RFC: System Buildpacks

text/0000-system-buildpacks-in-builder.md

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+# Meta
+[meta]: #meta
+- Name: System Buildpacks in Builder Images
+- Start Date: 2021-07-24
+- Author(s): [@jkutner](https://github.com/jkutner)
+- RFC Pull Request: (leave blank)
+- CNB Pull Request: (leave blank)
+- CNB Issue: (leave blank)
+- Supersedes: N/A
+
+# Summary
+[summary]: #summary
+
+This is a proposal for a mechanism that would allow a builder to contain a default set of buildpacks that participate in every detection group, regardless of the buildpack order passed by the platform.
+
+# Definitions
+[definitions]: #definitions
+
+* _system buildpacks_ - a standard buildpack, conforming to the Buildpack API, which participate in all groups
+
+# Motivation
+[motivation]: #motivation
+
+Forthcoming changes to the lifecycle (such as [removal of shell-specific logic](https://github.com/buildpacks/rfcs/pull/168)) will remove capabilities that users have come to expect. This includes mechanisms like `.profile`, which allows a buildpack user to customize the environment a process type runs in. We seek to replace these lost mechanisms with buildpacks, in an effort to preserve the capability while still removing complexity from the lifecycle.
+
+# What it is
+[what-it-is]: #what-it-is
+
+We introduce a `[system]` table in the `builder.toml` schema with the following structure:
+
+```
+[[system.pre.buildpacks]]
+  id = "<buildpack ID>"
+  version = "<buildpack version>"
+  optional = false
+
+[[system.post.buildpacks]]
+  id = "<buildpack ID>"
+  version = "<buildpack version>"
+  optional = false
+```
+
+The fields in the `system.pre.buildpacks` table and `system.post.buildpacks` table match the fields in the existing [`order.group` table](https://buildpacks.io/docs/reference/config/builder-config/#order-_list-required_).
+
+When a builder includes one or more `system.*.buildpacks` entry, the detect phase will prepend and append all `pre` and `post` buildpacks to each detection group in the provided order, respectively.
+
+**Note:** A non-`optional` system buildpack creates the possibility that a user provided group with all optional buildpacks could pass detection when it otherwise would not. We leave that up to the platform/builder owner. As long as the platform has a mechanism to disable system buildpacks (and `pack` will), then there is an escape valve for this situation.
+
+# How it Works
+[how-it-works]: #how-it-works
+
+System buildpacks conform to the [buildpack API](https://github.com/buildpacks/spec/blob/main/buildpack.md).
+
+The `system.*pre*.buildpacks` will be provided to the lifecycle into a new file, `system.toml`. The `[system]` table in `system.toml` will be processed by the lifecycle, and each `pre`/`post` buildpack will run during the detect phase. Those that pass detection will run during the build phase.
+
+## Detection
+
+The exit code of detection by system buildpacks MUST influence the selected buildpack group. If a system buildpack is non-optional and fails detection, then detection MUST for that group fail. If a system buildpack is optional and passes detection, then detection MAY pass for that group.
+
+System buildpacks may require/provide in the build plan following standard buildpack API specification.
+
+## Build
+
+System buildpacks that have passed detection will be added to `group.toml` and treated like any other buildpack for the remainder of the build.
+
+If a system buildpack exits with a status of `100`, the build will fail.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- If system buildpacks are hidden from the user before the build, their execution may be unexpected.
+
+# Alternatives
+[alternatives]: #alternatives
+
+## Do Nothing
+
+End users would have to add buildpacks like the `profile-buildpack` or other buildpacks that implement system/spec behaviors themselves.
+
+## Use the `[[order]]` table
+
+Instead of a new `[system]` table, we could put `pre` and `post` in the `[[order]]` table. However, this could imply that there is a interaction/override/etc between these buildpacks and the `pre`/`post` buildpacks in `project.toml`. But there is not.
+
+## Use the `[lifecycle]` table
+
+```
+[lifecycle]
+  version = "<string>"
+
+  [[lifecycle.pre.buildpacks]]
+    id = "<buildpack ID>"
+    version = "<buildpack version>"
+    optional = false
+
+  [[lifecycle.post.buildpacks]]
+    id = "<buildpack ID>"
+    version = "<buildpack version>"
+    optional = false
+```
+
+# Prior Art
+[prior-art]: #prior-art
+
+- [RFC-0078: Group additions to Builder order](https://github.com/buildpacks/rfcs/blob/main/text/0078-group-additions.md)
+
+# Unresolved Questions
+[unresolved-questions]: #unresolved-questions
+
+
+# Spec. Changes (OPTIONAL)
+[spec-changes]: #spec-changes
+
+## `detector` in Platform specifiction
+
+This proposal introduces a `--system` option on the `detector`.
+
+```
+/cnb/lifecycle/detector \
+  [--system <system>]\
+```
+
+Where:
+
+* the lifecycle SHALL merge the `pre` group with each group from `<system>` such that the `pre` buildpacks are placed at the beginning of each order group before running detection.
+* SHALL merge the `post` group with each group from `<system>` such that the `post` buildpacks are placed at the end of each order group before running detection.
+
+#### `system.toml` (TOML)
+
+```toml
+[[system.pre.buildpacks]]
+  id = "<buildpack ID>"
+  version = "<buildpack version>"
+  optional = true
+
+[[system.post.buildpacks]]
+  id = "<buildpack ID>"
+  version = "<buildpack version>"
+  optional = true
+```
+
+