feat: add authentication credentials, token cache, and auth policies (#8)

PR: #8

Author: OmarAlJarrah

src/Dexpace.Sdk.Core/Auth/AccessToken.cs

@@ -0,0 +1,53 @@
+// Copyright (c) 2026 dexpace and Omar Aljarrah.
+// Licensed under the MIT License. See LICENSE in the repository root for details.
+
+namespace Dexpace.Sdk.Core.Auth;
+
+/// <summary>
+/// An access token returned by a <see cref="TokenCredential"/>, together with its expiry
+/// and an optional proactive-refresh hint.
+/// </summary>
+public readonly struct AccessToken
+{
+    /// <summary>
+    /// Initializes an <see cref="AccessToken"/> with an expiry and no proactive-refresh hint.
+    /// </summary>
+    /// <param name="token">The raw token string.</param>
+    /// <param name="expiresOn">The time at which this token becomes invalid.</param>
+    /// <exception cref="ArgumentNullException"><paramref name="token"/> is <see langword="null"/>.</exception>
+    public AccessToken(string token, DateTimeOffset expiresOn)
+        : this(token, expiresOn, null)
+    {
+    }
+
+    /// <summary>
+    /// Initializes an <see cref="AccessToken"/> with an expiry and an optional proactive-refresh hint.
+    /// </summary>
+    /// <param name="token">The raw token string.</param>
+    /// <param name="expiresOn">The time at which this token becomes invalid.</param>
+    /// <param name="refreshOn">
+    /// An optional hint: when the clock reaches this value the token cache should proactively
+    /// refresh, even though <paramref name="expiresOn"/> has not been reached.
+    /// </param>
+    /// <exception cref="ArgumentNullException"><paramref name="token"/> is <see langword="null"/>.</exception>
+    public AccessToken(string token, DateTimeOffset expiresOn, DateTimeOffset? refreshOn)
+    {
+        ArgumentNullException.ThrowIfNull(token);
+        Token = token;
+        ExpiresOn = expiresOn;
+        RefreshOn = refreshOn;
+    }
+
+    /// <summary>The raw token value.</summary>
+    public string Token { get; }
+
+    /// <summary>The time at which this token becomes invalid.</summary>
+    public DateTimeOffset ExpiresOn { get; }
+
+    /// <summary>
+    /// An optional proactive-refresh hint. When non-<see langword="null"/>, the token cache
+    /// begins refreshing once the clock passes this value, even though <see cref="ExpiresOn"/>
+    /// has not yet been reached.
+    /// </summary>
+    public DateTimeOffset? RefreshOn { get; }
+}

