auth: add auth/git package for Git provider auth

Add package `auth/git` that provides a package level method
`GetCredentials()` which returns credentials to authenticate against a
Git provider.

Signed-off-by: Sanskar Jaiswal <jaiswalsanskar078@gmail.com>

auth/git/credentials.go

@@ -0,0 +1,146 @@
+/*
+Copyright 2023 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 git
+
+import (
+	"context"
+	"time"
+
+	"github.com/fluxcd/pkg/auth"
+	"github.com/fluxcd/pkg/auth/azure"
+	"github.com/fluxcd/pkg/auth/gcp"
+	"github.com/fluxcd/pkg/auth/github"
+)
+
+const GitHubAccessTokenUsername = "x-access-token"
+
+// Credentials contains the various authentication data needed
+// in order to access a Git repository.
+type Credentials struct {
+	Username    string `json:"username,omitempty"`
+	Password    string `json:"password,omitempty"`
+	BearerToken string `json:"bearerToken,omitempty"`
+}
+
+// ToSecretData returns the Credentials object in the format
+// of the data found in Kubernetes Generic Secret.
+func (c *Credentials) ToSecretData() map[string][]byte {
+	var data map[string][]byte
+
+	if c.BearerToken != "" {
+		data["bearerToken"] = []byte(c.BearerToken)
+	}
+	if c.Password != "" {
+		data["password"] = []byte(c.Password)
+	}
+	if c.Username != "" {
+		data["username"] = []byte(c.Username)
+	}
+	return data
+}
+
+// GetCredentials returns authentication credentials for accessing the provided
+// Git repository. If both authOpts and providerOpts try to configure the same
+// attribute of a provider, such as the Secret to use, the value specified
+// through providerOpts will be considered.
+//
+// If caching is enabled and cacheKey is not blank, the credentials are cached
+// according to the ttl advertised by the Git provider.
+func GetCredentials(ctx context.Context, provider string, authOpts *auth.AuthOptions) (*Credentials, error) {
+	var creds Credentials
+
+	cache := auth.GetCache()
+	if cache != nil && authOpts != nil && authOpts.CacheKey != "" {
+		val, found := cache.Get(authOpts.CacheKey)
+		if found {
+			creds = val.(Credentials)
+			return &creds, nil
+		}
+	}
+
+	var expiresIn time.Duration
+	switch provider {
+	case auth.ProviderAzure:
+		var opts []azure.ProviderOptFunc
+		if authOpts != nil {
+			opts = authOpts.ProviderOptions.AzureOpts
+		}
+		azureProvider := azure.NewProvider(opts...)
+
+		armToken, err := azureProvider.GetResourceManagerToken(ctx)
+		if err != nil {
+			return nil, err
+		}
+		creds = Credentials{
+			BearerToken: armToken.Token,
+		}
+		expiresIn = armToken.ExpiresOn.UTC().Sub(time.Now().UTC())
+	case auth.ProviderGCP:
+		var opts []gcp.ProviderOptFunc
+		if authOpts != nil {
+			opts = authOpts.ProviderOptions.GcpOpts
+		}
+		gcpProvider := gcp.NewProvider(opts...)
+
+		saToken, err := gcpProvider.GetServiceAccountToken(ctx)
+		if err != nil {
+			return nil, err
+		}
+		email, err := gcpProvider.GetServiceAccountEmail(ctx)
+		if err != nil {
+			return nil, err
+		}
+
+		creds = Credentials{
+			Username: email,
+			Password: saToken.AccessToken,
+		}
+		expiresIn = time.Duration(saToken.ExpiresIn)
+	case auth.ProviderGitHub:
+		var opts []github.ProviderOptFunc
+		if authOpts != nil {
+			if authOpts.Secret != nil {
+				opts = append(opts, github.WithSecret(*authOpts.Secret))
+			}
+			opts = append(opts, authOpts.ProviderOptions.GitHubOpts...)
+		}
+
+		ghProvider, err := github.NewProvider(opts...)
+		if err != nil {
+			return nil, err
+		}
+
+		appToken, err := ghProvider.GetAppToken(ctx)
+		if err != nil {
+			return nil, err
+		}
+		creds = Credentials{
+			Username: GitHubAccessTokenUsername,
+			Password: appToken.Token,
+		}
+		expiresIn = appToken.ExpiresIn
+	default:
+		return nil, nil
+	}
+
+	if cache != nil && authOpts != nil && authOpts.CacheKey != "" {
+		if err := cache.Set(authOpts.CacheKey, creds, expiresIn); err != nil {
+			return nil, err
+		}
+	}
+	return &creds, nil
+}