[PATCH proxmox-backup 1/2] fix #6373: HTTP level reader heartbeat for proxy connection keepalive

[Christian Ebner <c.ebner@proxmox.com>]

Backup readers can have long periods of idle connections, e.g. if a
backup snapshot has been mounted and all relevant chunks are locally
cached or a backup session with previous metadata archive not needing
to fetch new contents while the backup is ongoing.

Proxies like e.g. HAProxy might however close idle connections for
better resource handling [0,1], even multiplexed HTTP/2 connections as
are being used for the Proxmox Backup Sever backup/reader protocol.

This mainly affects the backup reader, while the backup writer will
do indexing and chunk uploads anyways.

Therefore, perform heartbeat traffic in the backup reader http2 client
when no other requests are being send. To do so, an new `heartbeat`
API endpoint is introduced as part of the backup reader protocol,
returning empty on GET requests. By making this part of the backup
reader protocol, this is limited to active reader HTTP/2 sessions.

On the client side the heartbeat is send out periodically whenever a
timeout is being reached, the timeout however being reset if other
requests are being performed via the http2 client.

Since older servers do not provide the new API path, ignore errors
as the response is not strictly necessary for the connection to
remain established.

The timeout is currently only being performed if the timeout value
in seconds is given via the PBS_READER_HEARTBEAT_TIMEOUT.

Testing was performed using HAProxy with the Proxmox Backup Server
as backend using the following 5 second connection idle timeouts
as configuration parameters in haproxy.cfg:
```
...
defaults
    ...
    timeout connect 5000
    timeout client  5000
    timeout server  5000
    ..

frontend http-in
    bind *:8007
    mode tcp
    default_backend pbs

backend pbs
    mode tcp
    http-reuse always
    server pbs <PBS-IP>:8007 verify none
```

As command invocation:
```
PBS_READER_HEARTBEAT_TIMEOUT=1 proxmox-backup-client mount <snapshot> <archive> \
<mountpoint> --repository <user-and-realm>@<PROXY-IP>:<datastore> --verbose
```

[0] https://www.haproxy.com/documentation/haproxy-configuration-manual/latest/#4-timeout%20client
[1] https://www.haproxy.com/documentation/haproxy-configuration-manual/latest/#4.2-timeout%20http-keep-alive

Fixes: https://bugzilla.proxmox.com/show_bug.cgi?id=6373
Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
---
 pbs-client/src/backup_reader.rs | 69 ++++++++++++++++++++++++++++++---
 src/api2/reader/mod.rs          |  9 ++++-
 2 files changed, 72 insertions(+), 6 deletions(-)

diff --git a/pbs-client/src/backup_reader.rs b/pbs-client/src/backup_reader.rs
index 88cba599b..95236bef2 100644
--- a/pbs-client/src/backup_reader.rs
+++ b/pbs-client/src/backup_reader.rs
@@ -1,10 +1,13 @@
 use anyhow::{format_err, Error};
 use std::fs::File;
 use std::io::{Seek, SeekFrom, Write};
+use std::str::FromStr;
 use std::sync::Arc;
+use std::time::Duration;
 
 use futures::future::AbortHandle;
 use serde_json::{json, Value};
+use tokio::sync::mpsc;
 
 use pbs_api_types::{BackupArchiveName, BackupDir, BackupNamespace, MANIFEST_BLOB_NAME};
 use pbs_datastore::data_blob::DataBlob;
@@ -18,26 +21,65 @@ use pbs_tools::sha::sha256;
 
 use super::{H2Client, HttpClient};
 
+enum HeartBeatMsg {
+    Reset,
+    Abort,
+}
+
 /// Backup Reader
 pub struct BackupReader {
     h2: H2Client,
     abort: AbortHandle,
     crypt_config: Option<Arc<CryptConfig>>,
+    heartbeat: Option<mpsc::Sender<HeartBeatMsg>>,
 }
 
 impl Drop for BackupReader {
     fn drop(&mut self) {
+        self.send_msg_heartbeat(HeartBeatMsg::Abort);
         self.abort.abort();
     }
 }
 
