// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package google

import (
	"context"
	"encoding/json"
	"fmt"
	"io/ioutil"
	"net/http"
	"os"
	"path/filepath"
	"runtime"

	"cloud.google.com/go/compute/metadata"
	"golang.org/x/oauth2"
	"golang.org/x/oauth2/authhandler"
)

// Credentials holds Google credentials, including "Application Default Credentials".
// For more details, see:
// https://developers.google.com/accounts/docs/application-default-credentials
// Credentials from external accounts (workload identity federation) are used to
// identify a particular application from an on-prem or non-Google Cloud platform
// including Amazon Web Services (AWS), Microsoft Azure or any identity provider
// that supports OpenID Connect (OIDC).
type Credentials struct {
	ProjectID   string // may be empty
	TokenSource oauth2.TokenSource

	// JSON contains the raw bytes from a JSON credentials file.
	// This field may be nil if authentication is provided by the
	// environment and not with a credentials file, e.g. when code is
	// running on Google Cloud Platform.
	JSON []byte
}

// DefaultCredentials is the old name of Credentials.
//
// Deprecated: use Credentials instead.
type DefaultCredentials = Credentials

// CredentialsParams holds user supplied parameters that are used together
// with a credentials file for building a Credentials object.
type CredentialsParams struct {
	// Scopes is the list OAuth scopes. Required.
	// Example: https://www.googleapis.com/auth/cloud-platform
	Scopes []string

	// Subject is the user email used for domain wide delegation (see
	// https://developers.google.com/identity/protocols/oauth2/service-account#delegatingauthority).
	// Optional.
	Subject string

	// AuthHandler is the AuthorizationHandler used for 3-legged OAuth flow. Optional.
	AuthHandler authhandler.AuthorizationHandler

	// State is a unique string used with AuthHandler. Optional.
	State string
}

func (params CredentialsParams) deepCopy() CredentialsParams {
	paramsCopy := params
	paramsCopy.Scopes = make([]string, len(params.Scopes))
	copy(paramsCopy.Scopes, params.Scopes)
	return paramsCopy
}

// DefaultClient returns an HTTP Client that uses the
// DefaultTokenSource to obtain authentication credentials.
func DefaultClient(ctx context.Context, scope ...string) (*http.Client, error) {
	ts, err := DefaultTokenSource(ctx, scope...)
	if err != nil {
		return nil, err
	}
	return oauth2.NewClient(ctx, ts), nil
}

// DefaultTokenSource returns the token source for
// "Application Default Credentials".
// It is a shortcut for FindDefaultCredentials(ctx, scope).TokenSource.
func DefaultTokenSource(ctx context.Context, scope ...string) (oauth2.TokenSource, error) {
	creds, err := FindDefaultCredentials(ctx, scope...)
	if err != nil {
		return nil, err
	}
	return creds.TokenSource, nil
}

// FindDefaultCredentialsWithParams searches for "Application Default Credentials".
//
// It looks for credentials in the following places,
// preferring the first location found:
//
//  1. A JSON file whose path is specified by the
//     GOOGLE_APPLICATION_CREDENTIALS environment variable.
//     For workload identity federation, refer to
//     https://cloud.google.com/iam/docs/how-to#using-workload-identity-federation on
//     how to generate the JSON configuration file for on-prem/non-Google cloud
//     platforms.
//  2. A JSON file in a location known to the gcloud command-line tool.
//     On Windows, this is %APPDATA%/gcloud/application_default_credentials.json.
//     On other systems, $HOME/.config/gcloud/application_default_credentials.json.
//  3. On Google App Engine standard first generation runtimes (<= Go 1.9) it uses
//     the appengine.AccessToken function.
//  4. On Google Compute Engine, Google App Engine standard second generation runtimes
//     (>= Go 1.11), and Google App Engine flexible environment, it fetches
//     credentials from the metadata server.
func FindDefaultCredentialsWithParams(ctx context.Context, params CredentialsParams) (*Credentials, error) {
	// Make defensive copy of the slices in params.
	params = params.deepCopy()

	// First, try the environment variable.
	const envVar = "GOOGLE_APPLICATION_CREDENTIALS"
	if filename := os.Getenv(envVar); filename != "" {
		creds, err := readCredentialsFile(ctx, filename, params)
		if err != nil {
			return nil, fmt.Errorf("google: error getting credentials using %v environment variable: %v", envVar, err)
		}
		return creds, nil
	}

