(Lianja) OAuth2 Token using IdentityServer4 with Client Credentials

Demonstrates how to get an OAuth2 access token using the client credential flow with IdentityServer4.

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// This example assumes the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

loHttp = createobject("CkHttp")

// The first step is to fetch your IdentityServer4's discovery document
// (OpenID Connect defines a discovery mechanism, called OpenID Connect Discovery, where an OpenID server publishes its metadata at a well-known URL, 
// typically https://server.com/.well-known/openid-configuration

loResp = loHttp.QuickRequest("GET","https://localhost:5000/.well-known/openid-configuration")
if (loHttp.LastMethodSuccess <> .T.) then
    ? loHttp.LastErrorText
    release loHttp
    return
endif

if (loResp.StatusCode <> 200) then
    ? "Received response status code " + str(loResp.StatusCode)
    ? "Response body containing error text or JSON:"
    ? loResp.BodyStr
    release loResp
    release loHttp
    return
endif

loJson = createobject("CkJsonObject")
llSuccess = loJson.Load(loResp.BodyStr)
release loResp

// We have the discovery document, which contains something like this:

// You can use this online tool to generate parsing code from sample JSON: 
// Generate Parsing Code from JSON

// {
//   "issuer": "https://localhost:5000",
//   "jwks_uri": "https://localhost:5000/.well-known/openid-configuration/jwks",
//   "authorization_endpoint": "https://localhost:5000/connect/authorize",
//   "token_endpoint": "https://localhost:5000/connect/token",
//   "userinfo_endpoint": "https://localhost:5000/connect/userinfo",
//   "end_session_endpoint": "https://localhost:5000/connect/endsession",
//   "check_session_iframe": "https://localhost:5000/connect/checksession",
//   "revocation_endpoint": "https://localhost:5000/connect/revocation",
//   "introspection_endpoint": "https://localhost:5000/connect/introspect",
//   "frontchannel_logout_supported": true,
//   "frontchannel_logout_session_supported": true,
//   "backchannel_logout_supported": true,
//   "backchannel_logout_session_supported": true,
//   "scopes_supported": [
//     "openid",
//     "profile",
//     "email",
//     "MyCompany.profile",
//     "MyCompany.Identity.WebApi",
//     "MyCompany.TriHub.WebApi",
//     "offline_access"
//   ],
//   "claims_supported": [
//     "sub",
//     "updated_at",
//     "locale",
//     "zoneinfo",
//     "birthdate",
//     "gender",
//     "website",
//     "profile",
//     "preferred_username",
//     "nickname",
//     "middle_name",
//     "given_name",
//     "family_name",
//     "name",
//     "picture",
//     "email_verified",
//     "email",
//     "userId",
//     "groups",
//     "fullname"
//   ],
//   "grant_types_supported": [
//     "authorization_code",
//     "client_credentials",
//     "refresh_token",
//     "implicit",
//     "password"
//   ],
//   "response_types_supported": [
//     "code",
//     "token",
//     "id_token",
//     "id_token token",
//     "code id_token",
//     "code token",
//     "code id_token token"
//   ],
//   "response_modes_supported": [
//     "form_post",
//     "query",
//     "fragment"
//   ],
//   "token_endpoint_auth_methods_supported": [
//     "client_secret_basic",
//     "client_secret_post"
//   ],
//   "subject_types_supported": [
//     "public"
//   ],
//   "id_token_signing_alg_values_supported": [
//     "RS256"
//   ],
//   "code_challenge_methods_supported": [
//     "plain",
//     "S256"
//   ]
// }
// 

// The next steps are to (1) get the token_endpoint,
// and (2) verify that the client_credentials grant type is supported.

lcTokenEndpoint = loJson.StringOf("token_endpoint")

loGrantTypes = loJson.ArrayOf("grant_types_supported")
lnClientCredentialsIdx = loGrantTypes.FindString("client_credentials",.T.)
release loGrantTypes

// If clientCredentialsIdx is less then zero (-1) then the "client_credentials" string was not found.
if (lnClientCredentialsIdx < 0) then
    ? "The client credentials grant type is not supported."
    release loHttp
    release loJson
    return
endif

// Request the access token using our Client ID and Client Secret.
// We're going to duplicate this CURL statement:

// curl --request POST \
//   --url '<tokenEndpoint>' \
//   --header 'content-type: application/x-www-form-urlencoded' \
//   --data 'grant_type=client_credentials&client_id=CLIENT_ID&client_secret=CLIENT_SECRET'

loReq = createobject("CkHttpRequest")
loReq.HttpVerb = "POST"
loReq.AddParam("grant_type","client_credentials")
loReq.AddParam("client_id","CLIENT_ID")
loReq.AddParam("client_secret","CLIENT_SECRET")

loResp = loHttp.PostUrlEncoded(lcTokenEndpoint,loReq)
if (loHttp.LastMethodSuccess = .F.) then
    ? loHttp.LastErrorText
    release loHttp
    release loJson
    release loReq
    return
endif

// Make sure we got a 200 response status code, otherwise it's an error.
if (loResp.StatusCode <> 200) then
    ? "POST to token endpoint failed."
    ? "Received response status code " + str(loResp.StatusCode)
    ? "Response body containing error text or JSON:"
    ? loResp.BodyStr
    release loResp
    release loHttp
    release loJson
    release loReq
    return
endif

llSuccess = loJson.Load(loResp.BodyStr)
release loResp

// Our JSON response should contain this:
// {
//   "access_token":"eyJz93a...k4laUWw",
//   "token_type":"Bearer",
//   "expires_in":86400
// }

// Get the access token:
lcAccessToken = loJson.StringOf("access_token")

// The access token is what gets added to "Authorization: Bearer <access_token>"
// for the subsequent REST API calls..


release loHttp
release loJson
release loReq