check: pin flutter version

* Introduce a git submodule to pin the version of flutter SDK under
  `vendor/flutter`.
* Use direnv to add `vendor/flutter/bin` to the current PATH so that
  subsequent calls to `flutter` on the shell uses the our pinned
  version of flutter.
* Update instructions to use pinned version of flutter.
* Pin to 18340ea16c of flutter which matches the current lowerbound
version in pubsec.yaml (3.21.0-12.0.pre.26).

NOTE: Users can still choose to opt-out and use their own version of
flutter. This just provides a recommended and tested version on our
CI.

Closes zulip#15

.envrc

@@ -0,0 +1,5 @@
+# This file is used by direnv to setup the environment when entering
+# to use vendored flutter SDK.
+
+# Comment out the next line if you want to use your system flutter.
+PATH_add vendor/flutter/bin

.github/workflows/ci.yml

@@ -6,17 +6,25 @@ jobs:
   check:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - name: Check out repository
+      uses: actions/checkout@v3
+      # We're not using `with: submodules` here because we can't do a
+      # depth=1 clone. See below for more details.
 
-    - name: Clone Flutter SDK
+    - name: Checkout flutter submodule
       # We can't do a depth-1 clone, because we need the most recent tag
       # so that Flutter knows its version and sees the constraint in our
       # pubspec is satisfied.  It's uncommon for flutter/flutter to go
-      # more than 100 commits between tags.  Fetch 1000 for good measure.
+      # more than 100 commits between tags. Fetch 1000 for good measure.
       run: |
-        git clone --depth=1000 https://github.com/flutter/flutter ~/flutter
-        TZ=UTC git --git-dir ~/flutter/.git log -1 --format='%h | %ci | %s' --date=iso8601-local
-        echo ~/flutter/bin >> "$GITHUB_PATH"
+        git submodule update --init --depth 1000
+
+    - name: Add vendored flutter to PATH
+      run: |
+        echo vendor/flutter/bin >> "$GITHUB_PATH"
+
+    - name: Run setup
+      run: tools/setup-vendor-flutter
 
     - name: Download Flutter SDK artifacts (flutter precache)
       run: flutter precache --universal

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "vendor/flutter"]
+	path = vendor/flutter
+	url = https://github.com/flutter/flutter.git
+	branch = main

.vscode/settings.json

@@ -17,4 +17,5 @@
 
   // This much more focused automatic fix is helpful, though.
   "files.trimTrailingWhitespace": true,
+  "dart.flutterSdkPaths": ["vendor/flutter"],
 }

README.md

@@ -92,26 +92,75 @@ Two specific points to expand on:
 
 ### Setting up
 
-1. Follow the [Flutter installation guide](https://docs.flutter.dev/get-started/install)
-   for your platform of choice.
-2. Switch to the latest version of Flutter by running `flutter channel main`
-   and `flutter upgrade` (see [Flutter version](#flutter-version) below).
-3. Ensure Flutter is correctly configured by running `flutter doctor`.
-4. Start the app with `flutter run`, or from your IDE.
+1. Setup the pinned version of flutter.
+
+    1.1. Make sure you have initialized submodule by doing:
+
+    ```sh
+    # Update and initialize submodule
+    git submodule update --init
+    # Run setup-vendor-flutter
+    tools/setup-vendor-flutter
+    ```
+
+    1.2. Install [direnv][direnv], and set up its hook:
+
+    ```sh
+    # On Debian or Ubuntu (bash):
+    sudo apt install direnv && echo 'eval "$(direnv hook bash)"' >>~/.bashrc
+
+    # Or using Homebrew:
+    brew install direnv
+
+    # Or see: https://direnv.net/docs/installation.html
+    # Then: https://direnv.net/docs/hook.html
+
+    # After installing and setting up the hook, run this in the zulip-flutter
+    # directory to allow the .envrc file.
+    direnv allow
+    ```
+
+   1.3. Follow remaining instructions in [Flutter installation guide](https://docs.flutter.dev/get-started/install)
+   for your platform of choice. You can skip the Flutter SDK installation as that is now done.
+
+2. Ensure Flutter is correctly configured by running `flutter doctor`.
+3. Start the app with `flutter run`, or from your IDE.
+   If you're using VSCode, you're set.
+   If you're using Android Studio, please read the next step.
+
+   3.1 For Android Studio users only (unless you are using your own
+       custom version of flutter):
+
+   After you first open, the project, go to:
+     `Settings -> Languages & Frameworks -> Flutter`
+   Under `Flutter SDK Path`, enter `<REPO_DIR>/vendor/flutter` (where
+   `<REPO_DIR>` is the location of the checkout of the repo).
 