src/Dexpace.Sdk.Core/Auth/AccessTokenCache.cs

@@ -0,0 +1,172 @@
+// Copyright (c) 2026 dexpace and Omar Aljarrah.
+// Licensed under the MIT License. See LICENSE in the repository root for details.
+
+namespace Dexpace.Sdk.Core.Auth;
+
+/// <summary>
+/// An in-memory token cache that wraps a <see cref="TokenCredential"/> and provides
+/// proactive refresh, expiry-based invalidation, and single-flight protection against
+/// concurrent refresh stampedes.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Tokens are cached keyed by the <see cref="TokenRequestContext.CacheKey"/>. A cached token
+/// is served without calling the underlying credential as long as both:
+/// <list type="bullet">
+///   <item><c>now &lt; ExpiresOn</c></item>
+///   <item><c>RefreshOn</c> is <see langword="null"/> OR <c>now &lt; RefreshOn</c></item>
+/// </list>
+/// </para>
+/// <para>
+/// Once either condition fails, a single caller acquires the per-key semaphore and calls
+/// the credential. All concurrent callers wait on the semaphore and perform a double-checked
+/// read after acquiring it. Only one network round-trip fires per key per refresh cycle.
+/// </para>
+/// <para>
+/// <strong>Failure while valid:</strong> if the credential throws but a token remains valid
+/// (<c>now &lt; ExpiresOn</c>), the cached token is returned silently. If no valid token
+/// exists the exception propagates.
+/// </para>
+/// <para>
+/// This class is thread-safe. Inject a custom <see cref="TimeProvider"/> for deterministic
+/// unit testing.
+/// </para>
+/// </remarks>
+public sealed class AccessTokenCache
+{
+    private readonly TokenCredential _credential;
+    private readonly TimeProvider _time;
+
+    // Per-key state: the current cached token (null = never fetched) + a semaphore for
+    // single-flight. One CacheEntry per unique TokenRequestContext.CacheKey.
+    private readonly System.Collections.Concurrent.ConcurrentDictionary<string, CacheEntry> _entries = new();
+
+    /// <summary>
+    /// Initializes an <see cref="AccessTokenCache"/> backed by the given credential.
+    /// </summary>
+    /// <param name="credential">The underlying credential to call when a token is needed.</param>
+    /// <param name="timeProvider">
+    /// A <see cref="TimeProvider"/> used to determine "now". Defaults to
+    /// <see cref="TimeProvider.System"/> when <see langword="null"/>.
+    /// </param>
+    /// <exception cref="ArgumentNullException"><paramref name="credential"/> is <see langword="null"/>.</exception>
+    public AccessTokenCache(TokenCredential credential, TimeProvider? timeProvider = null)
+    {
+        ArgumentNullException.ThrowIfNull(credential);
+        _credential = credential;
+        _time = timeProvider ?? TimeProvider.System;
+    }
+
+    /// <summary>
+    /// Returns a valid <see cref="AccessToken"/> for the given <paramref name="context"/>,
+    /// fetching and caching one if necessary.
+    /// </summary>
+    /// <param name="context">The token request context identifying the scopes and claims.</param>
+    /// <param name="ct">A token to cancel an in-progress credential call.</param>
+    /// <returns>
+    /// A <see cref="ValueTask{AccessToken}"/> resolving to a valid access token.
+    /// </returns>
+    public async ValueTask<AccessToken> GetAsync(TokenRequestContext context, CancellationToken ct = default)
+    {
+        var entry = _entries.GetOrAdd(context.CacheKey, static _ => new CacheEntry());
+        var now = _time.GetUtcNow();
+
+        // Fast path: read the holder once (volatile acquire) and return immediately when the
+        // token is still valid and does not need a proactive refresh. The volatile read ensures
+        // that any holder published by a slow-path writer is fully visible to this thread.
+        var fastHolder = entry.Holder;
+        if (fastHolder is not null && IsValid(fastHolder.Token, now) && !NeedsRefresh(fastHolder.Token, now))
+        {
+            return fastHolder.Token;
+        }
+
+        // Slow path: acquire the semaphore for this key.
+        await entry.Semaphore.WaitAsync(ct).ConfigureAwait(false);
+        try
+        {
+            // Double-check after acquiring.
+            now = _time.GetUtcNow();
+            var recheckedHolder = entry.Holder;
+            if (recheckedHolder is not null && IsValid(recheckedHolder.Token, now) && !NeedsRefresh(recheckedHolder.Token, now))
+            {
+                return recheckedHolder.Token;
+            }
+
+            // Try to refresh from the credential.
+            try
+            {
+                var fresh = await _credential.GetTokenAsync(context, ct).ConfigureAwait(false);
+                // Volatile write releases the fully-constructed holder to all threads.
+                entry.Holder = new TokenHolder(fresh);
+                return fresh;
+            }
+            catch
+            {
+                // Re-sample time: GetTokenAsync may have taken a long time. Validate the
+                // cached token against the clock *after* the (slow) failing call, so a token
+                // that expired during the attempt is not returned as valid.
+                var nowAfterFailure = _time.GetUtcNow();
+                var fallbackHolder = entry.Holder;
+                if (fallbackHolder is not null && IsValid(fallbackHolder.Token, nowAfterFailure))
+                {
+                    return fallbackHolder.Token;
+                }
+
+                throw;
+            }
+        }
+        finally
+        {
+            entry.Semaphore.Release();
+        }
+    }
+
+    // A token is "valid" while the clock hasn't yet reached ExpiresOn.
+    private static bool IsValid(AccessToken token, DateTimeOffset now) =>
+        now < token.ExpiresOn;
+
+    // A token "needs refresh" once RefreshOn has been reached (proactive hint).
+    private static bool NeedsRefresh(AccessToken token, DateTimeOffset now) =>
+        token.RefreshOn is { } refreshOn && now >= refreshOn;
+
+    // Immutable wrapper so the token can be published through a single volatile reference,
+    // giving acquire/release ordering on all platforms. AccessToken is a multi-field struct;
+    // publishing it directly would allow fast-path readers to observe a torn or partially
+    // written value. Boxing it inside a reference type (TokenHolder) reduces the published
+    // value to a single pointer-width write, which the .NET memory model guarantees to be
+    // atomic and not torn.
+    private sealed class TokenHolder
+    {
+        /// <summary>Initializes a <see cref="TokenHolder"/> wrapping the given token.</summary>
+        /// <param name="token">The token to publish atomically.</param>
+        public TokenHolder(AccessToken token) => Token = token;
+
+        /// <summary>The wrapped access token.</summary>
+        public AccessToken Token { get; }
+    }
+
+    private sealed class CacheEntry
+    {
+        // volatile ensures that a fast-path reader always observes the most recently published
+        // holder (acquire semantics on read, release semantics on write). Combined with the
+        // pointer-width atomicity of reference reads/writes, this is safe without a lock on
+        // the fast path.
+        private volatile TokenHolder? _holder;
+
+        /// <summary>
+        /// The most recently published token holder, or <see langword="null"/> if no token
+        /// has been fetched yet. Reads and writes are reference-atomic and carry
+        /// acquire/release ordering via the <see langword="volatile"/> modifier.
+        /// </summary>
+        public TokenHolder? Holder
+        {
+            get => _holder;
+            set => _holder = value;
+        }
+
+        /// <summary>
+        /// A semaphore (initial count = 1) that serializes refresh calls for this key.
+        /// </summary>
+        public SemaphoreSlim Semaphore { get; } = new SemaphoreSlim(1, 1);
+    }
+}

