Update Haste Map to support unix epoch Clock spec

[majames]

🚀 Feature Proposal

Context

As previously mentioned, we use jest-haste-map at Airbnb (both directly and indirectly through the Metro bundler). Developers currently run Metro in a docker container as part of their development environment. This container is cycled frequently as part of their development workflow.

We can preserve the previously computed Haste Map file in a docker volume for re-use between cycles, however this doesn't help with subsequent Haste Map build performance as the previously stored logical clock is tied to a specific instance of Watchman (which changes on each container cycle). isFresh will always be true on container cycle which results in all files in the Haste Map being re-processed even if none of them have changed between cycles!

Suggested Solution

To workaround this issue, we could instead use a simple unix timestamp for determining which files to surface in the Watchman query. Using unix timestamps with since can suffer from race conditions, however given that the jest-haste-map only issues the query on build() (i.e. container boot in our use-case) there's next to no chance of this happening.

Implementation

jest-haste-map consumers could opt-in to this behavior by passing the clockType: 'unix-timestamp' option into the HasteMap constructor and, when said argument is provided it would override response.clock below this line with a unix timestamp.

Motivation

Allow haste maps to be re-used between container cycles increasing performance!

Example

We would update our internal fork of Metro to pass clockType: 'unix-timestamp' to the Haste Map constructor.

Pitch

It belongs in Jest core because it directly effects the jest-haste-map package.

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