Incremental update of huge tags db -- parse the VCS diff

[amerlyq]

Idea: parse changeset removed/added lines and modify tags database in-place accordingly by removing tags from - lines and adding tags from + lines. In theory it may significantly reduce time to update tagfile and will allow to automatically run ctags from pre-commit and post-checkout hooks without user workflow disruption.

Suggested command:

git diff-index -U0 $(git rev-list -1 --before=$(date +%s -r tags) master) | ctags --incremental ...

The question: is it feasible?

• Won't underlying filesystem ruin theoretical performance due to necessity of reading whole file and dumping it whole back after modifications?
• Are tagfile parser and formatter fast enough to parse large tags file and dump them back after modifications, or they will be as slow, as recursively parsing directly from sources?
• Do we need some novel logfile-like append-only DB format to reduce the performance hit, CoW cloning and SSD wear?

[masatake]

You may be interested in the discssion we had at universal-ctags/ctags#423 .