src/Dexpace.Sdk.Core/Auth/ApiKeyCredential.cs

@@ -0,0 +1,56 @@
+// Copyright (c) 2026 dexpace and Omar Aljarrah.
+// Licensed under the MIT License. See LICENSE in the repository root for details.
+
+using Dexpace.Sdk.Core.Http.Common;
+
+namespace Dexpace.Sdk.Core.Auth;
+
+/// <summary>
+/// A credential that authenticates requests by stamping a static API key into an HTTP header.
+/// </summary>
+/// <remarks>
+/// By default the key is sent in the <c>Authorization</c> header with no scheme prefix, i.e.
+/// the header value is exactly <see cref="Key"/>. Specify a scheme to add a prefix, e.g.
+/// <c>"Bearer"</c> produces <c>Authorization: Bearer &lt;key&gt;</c>. Pass a custom header
+/// name to use a non-standard header such as <c>X-Api-Key</c>.
+/// </remarks>
+public sealed class ApiKeyCredential
+{
+    /// <summary>
+    /// Initializes an <see cref="ApiKeyCredential"/>.
+    /// </summary>
+    /// <param name="key">The API key value. Must not be null or empty.</param>
+    /// <param name="header">
+    /// The header to stamp. Defaults to <see cref="HttpHeaderName.WellKnown.Authorization"/>.
+    /// </param>
+    /// <param name="scheme">
+    /// An optional scheme prefix (e.g. <c>"Bearer"</c>). When <see langword="null"/> the key
+    /// is used as the entire header value.
+    /// </param>
+    /// <exception cref="ArgumentNullException"><paramref name="key"/> is <see langword="null"/>.</exception>
+    /// <exception cref="ArgumentException"><paramref name="key"/> is empty.</exception>
+    public ApiKeyCredential(string key, HttpHeaderName? header = null, string? scheme = null)
+    {
+        ArgumentNullException.ThrowIfNull(key);
+        if (key.Length == 0)
+        {
+            throw new ArgumentException("API key must not be empty.", nameof(key));
+        }
+
+        Key = key;
+        HeaderName = header ?? HttpHeaderName.WellKnown.Authorization;
+        Scheme = scheme;
+    }
+
+    /// <summary>The raw API key value.</summary>
+    public string Key { get; }
+
+    /// <summary>The HTTP header into which the key is stamped.</summary>
+    public HttpHeaderName HeaderName { get; }
+
+    /// <summary>
+    /// The optional scheme prefix. When non-<see langword="null"/>, the header value is
+    /// <c>"&lt;Scheme&gt; &lt;Key&gt;"</c>; otherwise the header value is exactly <see cref="Key"/>.
+    /// </summary>
+    public string? Scheme { get; }
+}

