Notify safekeeper readiness with systemd.

To avoid downtime during deploy, as in busy regions initial load can currently
take ~30s.

Cargo.lock

@@ -4146,6 +4146,7 @@ dependencies = [
  "reqwest",
  "safekeeper_api",
  "scopeguard",
+ "sd-notify",
  "serde",
  "serde_json",
  "serde_with",
@@ -4208,6 +4209,12 @@ dependencies = [
  "untrusted",
 ]
 
+[[package]]
+name = "sd-notify"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "621e3680f3e07db4c9c2c3fb07c6223ab2fab2e54bd3c04c3ae037990f428c32"
+
 [[package]]
 name = "security-framework"
 version = "2.9.1"

Cargo.toml

@@ -122,6 +122,7 @@ rustls-pemfile = "1"
 rustls-split = "0.3"
 scopeguard = "1.1"
 sysinfo = "0.29.2"
+sd-notify = "0.4.1"
 sentry = { version = "0.31", default-features = false, features = ["backtrace", "contexts", "panic", "rustls", "reqwest" ] }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1"

safekeeper/Cargo.toml

@@ -46,6 +46,7 @@ postgres_ffi.workspace = true
 pq_proto.workspace = true
 remote_storage.workspace = true
 safekeeper_api.workspace = true
+sd-notify.workspace = true
 storage_broker.workspace = true
 tokio-stream.workspace = true
 utils.workspace = true

safekeeper/src/bin/safekeeper.rs

@@ -8,6 +8,7 @@ use futures::future::BoxFuture;
 use futures::stream::FuturesUnordered;
 use futures::{FutureExt, StreamExt};
 use remote_storage::RemoteStorageConfig;
+use sd_notify::NotifyState;
 use tokio::runtime::Handle;
 use tokio::signal::unix::{signal, SignalKind};
 use tokio::task::JoinError;
@@ -434,6 +435,12 @@ async fn start_safekeeper(conf: SafeKeeperConf) -> Result<()> {
     let mut sigint_stream = signal(SignalKind::interrupt())?;
     let mut sigterm_stream = signal(SignalKind::terminate())?;
 
+    // Notify systemd that we are ready. This is important as currently loading
+    // timelines takes significant time (~30s in busy regions).
+    if let Err(e) = sd_notify::notify(true, &[NotifyState::Ready]) {
+        warn!("systemd notify failed: {:?}", e);
+    }
+
     tokio::select! {
         Some((task_name, res)) = tasks_handles.next()=> {
             error!("{} task failed: {:?}, exiting", task_name, res);