Use DefaultCredentialsChain AWS authentication in remote_storage (#8440)

PR #8299 has switched the storage scrubber to use
`DefaultCredentialsChain`. Now we do this for `remote_storage`, as it
allows us to use `remote_storage` from inside kubernetes. Most of the
diff is due to `GenericRemoteStorage::from_config` becoming `async fn`.

safekeeper/src/bin/safekeeper.rs

@@ -418,7 +418,7 @@ async fn start_safekeeper(conf: SafeKeeperConf) -> Result<()> {
     let timeline_collector = safekeeper::metrics::TimelineCollector::new();
     metrics::register_internal(Box::new(timeline_collector))?;
 
-    wal_backup::init_remote_storage(&conf);
+    wal_backup::init_remote_storage(&conf).await;
 
     // Keep handles to main tasks to die if any of them disappears.
     let mut tasks_handles: FuturesUnordered<BoxFuture<(String, JoinTaskRes)>> =

safekeeper/src/wal_backup.rs

@@ -22,7 +22,7 @@ use tokio::fs::File;
 
 use tokio::select;
 use tokio::sync::mpsc::{self, Receiver, Sender};
-use tokio::sync::watch;
+use tokio::sync::{watch, OnceCell};
 use tokio::time::sleep;
 use tracing::*;
 
@@ -33,8 +33,6 @@ use crate::timeline::{PeerInfo, WalResidentTimeline};
 use crate::timeline_manager::{Manager, StateSnapshot};
 use crate::{SafeKeeperConf, WAL_BACKUP_RUNTIME};
 
-use once_cell::sync::OnceCell;
-
 const UPLOAD_FAILURE_RETRY_MIN_MS: u64 = 10;
 const UPLOAD_FAILURE_RETRY_MAX_MS: u64 = 5000;
 
@@ -167,7 +165,7 @@ fn determine_offloader(
     }
 }
 
-static REMOTE_STORAGE: OnceCell<Option<GenericRemoteStorage>> = OnceCell::new();
+static REMOTE_STORAGE: OnceCell<Option<GenericRemoteStorage>> = OnceCell::const_new();
 
 // Storage must be configured and initialized when this is called.
 fn get_configured_remote_storage() -> &'static GenericRemoteStorage {
@@ -178,14 +176,22 @@ fn get_configured_remote_storage() -> &'static GenericRemoteStorage {
         .unwrap()
 }
 
-pub fn init_remote_storage(conf: &SafeKeeperConf) {
+pub async fn init_remote_storage(conf: &SafeKeeperConf) {
     // TODO: refactor REMOTE_STORAGE to avoid using global variables, and provide
     // dependencies to all tasks instead.
-    REMOTE_STORAGE.get_or_init(|| {
-        conf.remote_storage
-            .as_ref()
-            .map(|c| GenericRemoteStorage::from_config(c).expect("failed to create remote storage"))
-    });
+    REMOTE_STORAGE
+        .get_or_init(|| async {
+            if let Some(conf) = conf.remote_storage.as_ref() {
+                Some(
+                    GenericRemoteStorage::from_config(conf)
+                        .await
+                        .expect("failed to create remote storage"),
+                )
+            } else {
+                None
+            }
+        })
+        .await;
 }
 
 struct WalBackupTask {