src/Dexpace.Sdk.Core/Auth/BasicCredential.cs

@@ -0,0 +1,50 @@
+// Copyright (c) 2026 dexpace and Omar Aljarrah.
+// Licensed under the MIT License. See LICENSE in the repository root for details.
+
+using System.Text;
+
+namespace Dexpace.Sdk.Core.Auth;
+
+/// <summary>
+/// A credential that authenticates requests using the HTTP Basic scheme (RFC 7617).
+/// </summary>
+/// <remarks>
+/// The credential stores the username and password in plain text. Call <see cref="ToBase64"/>
+/// to obtain the Base64-encoded <c>username:password</c> token suitable for the
+/// <c>Authorization: Basic &lt;token&gt;</c> header value.
+/// </remarks>
+public sealed class BasicCredential
+{
+    /// <summary>
+    /// Initializes a <see cref="BasicCredential"/> with the given username and password.
+    /// </summary>
+    /// <param name="username">The username. Must not be <see langword="null"/>.</param>
+    /// <param name="password">The password. Must not be <see langword="null"/>.</param>
+    /// <exception cref="ArgumentNullException">
+    /// <paramref name="username"/> or <paramref name="password"/> is <see langword="null"/>.
+    /// </exception>
+    public BasicCredential(string username, string password)
+    {
+        ArgumentNullException.ThrowIfNull(username);
+        ArgumentNullException.ThrowIfNull(password);
+        Username = username;
+        Password = password;
+    }
+
+    /// <summary>The username.</summary>
+    public string Username { get; }
+
+    /// <summary>The password.</summary>
+    public string Password { get; }
+
+    /// <summary>
+    /// Returns the Base64-encoded UTF-8 <c>username:password</c> token for use in the
+    /// <c>Authorization: Basic &lt;token&gt;</c> header value.
+    /// </summary>
+    /// <returns>The Base64-encoded credentials.</returns>
+    public string ToBase64()
+    {
+        var bytes = Encoding.UTF8.GetBytes($"{Username}:{Password}");
+        return Convert.ToBase64String(bytes);
+    }
+}

src/Dexpace.Sdk.Core/Auth/TokenCredential.cs

@@ -0,0 +1,48 @@
+// Copyright (c) 2026 dexpace and Omar Aljarrah.
+// Licensed under the MIT License. See LICENSE in the repository root for details.
+
+namespace Dexpace.Sdk.Core.Auth;
+
+/// <summary>
+/// Base class for credential implementations that produce <see cref="AccessToken"/> instances.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Subclasses implement <see cref="GetTokenAsync"/> and may optionally override
+/// <see cref="GetToken"/> for a non-blocking synchronous path. The default
+/// <see cref="GetToken"/> implementation is a blocking bridge over
+/// <see cref="GetTokenAsync"/>; override it when a truly synchronous code path exists.
+/// </para>
+/// <para>
+/// Tokens are typically obtained through an <c>AccessTokenCache</c> rather than calling
+/// this type directly, so that caching, proactive refresh, and single-flight behaviour are
+/// applied automatically.
+/// </para>
+/// </remarks>
+public abstract class TokenCredential
+{
+    /// <summary>
+    /// Asynchronously obtains an <see cref="AccessToken"/> for the requested context.
+    /// </summary>
+    /// <param name="context">The scopes and optional claims for the token request.</param>
+    /// <param name="ct">A token to cancel the request.</param>
+    /// <returns>
+    /// A <see cref="ValueTask{AccessToken}"/> that resolves to the token on success.
+    /// </returns>
+    public abstract ValueTask<AccessToken> GetTokenAsync(
+        TokenRequestContext context,
+        CancellationToken ct = default);
+
+    /// <summary>
+    /// Synchronously obtains an <see cref="AccessToken"/> for the requested context.
+    /// </summary>
+    /// <remarks>
+    /// The default implementation is a blocking bridge over <see cref="GetTokenAsync"/>.
+    /// Override this method when a non-blocking synchronous path is available.
+    /// </remarks>
+    /// <param name="context">The scopes and optional claims for the token request.</param>
+    /// <param name="ct">A token to cancel the request.</param>
+    /// <returns>The access token.</returns>
+    public virtual AccessToken GetToken(TokenRequestContext context, CancellationToken ct = default)
+        => GetTokenAsync(context, ct).AsTask().GetAwaiter().GetResult();
+}