 
 ### Flutter version
 
-While in the beta phase, we use the latest Flutter from Flutter's
-main branch.  Use `flutter channel main` and `flutter upgrade`.
+We pin to a particular version of flutter SDK via git submodules in
+[vendor/flutter/](vendor/flutter/).
+If your local checkout of this repository does not have this submodule
+checked out at the same commit, the build may fail.
+
+However, if you want to manage your own flutter SDK version you can
+opt out of this behavior by either of:
+- Skip installing direnv or don't do `direnv allow`.
+- Comment out the lines in `.envrc`.
+
+Do note that if you do this, you need to manually make sure that the
+flutter SDK version matches or is compatible with the version
+indicated by the submodule commit SHA.
 
-We don't pin a specific version, because Flutter itself doesn't offer
-a way to do so.  So far that hasn't been a problem.  When it becomes one,
-we'll figure it out; there are several tools for this in the Flutter
-community.  See [issue #15][].
+Otherwise, the build can fail as we have not tested the current code
+with that particular flutter SDK version.
 
-[issue #15]: https://github.com/zulip/zulip-flutter/issues/15
+If you want manage your own flutter SDK version, follow the [Flutter
+installation guide](https://docs.flutter.dev/get-started/install) for
+your platform of choice.
 
+[direnv]: https://direnv.net/
 
 ### Tests
 
@@ -242,13 +291,16 @@ that's a good prompt to do this.  We also do this when there's a
 new PR merged that we particularly want to take.
 
 To update the version bounds:
+* First, make sure you're on the `main` channel, run:
+  `flutter channel main`.
 * Use `flutter upgrade` to upgrade your local Flutter and Dart.
 * Update the lower bounds at `environment` in `pubspec.yaml`
   to the new versions, as seen in `flutter --version`.
 * Run `flutter pub get`, which will update `pubspec.lock`.
 * Make a quick check that things work: `tools/check`,
   and do a quick smoke-test of the app.
-* Commit and push the changes in `pubspec.yaml` and `pubspec.lock`.
+* Commit and push the changes in the submodule `vendor/flutter`,
+  `pubspec.yaml` and `pubspec.lock`.
 
 
 ### Upgrading dependencies

analysis_options.yaml

@@ -12,6 +12,9 @@ analyzer:
     # TODO(pigeon) re-enable lints once clean: https://github.com/flutter/flutter/issues/145633
     - lib/host/*.g.dart
 
+    # Skip analysis on `vendor/`, which has outside code like the Flutter SDK.
+    - vendor/**
+
 linter:
   # For a list of all available lints, with docs, see:
   #   https://dart-lang.github.io/linter/lints/index.html.

tools/check

@@ -353,7 +353,7 @@ run_icons() {
 
 run_shellcheck() {
     # Omitted from this check: nothing (nothing known, anyway).
-    files_check tools/ '!*.'{dart,js,json} \
+    files_check tools/ '!*.'{dart,js,json} .envrc \
     || return 0
 
     # Shellcheck is fast, <1s; so if we touched any possible targets at all,
@@ -362,6 +362,7 @@ run_shellcheck() {
     targets=(
         $(git grep -l '#!.*sh\b' -- tools/)
         $(git ls-files -- tools/'*.sh')
+        .envrc
     )
 
     if ! type shellcheck >/dev/null 2>&1; then

tools/lib/git.sh

@@ -90,3 +90,13 @@ git_base_commit() {
 git_changed_files() {
     git diff --name-only --diff-filter=d "$@"
 }
+
+# Note that this assumes ONE submodule. If we ever have more than one,
+# we'll need to generalize this.
+submodule_is_clean()
+{
+    # first character of every line status indicates the status of the
+    # submodule. If there is a space, it means the submodule is clean
+    # and initiated.
+    ! git submodule status -- "$@" | grep -q "^[^ ]"
+}

tools/setup-vendor-flutter

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+this_dir=${BASH_SOURCE[0]%/*}
+
+# shellcheck source=tools/lib/git.sh
+. "${this_dir}"/lib/git.sh
+
+
+# One-time setup to let git submodule + flutter SDK know that we're
+# using the main channel (main branch).
+# We should do a quick check that there's no changes.
+if ! submodule_is_clean "vendor/flutter" ; then
+    echo >&2 "There are changes in 'vendor/flutter' tree or it is not initialized."
+    echo >&2 "Please run 'git submodule update --init' then run this script again."
+    exit 1
+fi
+
+git -C vendor/flutter checkout -B main HEAD