	// Second, try a well-known file.
	filename := wellKnownFile()
	if creds, err := readCredentialsFile(ctx, filename, params); err == nil {
		return creds, nil
	} else if !os.IsNotExist(err) {
		return nil, fmt.Errorf("google: error getting credentials using well-known file (%v): %v", filename, err)
	}

	// Third, if we're on a Google App Engine standard first generation runtime (<= Go 1.9)
	// use those credentials. App Engine standard second generation runtimes (>= Go 1.11)
	// and App Engine flexible use ComputeTokenSource and the metadata server.
	if appengineTokenFunc != nil {
		return &DefaultCredentials{
			ProjectID:   appengineAppIDFunc(ctx),
			TokenSource: AppEngineTokenSource(ctx, params.Scopes...),
		}, nil
	}

	// Fourth, if we're on Google Compute Engine, an App Engine standard second generation runtime,
	// or App Engine flexible, use the metadata server.
	if metadata.OnGCE() {
		id, _ := metadata.ProjectID()
		return &DefaultCredentials{
			ProjectID:   id,
			TokenSource: ComputeTokenSource("", params.Scopes...),
		}, nil
	}

	// None are found; return helpful error.
	const url = "https://developers.google.com/accounts/docs/application-default-credentials"
	return nil, fmt.Errorf("google: could not find default credentials. See %v for more information.", url)
}

// FindDefaultCredentials invokes FindDefaultCredentialsWithParams with the specified scopes.
func FindDefaultCredentials(ctx context.Context, scopes ...string) (*Credentials, error) {
	var params CredentialsParams
	params.Scopes = scopes
	return FindDefaultCredentialsWithParams(ctx, params)
}

// CredentialsFromJSONWithParams obtains Google credentials from a JSON value. The JSON can
// represent either a Google Developers Console client_credentials.json file (as in ConfigFromJSON),
// a Google Developers service account key file, a gcloud user credentials file (a.k.a. refresh
// token JSON), or the JSON configuration file for workload identity federation in non-Google cloud
// platforms (see https://cloud.google.com/iam/docs/how-to#using-workload-identity-federation).
func CredentialsFromJSONWithParams(ctx context.Context, jsonData []byte, params CredentialsParams) (*Credentials, error) {
	// Make defensive copy of the slices in params.
	params = params.deepCopy()

	// First, attempt to parse jsonData as a Google Developers Console client_credentials.json.
	config, _ := ConfigFromJSON(jsonData, params.Scopes...)
	if config != nil {
		return &Credentials{
			ProjectID:   "",
			TokenSource: authhandler.TokenSource(ctx, config, params.State, params.AuthHandler),
			JSON:        jsonData,
		}, nil
	}

	// Otherwise, parse jsonData as one of the other supported credentials files.
	var f credentialsFile
	if err := json.Unmarshal(jsonData, &f); err != nil {
		return nil, err
	}
	ts, err := f.tokenSource(ctx, params)
	if err != nil {
		return nil, err
	}
	return &DefaultCredentials{
		ProjectID:   f.ProjectID,
		TokenSource: ts,
		JSON:        jsonData,
	}, nil
}

// CredentialsFromJSON invokes CredentialsFromJSONWithParams with the specified scopes.
func CredentialsFromJSON(ctx context.Context, jsonData []byte, scopes ...string) (*Credentials, error) {
	var params CredentialsParams
	params.Scopes = scopes
	return CredentialsFromJSONWithParams(ctx, jsonData, params)
}

func wellKnownFile() string {
	const f = "application_default_credentials.json"
	if runtime.GOOS == "windows" {
		return filepath.Join(os.Getenv("APPDATA"), "gcloud", f)
	}
	return filepath.Join(guessUnixHomeDir(), ".config", "gcloud", f)
}

func readCredentialsFile(ctx context.Context, filename string, params CredentialsParams) (*DefaultCredentials, error) {
	b, err := ioutil.ReadFile(filename)
	if err != nil {
		return nil, err
	}
	return CredentialsFromJSONWithParams(ctx, b, params)
}