 impl BackupReader {
     fn new(h2: H2Client, abort: AbortHandle, crypt_config: Option<Arc<CryptConfig>>) -> Arc<Self> {
-        Arc::new(Self {
-            h2,
-            abort,
-            crypt_config,
-        })
+        let timeout = match std::env::var("PBS_READER_HEARTBEAT_TIMEOUT") {
+            Ok(val) => u64::from_str(&val).map(Duration::from_secs).ok(),
+            Err(_err) => None,
+        };
+
+        if let Some(timeout) = timeout {
+            let (send, mut recv) = mpsc::channel(1);
+            let backup_reader = Arc::new(Self {
+                h2,
+                abort,
+                crypt_config,
+                heartbeat: Some(send),
+            });
+            let reader_cloned = Arc::clone(&backup_reader);
+
+            tokio::spawn(async move {
+                loop {
+                    match tokio::time::timeout(timeout, recv.recv()).await {
+                        Ok(Some(HeartBeatMsg::Reset)) => (),
+                        Ok(Some(HeartBeatMsg::Abort)) | Ok(None) => break,
+                        Err(_elapsed) => {
+                            // connection idle timeout reached, send heatbeat
+                            let _ = reader_cloned.h2.get("heartbeat", None).await;
+                        }
+                    }
+                }
+            });
+
+            backup_reader
+        } else {
+            Arc::new(Self {
+                h2,
+                abort,
+                crypt_config,
+                heartbeat: None,
+            })
+        }
     }
 
     /// Create a new instance by upgrading the connection at '/api2/json/reader'
@@ -79,21 +121,25 @@ impl BackupReader {
 
     /// Execute a GET request
     pub async fn get(&self, path: &str, param: Option<Value>) -> Result<Value, Error> {
+        self.send_msg_heartbeat(HeartBeatMsg::Reset);
         self.h2.get(path, param).await
     }
 
     /// Execute a PUT request
     pub async fn put(&self, path: &str, param: Option<Value>) -> Result<Value, Error> {
+        self.send_msg_heartbeat(HeartBeatMsg::Reset);
         self.h2.put(path, param).await
     }
 
     /// Execute a POST request
     pub async fn post(&self, path: &str, param: Option<Value>) -> Result<Value, Error> {
+        self.send_msg_heartbeat(HeartBeatMsg::Reset);
         self.h2.post(path, param).await
     }
 
     /// Execute a GET request and send output to a writer
     pub async fn download<W: Write + Send>(&self, file_name: &str, output: W) -> Result<(), Error> {
+        self.send_msg_heartbeat(HeartBeatMsg::Reset);
         let path = "download";
         let param = json!({ "file-name": file_name });
         self.h2.download(path, Some(param), output).await
@@ -103,6 +149,7 @@ impl BackupReader {
     ///
     /// This writes random data, and is only useful to test download speed.
     pub async fn speedtest<W: Write + Send>(&self, output: W) -> Result<(), Error> {
+        self.send_msg_heartbeat(HeartBeatMsg::Reset);
         self.h2.download("speedtest", None, output).await
     }
 
@@ -112,12 +159,14 @@ impl BackupReader {
         digest: &[u8; 32],
         output: W,
     ) -> Result<(), Error> {
+        self.send_msg_heartbeat(HeartBeatMsg::Reset);
         let path = "chunk";
         let param = json!({ "digest": hex::encode(digest) });
         self.h2.download(path, Some(param), output).await
     }
 
     pub fn force_close(self) {
+        self.send_msg_heartbeat(HeartBeatMsg::Abort);
         self.abort.abort();
     }
 
@@ -205,4 +254,14 @@ impl BackupReader {
 
         Ok(index)
     }
+
+    /// Send given message to the heartbeat closure.
+    ///
+    /// All errors are being ignored, since they cannot be handled anyways.
+    fn send_msg_heartbeat(&self, msg: HeartBeatMsg) {
+        let _ = self
+            .heartbeat
+            .as_ref()
+            .map(|heartbeat| heartbeat.try_send(msg));
+    }
 }
diff --git a/src/api2/reader/mod.rs b/src/api2/reader/mod.rs
index f7adc366f..1c673d7a6 100644
--- a/src/api2/reader/mod.rs
+++ b/src/api2/reader/mod.rs
@@ -18,7 +18,7 @@ use proxmox_router::{
     http_err, list_subdirs_api_method, ApiHandler, ApiMethod, ApiResponseFuture, Permission,
     Router, RpcEnvironment, SubdirMap,
 };
-use proxmox_schema::{BooleanSchema, ObjectSchema};
+use proxmox_schema::{api, BooleanSchema, ObjectSchema};
 use proxmox_sortable_macro::sortable;
 
 use pbs_api_types::{
@@ -227,6 +227,7 @@ const READER_API_SUBDIRS: SubdirMap = &[
         "download",
         &Router::new().download(&API_METHOD_DOWNLOAD_FILE),
     ),
+    ("heartbeat", &Router::new().download(&API_METHOD_HEARTBEAT)),
     ("speedtest", &Router::new().download(&API_METHOD_SPEEDTEST)),
 ];
 
@@ -433,3 +434,9 @@ fn speedtest(
 
     future::ok(response).boxed()
 }
+
+#[api()]
+/// HTTP level heartbeat to avoid proxies closing long running idle backup reader connections.
+pub async fn heartbeat(_rpcenv: &mut dyn RpcEnvironment) -> Result<(), Error> {
+    Ok(())
+}
-- 
2.47.3