WIP feat: agent-level HTTP client

Cargo.toml

@@ -46,6 +46,7 @@ members = [
   "libdd-tinybytes",
   "libdd-dogstatsd-client",
   "libdd-http-client",
+  "libdd-agent-client",
   "libdd-log",
   "libdd-log-ffi", "libdd-libunwind-sys",
 ]

libdd-agent-client/Cargo.toml

@@ -0,0 +1,27 @@
+# Copyright 2026-Present Datadog, Inc. https://www.datadoghq.com/
+# SPDX-License-Identifier: Apache-2.0
+
+[package]
+name = "libdd-agent-client"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+authors.workspace = true
+description = "Datadog-agent-specialized HTTP client: language metadata injection, per-endpoint send methods, retry, and compression"
+homepage = "https://github.com/DataDog/libdatadog/tree/main/libdd-agent-client"
+repository = "https://github.com/DataDog/libdatadog/tree/main/libdd-agent-client"
+
+[lib]
+bench = false
+
+[dependencies]
+bytes = "1.4"
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+thiserror = "2"
+tokio = { version = "1.23", features = ["rt"] }
+libdd-http-client = { path = "../libdd-http-client" }
+
+[dev-dependencies]
+tokio = { version = "1.23", features = ["rt", "macros"] }

libdd-agent-client/src/agent_info.rs

@@ -0,0 +1,32 @@
+// Copyright 2026-Present Datadog, Inc. https://www.datadoghq.com/
+// SPDX-License-Identifier: Apache-2.0
+
+//! Types for [`crate::AgentClient::agent_info`].
+
+/// Parsed response from a `GET /info` probe.
+///
+/// Returned by [`crate::AgentClient::agent_info`]. Contains agent capabilities and the
+/// headers that dd-trace-py currently processes via the side-effectful `process_info_headers`
+/// function (`agent.py:17-23`) — here they are explicit typed fields instead.
+#[derive(Debug, Clone)]
+pub struct AgentInfo {
+    /// Available agent endpoints, e.g. `["/v0.4/traces", "/v0.5/traces"]`.
+    pub endpoints: Vec<String>,
+    /// Whether the agent supports client-side P0 dropping.
+    pub client_drop_p0s: bool,
+    /// Raw agent configuration block.
+    pub config: serde_json::Value,
+    /// Agent version string, if reported.
+    pub version: Option<String>,
+    /// Parsed from the `Datadog-Container-Tags-Hash` response header.
+    ///
+    /// Used by dd-trace-py to compute the base tag hash (`agent.py:17-23`).
+    pub container_tags_hash: Option<String>,
+    /// Value of the `Datadog-Agent-State` response header from the last `/info` fetch.
+    ///
+    /// The agent updates this opaque token whenever its internal state changes (e.g. a
+    /// configuration reload). Clients that poll `/info` periodically can skip re-parsing
+    /// the response body by comparing this value to the one returned by the previous call
+    /// and only acting when it differs.
+    pub state_hash: Option<String>,
+}

