[RFC-007] Implement GitHub app authentication for git repositories.

- Add github app based authentication method to fetch installation token in auth package.
- Add unit tests to test the github app authentication
- Add github provider options in git package.
- Use the github provider to clone from go-git package.
- Add unit tests to fetch git credentials and cloning the repository using github app authentication.
- Add e2e tests to test pull/push to git repositories using github app authentication.
- Update the github workflow to run e2etests from CI.

Signed-off-by: Dipti Pai <diptipai89@outlook.com>

.github/workflows/e2e.yaml

@@ -61,6 +61,9 @@ jobs:
               export GITHUB_USER="fluxcd-gitprovider-bot"
               export GITHUB_ORG="fluxcd-testing"
               export GITHUB_TOKEN="${{ secrets.GITPROVIDER_BOT_TOKEN }}"
+              export GHAPP_ID="${{ secrets.GHAPP_ID }}"
+              export GHAPP_INSTALL_ID="${{ secrets.GHAPP_INSTALL_ID }}"
+              export GHAPP_PRIVATE_KEY="${{ secrets.GHAPP_PRIVATE_KEY }}"
             elif [[ ${{ matrix.provider }} = "gitlab" ]]; then
               export GO_TEST_PREFIX="TestGitLabE2E"
               export GITLAB_USER="fluxcd-gitprovider-bot"

auth/github/client.go

@@ -0,0 +1,169 @@
+/*
+Copyright 2024 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package github
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strconv"
+	"time"
+
+	"github.com/bradleyfalzon/ghinstallation/v2"
+	"golang.org/x/net/http/httpproxy"
+)
+
+const (
+	AppIDKey             = "githubAppID"
+	AppInstallationIDKey = "githubAppInstallationID"
+	AppPrivateKey        = "githubAppPrivateKey"
+	ApiURLKey            = "githubApiURL"
+)
+
+// Client is an authentication provider for GitHub Apps.
+type Client struct {
+	appID          int
+	installationID int
+	privateKey     []byte
+	apiURL         string
+	proxyConfig    *httpproxy.Config
+	ghTransport    *ghinstallation.Transport
+}
+
+// OptFunc enables specifying options for the provider.
+type OptFunc func(*Client) error
+
+// New returns a new authentication provider for GitHub Apps.
+func New(opts ...OptFunc) (*Client, error) {
+	var err error
+
+	p := &Client{}
+	for _, opt := range opts {
+		err = opt(p)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	transport := http.DefaultTransport.(*http.Transport).Clone()
+	if p.proxyConfig != nil {
+		proxyFunc := func(req *http.Request) (*url.URL, error) {
+			return p.proxyConfig.ProxyFunc()(req.URL)
+		}
+		transport.Proxy = proxyFunc
+	}
+	p.ghTransport, err = ghinstallation.New(transport, int64(p.appID), int64(p.installationID), p.privateKey)
+	if err != nil {
+		return nil, err
+	}
+
+	if p.apiURL != "" {
+		p.ghTransport.BaseURL = p.apiURL
+	}
+
+	return p, nil
+}
+
+// WithInstallationID configures the installation ID of the GitHub App.
+func WithInstllationID(installationID int) OptFunc {
+	return func(p *Client) error {
+		p.installationID = installationID
+		return nil
+	}
+}
+
+// WithAppID configures the app ID of the GitHub App.
+func WithAppID(appID int) OptFunc {
+	return func(p *Client) error {
+		p.appID = appID
+		return nil
+	}
+}
+
+// WithPrivateKey configures the private key of the GitHub App.
+func WithPrivateKey(pk []byte) OptFunc {
+	return func(p *Client) error {
+		p.privateKey = pk
+		return nil
+	}
+}
+
+// WithApiURL configures the GitHub API endpoint to use to fetch GitHub App
+// installation token.
+func WithApiURL(apiURL string) OptFunc {
+	return func(p *Client) error {
+		p.apiURL = apiURL
+		return nil
+	}
+}
+
+// WithAppData configures the client using data from a map
+func WithAppData(appData map[string][]byte) OptFunc {
+	return func(p *Client) error {
+		var err error
+		for _, key := range []string{AppIDKey, AppInstallationIDKey, AppPrivateKey} {
+			if _, exists := appData[key]; !exists {
+				return fmt.Errorf("github app data must contain key : %s", key)
+			}
+		}
+		p.appID, err = strconv.Atoi(string(appData[AppIDKey]))
+		if err != nil {
+			return fmt.Errorf("github app data error for key : %s, err: %v", AppIDKey, err)
+		}
+		p.installationID, err = strconv.Atoi(string(appData[AppInstallationIDKey]))
+		if err != nil {
+			return fmt.Errorf("github app data error for key : %s, err: %v", AppInstallationIDKey, err)
+		}
+		p.privateKey = appData[AppPrivateKey]
+		p.apiURL = string(appData[ApiURLKey])
+		return nil
+	}
+}
+
+func WithProxyConfig(proxyConfig *httpproxy.Config) OptFunc {
+	return func(p *Client) error {
+		p.proxyConfig = proxyConfig
+		return nil
+	}
+}
+
+// AppToken contains a GitHub App installation token and its expiry.
+type AppToken struct {
+	Token     string
+	ExpiresAt time.Time
+}
+
+// GetToken returns the token that can be used to authenticate
+// as a GitHub App installation.
+// Ref: https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/authenticating-as-a-github-app-installation
+func (p *Client) GetToken(ctx context.Context) (*AppToken, error) {
+	token, err := p.ghTransport.Token(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	expiresAt, _, err := p.ghTransport.Expiry()
+	if err != nil {
+		return nil, err
+	}
+
+	return &AppToken{
+		Token:     token,
+		ExpiresAt: expiresAt,
+	}, nil
+}