[pbs-devel] [PATCH proxmox-backup 7/9] file-restore-daemon: limit concurrent download calls

[Stefan Reiter <s.reiter@proxmox.com>]

While the issue with vsock packets starving kernel memory is mostly
worked around by the '64k -> 4k buffer' patch in
'proxmox-backup-restore-image', let's be safe and also limit the number
of concurrent transfers. 8 downloads per VM seems like a fair value.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
---

Mostly optional, included since I had it lying around for a while, but only now
is it useable, since it suffered from the same tokio issue as the watchdog
inhibit.

 src/bin/proxmox_restore_daemon/api.rs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/bin/proxmox_restore_daemon/api.rs b/src/bin/proxmox_restore_daemon/api.rs
index c578b2c0..f2732e64 100644
--- a/src/bin/proxmox_restore_daemon/api.rs
+++ b/src/bin/proxmox_restore_daemon/api.rs
@@ -6,6 +6,7 @@ use hyper::{header, Body, Response, StatusCode};
 use log::error;
 use pathpatterns::{MatchEntry, MatchPattern, MatchType, Pattern};
 use serde_json::Value;
+use tokio::sync::Semaphore;
 
 use std::ffi::OsStr;
 use std::fs;
@@ -41,6 +42,8 @@ pub const ROUTER: Router = Router::new()
     .get(&list_subdirs_api_method!(SUBDIRS))
     .subdirs(SUBDIRS);
 
+static DOWNLOAD_SEM: Semaphore = Semaphore::const_new(8);
+
 fn read_uptime() -> Result<f32, Error> {
     let uptime = fs::read_to_string("/proc/uptime")?;
     // unwrap the Option, if /proc/uptime is empty we have bigger problems
@@ -252,6 +255,12 @@ fn extract(
     let _inhibitor = watchdog_inhibit();
     async move {
         let _inhibitor = _inhibitor;
+
+        let _permit = match DOWNLOAD_SEM.try_acquire() {
+            Ok(permit) => permit,
+            Err(_) => bail!("maximum concurrent download limit reached, please wait for another restore to finish before attempting a new one"),
+        };
+
         let path = tools::required_string_param(&param, "path")?;
         let mut path = base64::decode(path)?;
         if let Some(b'/') = path.last() {
@@ -286,6 +295,7 @@ fn extract(
         if pxar {
             tokio::spawn(async move {
                 let _inhibitor = _inhibitor;
+                let _permit = _permit;
                 let result = async move {
                     // pxar always expects a directory as it's root, so to accommodate files as
                     // well we encode the parent dir with a filter only matching the target instead
@@ -344,6 +354,7 @@ fn extract(
         } else {
             tokio::spawn(async move {
                 let _inhibitor = _inhibitor;
+                let _permit = _permit;
                 let result = async move {
                     if vm_path.is_dir() {
                         zip_directory(&mut writer, &vm_path).await?;
-- 
2.20.1