libdd-agent-client/src/builder.rs

@@ -0,0 +1,304 @@
+// Copyright 2026-Present Datadog, Inc. https://www.datadoghq.com/
+// SPDX-License-Identifier: Apache-2.0
+
+//! Builder for [`crate::AgentClient`].
+
+use std::collections::HashMap;
+use std::time::Duration;
+
+use libdd_http_client::RetryConfig;
+
+use crate::{error::BuildError, language_metadata::LanguageMetadata, AgentClient};
+
+/// Default timeout for agent requests: 2 000 ms.
+///
+/// Matches dd-trace-py's `DEFAULT_TIMEOUT = 2.0 s` (`constants.py:97`).
+pub const DEFAULT_TIMEOUT_MS: u64 = 2_000;
+
+/// Default retry configuration: 2 retries (3 total attempts), 100 ms initial delay,
+/// exponential backoff with full jitter.
+///
+/// This approximates dd-trace-py's `fibonacci_backoff_with_jitter` pattern used in
+/// `writer.py:245-249`, `stats.py:123-126`, and `datastreams/processor.py:140-143`.
+pub fn default_retry_config() -> RetryConfig {
+    RetryConfig::new()
+        .max_retries(2)
+        .initial_delay(Duration::from_millis(100))
+        .with_jitter(true)
+}
+
+/// Transport configuration for the agent client.
+///
+/// Determines how the client connects to the Datadog agent (or an intake endpoint).
+/// Set via [`AgentClientBuilder::transport`] or the convenience helpers
+/// [`AgentClientBuilder::http`], [`AgentClientBuilder::https`],
+/// [`AgentClientBuilder::unix_socket`], etc.
+#[derive(Debug, Clone)]
+pub enum AgentTransport {
+    /// HTTP over TCP to `http://{host}:{port}`.
+    Http {
+        /// Hostname or IP address.
+        host: String,
+        /// Port number.
+        port: u16,
+    },
+    /// HTTPS over TCP to `https://{host}:{port}` (e.g. for intake endpoints).
+    Https {
+        /// Hostname or IP address.
+        host: String,
+        /// Port number.
+        port: u16,
+    },
+    /// Unix Domain Socket.
+    ///
+    /// HTTP requests are still formed with `Host: localhost`; the socket path
+    /// governs only the transport layer.
+    #[cfg(unix)]
+    UnixSocket {
+        /// Filesystem path to the socket file.
+        path: std::path::PathBuf,
+    },
+    /// Windows Named Pipe.
+    #[cfg(windows)]
+    NamedPipe {
+        /// Named pipe path, e.g. `\\.\pipe\DD_APM_DRIVER`.
+        path: std::ffi::OsString,
+    },
+    /// Probe at build time: use UDS if the socket file exists, otherwise fall back to HTTP.
+    ///
+    /// Mirrors the auto-detect logic in dd-trace-py's `_agent.py:32-49`.
+    #[cfg(unix)]
+    AutoDetect {
+        /// UDS path to probe.
+        uds_path: std::path::PathBuf,
+        /// Fallback host when the socket is absent.
+        fallback_host: String,
+        /// Fallback port when the socket is absent (typically 8126).
+        fallback_port: u16,
+    },
+}
+
+impl Default for AgentTransport {
+    fn default() -> Self {
+        AgentTransport::Http {
+            host: "localhost".to_owned(),
+            port: 8126,
+        }
+    }
+}
+
+/// Connection mode for the underlying HTTP client.
+///
+/// # Correctness note
+///
+/// The Datadog agent has a low keep-alive timeout that causes "pipe closed" errors on every
+/// second connection when connection reuse is enabled. [`ClientMode::Periodic`] (the default)
+/// disables connection pooling and is **correct** for all periodic-flush writers (traces, stats,
+/// data streams). Only high-frequency continuous senders (e.g. a streaming profiling exporter)
+/// should opt into [`ClientMode::Persistent`].
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub enum ClientMode {
+    /// No connection pooling. Correct for periodic flushes to the agent.
+    #[default]
+    Periodic,
+    /// Keep connections alive across requests.
+    ///
+    /// Use only for high-frequency continuous senders.
+    Persistent,
+}
+
+/// Builder for [`AgentClient`].
+///
+/// Obtain via [`AgentClient::builder`].
+///
+/// # Required fields
+///
+/// - Transport: set via [`AgentClientBuilder::transport`] or a convenience method
+///   ([`AgentClientBuilder::http`], [`AgentClientBuilder::https`],
+///   [`AgentClientBuilder::unix_socket`], [`AgentClientBuilder::windows_named_pipe`],
+///   [`AgentClientBuilder::auto_detect`]).
+/// - [`AgentClientBuilder::language_metadata`].
+///
+/// # Agentless mode
+///
+/// Call [`AgentClientBuilder::api_key`] with your Datadog API key and point the transport to
+/// the intake endpoint via [`AgentClientBuilder::https`]. The client injects `dd-api-key` on
+/// every request.
+///
+/// # Testing
+///
+/// Call [`AgentClientBuilder::test_token`] to inject `x-datadog-test-session-token` on every
+/// request. This replaces dd-trace-py's `AgentWriter.set_test_session_token` (`writer.py:754-755`).
+///
+/// # Fork safety
+///
+/// The underlying `libdd-http-client` uses `hickory-dns` by default — an in-process, fork-safe
+/// DNS resolver that avoids the class of bugs where a forked child inherits open sockets from a
+/// parent's DNS thread pool. This is important for host processes that fork (Django, Flask,
+/// Celery workers, PHP-FPM, etc.).
+#[derive(Debug, Default)]
+pub struct AgentClientBuilder {
+    transport: Option<AgentTransport>,
+    api_key: Option<String>,
+    test_token: Option<String>,
+    timeout: Option<Duration>,
+    language: Option<LanguageMetadata>,
+    retry: Option<RetryConfig>,
+    client_mode: ClientMode,
+    extra_headers: HashMap<String, String>,
+}
+
+impl AgentClientBuilder {
