docs: add thoughts on future design (#157)

* docs: add thoughts on future design

* chore: fix bsdiff to bidiff

* chore: run formatter

README.md

@@ -16,11 +16,16 @@ into the Flutter engine during build time.
 
 ## Parts
 
-- `dart_cli`: Test ffi wrapping of updater library.
-- `library`: The rust library that does the actual update work.
-- `dart_bindings`: The Dart bindings for the updater library.
+- `library`: Runtime library linked into Flutter Engine to unpack and apply
+  updates. Build into libupdater.a and linked into libflutter.so.
+- `patch`: Developer tooling to package Shorebird updates ("patches"). Built
+  into `patch.exe` and downloaded and run by `shorebird` command line tool:
+  https://github.com/shorebirdtech/shorebird/tree/main/packages/shorebird_cli.
+- `shorebird_code_push`: The Dart bindings for communicating with the updater
+  library from within a Flutter app. Published to pub.dev, usage is optional by
+  developers.
 
-All of the interesting code is in the `library` directory. There is also
+Most interesting code is in the `library` directory. There is also
 a [README.md](library/README.md) in that directory explaining the design.
 
 ## Developing

patch/README.md

@@ -1,22 +1,43 @@
 # patch command line tool
 
-This is the tool used by the `shorebird` command line to compute the patch
-file for uploading to the server.
-
-This currently uses the rust `bidiff` crate to compute the patch file.
-and could just use the `bic` command line tool included in that crate. However
-we're explicitly writing our own command line to allow us to change the
-underlying compression without affecting the `shorebird` command line callers.
+This is the tool used by the `shorebird` command line to package up a patch
+file for uploading to Shorebird's servers.
 
 ## Usage
 
     patch <old> <new> <patch>
 
+## Context, design and future thoughts.
+
+Originally `patch` was written on top of the `bidiff` tool, but really no longer
+needs to be. For expediency we used the `comde` crate to have a generic api
+across multiple compression algorithms. However, we've since decided to use the
+`zstd` crate for compression and decompression and could remove our `comde` and
+`bidiff` dependencies eventually.
+
+Because `bidiff` does not check what it's patching, it's possible to apply a
+patch to the wrong file. To avoid this we currently have a separate hash
+which we save in Shorebird's database and then `library` validates that the
+patched file matches what we expected after inflation. This is the wrong design
+and we intend to move to a system whereby `patch` is responsible for including
+its own hash in the patch file and validating during application.
+
+In that world we might end up with two hashes. One who's purpose is to validate
+the patch file itself and another to validate the resulting inflated file or
+the original file (both are equivalent). Both of these hashes could be stored
+inside the patch container (.vmcode) and validated by the `library` code.
+
+We should also probably rename `patch` to `packager` or similar since it should
+do more than just bidiff. Also some of the apply/inflate code in library might
+want to move into this directory to be more of the same place.
+
+We also probably eventually want to move to something like sigstore.dev rather
+than rolling our own packaging/signing system.
 
 ## Generating test expectations
 
-The string_patch target can be used to generate test expectations for testing
-the updater.  It takes two strings as arguments and prints the necessary
+The `string_patch` target can be used to generate test expectations for testing
+the updater. It takes two strings as arguments and prints the necessary
 variables you will need in your test to stdout.
 
 ```
@@ -28,9 +49,10 @@ Hash (new): fcde2b2edba56bf408601fb721fe9b5c338d10ee429ea04fae5511b68fbf8fb9
 ```
 
 Which will translate into:
+
 ```rust
 let base = "foo";
 let new = "bar";
 let patch: Vec<u8> = vec![40, 181, 47, 253, 0, 128, 113, 0, 0, 223, 177, 0, 0, 0, 16, 0, 0, 0, 3, 98, 97, 114, 0];
 let hash = "fcde2b2edba56bf408601fb721fe9b5c338d10ee429ea04fae5511b68fbf8fb9";
-```
+```