Mghash

This is mghash, an extension to Mage for computing hash-based dependencies.

Mage already includes functions for skipping recompilation of targets whose file modtimes are newer than those of their sources. (See github.com/magefile/mage/target.) This library does the same thing, but based on content hashes, not file modtimes.

Example

This Magefile snippet defines a rule called Generate for generating foo.pb.go from foo.proto. Before running protoc, the contents of foo.proto and foo.pb.go are hashed together with the text of the protoc command. If the resulting hash is present in the hashes.sqlite databaase then foo.pb.go is considered to be up to date and is not rebuilt. Otherwise the protoc command runs, after which the hash is recomputed and added to the database.

import (
  "github.com/bobg/mghash"
  "github.com/bobg/mghash/sqlite"
)

func Generate() error {
  db, err := sqlite.Open("hashes.sqlite")
  if err != nil { ... }
  defer db.Close()
  
  mg.Deps(&mghash.Fn{DB: db, Rule: mghash.JRule{
    Sources: []string{"foo.proto"},
    Targets: []string{"foo.pb.go"},
    Command: []string{"protoc", "-I.", "--go_out=.", "foo.proto"},
  }})
}