operations are NO-OP:

• any ingests that occur in TableWriter
• any purge operation such as delete columns or partitions (ColumnPurgeOperator and O3PartitionPurgeJob)

These operations will SUCCEED:

• detach partition (if via soft link, just unlink and no creation of further .detached partition)
• drop partition (if via soft link, just unlink)
• rename column
• add column
• drop column (leaves column files, does not remove them)
• add index
• drop index

Class Path made me constantly make mistakes all due to the requirement that all strings need to be null terminated. Under the scenes we would add a null char to the buffer and move the write pointer forward (a call to .$()). To later use the path again you would have to .chop() it first, to move the write pointer a position back. Now we do not move the pointer when we add the null char, and .chop() no longer exists. The path is ready for .concat() right after a call to .$() (which btw is now idempotent), the write pointer is pointing at the null so it will be overriden.