Source: settings.go in package google.golang.org/api/internal

Package internal supports the options and transport packages.

package internal

import (
	"crypto/tls"
	"errors"
	"net/http"

	"golang.org/x/oauth2"
	"golang.org/x/oauth2/google"
	"google.golang.org/api/internal/impersonate"
	"google.golang.org/grpc"
)

DialSettings holds information needed to establish a connection with a Google API service.

type DialSettings struct {
	Endpoint            string
	DefaultEndpoint     string
	DefaultMTLSEndpoint string
	Scopes              []string
	TokenSource         oauth2.TokenSource
	Credentials         *google.Credentials
	CredentialsFile     string // if set, Token Source is ignored.
	CredentialsJSON     []byte
	UserAgent           string
	APIKey              string
	Audiences           []string
	HTTPClient          *http.Client
	GRPCDialOpts        []grpc.DialOption
	GRPCConn            *grpc.ClientConn
	GRPCConnPool        ConnPool
	GRPCConnPoolSize    int
	NoAuth              bool
	TelemetryDisabled   bool
	ClientCertSource    func(*tls.CertificateRequestInfo) (*tls.Certificate, error)
	CustomClaims        map[string]interface{}
	SkipValidation      bool
	ImpersonationConfig *impersonate.Config

Google API system parameters. For more information please read: https://cloud.google.com/apis/docs/system-parameters

	QuotaProject  string
	RequestReason string
}

Validate reports an error if ds is invalid.

func (ds *DialSettings) Validate() error {
	if ds.SkipValidation {
		return nil
	}
	hasCreds := ds.APIKey != "" || ds.TokenSource != nil || ds.CredentialsFile != "" || ds.Credentials != nil
	if ds.NoAuth && hasCreds {
		return errors.New("options.WithoutAuthentication is incompatible with any option that provides credentials")

Credentials should not appear with other options. We currently allow TokenSource and CredentialsFile to coexist. TODO(jba): make TokenSource & CredentialsFile an error (breaking change).

	nCreds := 0
	if ds.Credentials != nil {
		nCreds++
	}
	if ds.CredentialsJSON != nil {
		nCreds++
	}
	if ds.CredentialsFile != "" {
		nCreds++
	}
	if ds.APIKey != "" {
		nCreds++
	}
	if ds.TokenSource != nil {
		nCreds++
	}
	if len(ds.Scopes) > 0 && len(ds.Audiences) > 0 {
		return errors.New("WithScopes is incompatible with WithAudience")

Accept only one form of credentials, except we allow TokenSource and CredentialsFile for backwards compatibility.

	if nCreds > 1 && !(nCreds == 2 && ds.TokenSource != nil && ds.CredentialsFile != "") {
		return errors.New("multiple credential options provided")
	}
	if ds.GRPCConn != nil && ds.GRPCConnPool != nil {
		return errors.New("WithGRPCConn is incompatible with WithConnPool")
	}
	if ds.HTTPClient != nil && ds.GRPCConnPool != nil {
		return errors.New("WithHTTPClient is incompatible with WithConnPool")
	}
	if ds.HTTPClient != nil && ds.GRPCConn != nil {
		return errors.New("WithHTTPClient is incompatible with WithGRPCConn")
	}
	if ds.HTTPClient != nil && ds.GRPCDialOpts != nil {
		return errors.New("WithHTTPClient is incompatible with gRPC dial options")
	}
	if ds.HTTPClient != nil && ds.QuotaProject != "" {
		return errors.New("WithHTTPClient is incompatible with QuotaProject")
	}
	if ds.HTTPClient != nil && ds.RequestReason != "" {
		return errors.New("WithHTTPClient is incompatible with RequestReason")
	}
	if ds.HTTPClient != nil && ds.ClientCertSource != nil {
		return errors.New("WithHTTPClient is incompatible with WithClientCertSource")
	}
	if ds.ClientCertSource != nil && (ds.GRPCConn != nil || ds.GRPCConnPool != nil || ds.GRPCConnPoolSize != 0 || ds.GRPCDialOpts != nil) {
		return errors.New("WithClientCertSource is currently only supported for HTTP. gRPC settings are incompatible")
	}
	if ds.ImpersonationConfig != nil && len(ds.ImpersonationConfig.Scopes) == 0 && len(ds.Scopes) == 0 {
		return errors.New("WithImpersonatedCredentials requires scopes being provided")
	}
	return nil