+    /// Create a new builder with default settings.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    // ── Transport ─────────────────────────────────────────────────────────────
+
+    /// Set the transport configuration.
+    pub fn transport(mut self, transport: AgentTransport) -> Self {
+        self.transport = Some(transport);
+        self
+    }
+
+    /// Convenience: HTTP over TCP.
+    pub fn http(self, host: impl Into<String>, port: u16) -> Self {
+        self.transport(AgentTransport::Http {
+            host: host.into(),
+            port,
+        })
+    }
+
+    /// Convenience: HTTPS over TCP.
+    pub fn https(self, host: impl Into<String>, port: u16) -> Self {
+        self.transport(AgentTransport::Https {
+            host: host.into(),
+            port,
+        })
+    }
+
+    /// Convenience: Unix Domain Socket.
+    #[cfg(unix)]
+    pub fn unix_socket(self, path: impl Into<std::path::PathBuf>) -> Self {
+        self.transport(AgentTransport::UnixSocket { path: path.into() })
+    }
+
+    /// Convenience: Windows Named Pipe.
+    #[cfg(windows)]
+    pub fn windows_named_pipe(self, path: impl Into<std::ffi::OsString>) -> Self {
+        self.transport(AgentTransport::NamedPipe { path: path.into() })
+    }
+
+    /// Convenience: auto-detect transport (UDS if socket file exists, else HTTP).
+    ///
+    /// Mirrors the logic in dd-trace-py's `_agent.py:32-49`.
+    #[cfg(unix)]
+    pub fn auto_detect(
+        self,
+        uds_path: impl Into<std::path::PathBuf>,
+        fallback_host: impl Into<String>,
+        fallback_port: u16,
+    ) -> Self {
+        self.transport(AgentTransport::AutoDetect {
+            uds_path: uds_path.into(),
+            fallback_host: fallback_host.into(),
+            fallback_port,
+        })
+    }
+
+    // ── Authentication / routing ──────────────────────────────────────────────
+
+    /// Set the Datadog API key (agentless mode).
+    ///
+    /// When set, `dd-api-key: <key>` is injected on every request.
+    /// Point the transport to the intake endpoint via [`AgentClientBuilder::https`].
+    pub fn api_key(mut self, key: impl Into<String>) -> Self {
+        self.api_key = Some(key.into());
+        self
+    }
+
+    /// Set the test session token.
+    ///
+    /// When set, `x-datadog-test-session-token: <token>` is injected on every request.
+    /// Replaces dd-trace-py's `AgentWriter.set_test_session_token` (`writer.py:754-755`).
+    pub fn test_token(mut self, token: impl Into<String>) -> Self {
+        self.test_token = Some(token.into());
+        self
+    }
+
+    // ── Timeout / retries ─────────────────────────────────────────────────────
+
+    /// Set the request timeout.
+    ///
+    /// Defaults to [`DEFAULT_TIMEOUT_MS`] (2 000 ms) when not set.
+    pub fn timeout(mut self, timeout: Duration) -> Self {
+        self.timeout = Some(timeout);
+        self
+    }
+
+    /// Read the timeout from `DD_TRACE_AGENT_TIMEOUT_SECONDS`, falling back to
+    /// [`DEFAULT_TIMEOUT_MS`] if the variable is unset or unparseable.
+    pub fn timeout_from_env(self) -> Self {
+        todo!()
+    }
+
+    /// Override the default retry configuration.
+    ///
+    /// Defaults to [`default_retry_config`]: 2 retries, 100 ms initial delay, exponential
+    /// backoff with full jitter.
+    pub fn retry(mut self, config: RetryConfig) -> Self {
+        self.retry = Some(config);
+        self
+    }
+
+    // ── Language metadata ─────────────────────────────────────────────────────
+
+    /// Set the language/runtime metadata injected into every request.
+    ///
+    /// Required. Drives `Datadog-Meta-Lang`, `Datadog-Meta-Lang-Version`,
+    /// `Datadog-Meta-Lang-Interpreter`, `Datadog-Meta-Tracer-Version`, and `User-Agent`.
+    pub fn language_metadata(mut self, meta: LanguageMetadata) -> Self {
+        self.language = Some(meta);
+        self
+    }
+
+    // ── Connection pooling ────────────────────────────────────────────────────
+
+    /// Set the connection mode. Defaults to [`ClientMode::Periodic`].
+    ///
+    /// See [`ClientMode`] for the correctness rationale behind the default.
+    pub fn client_mode(mut self, mode: ClientMode) -> Self {
+        self.client_mode = mode;
+        self
+    }
+
+    // ── Compression ───────────────────────────────────────────────────────────
+    //
+    // Not exposed in v1. Gzip compression (level 6, matching dd-trace-py's trace writer at
+    // `writer.py:490`) will be added in a follow-up once the core send paths are stable.
+    // Per-method defaults (e.g. unconditional gzip for `send_pipeline_stats`) are already
+    // baked in; only the opt-in client-level `gzip(level)` builder knob is deferred.
+
+    // ── Extra headers ─────────────────────────────────────────────────────────
+
+    /// Merge additional headers into every request.
+    ///
+    /// Intended for `_DD_TRACE_WRITER_ADDITIONAL_HEADERS` in dd-trace-py.
+    pub fn extra_headers(mut self, headers: HashMap<String, String>) -> Self {
+        self.extra_headers = headers;
+        self
+    }
+
+    // ── Build ─────────────────────────────────────────────────────────────────
+
+    /// Build the [`AgentClient`].
+    ///
+    /// # Errors
+    ///
+    /// - [`BuildError::MissingTransport`] — no transport was configured.
+    /// - [`BuildError::MissingLanguageMetadata`] — no language metadata was configured.
+    pub fn build(self) -> Result<AgentClient, BuildError> {
+        todo!()
+    }
+}