[pbs-devel] [PATCH backup] http_client: fallback if XDG_RUNTIME_DIR is not set

[Maximiliano Sandoval]

xdg::BaseDirectory's [place_directory_file] errors out if
`XDG_RUNTIME_DIR` is not set.

This is not ideal, as per the the base directory [specification]:

> If $XDG_RUNTIME_DIR is not set applications should fall back to a
replacement directory with similar capabilities and print a warning
message. Applications should use this directory for communication and
synchronization purposes and should not place larger files in it, since
it might reside in runtime memory and cannot necessarily be swapped out
to disk.

At the moment, running the proxmox-backup-client as root will print an error:
```
storing login ticket failed: $XDG_RUNTIME_DIR must be set
```

We add a helper that places a runtime file `basename` which fallbacks to
either `/run/{prefix}/{basename}` or
`/run/user/{uid}/{prefix}/{basename}` depending on whether the client is
running as root or as a different user.

[place_directory_file] https://docs.rs/xdg/latest/xdg/struct.BaseDirectories.html#method.place_runtime_file
[specification]: https://specifications.freedesktop.org/basedir-spec/latest/

Signed-off-by: Maximiliano Sandoval <m.sandoval at proxmox.com>
---
 pbs-client/src/http_client.rs | 55 ++++++++++++++++++++++++++---------
 1 file changed, 41 insertions(+), 14 deletions(-)

diff --git a/pbs-client/src/http_client.rs b/pbs-client/src/http_client.rs
index c95def07b..433c91477 100644
--- a/pbs-client/src/http_client.rs
+++ b/pbs-client/src/http_client.rs
@@ -1,8 +1,9 @@
 use std::io::{IsTerminal, Write};
+use std::path::PathBuf;
 use std::sync::{Arc, Mutex, RwLock};
 use std::time::Duration;
 
-use anyhow::{bail, format_err, Error};
+use anyhow::{bail, format_err, Context, Error};
 use futures::*;
 #[cfg(not(target_feature = "crt-static"))]
 use hyper::client::connect::dns::GaiResolver;
@@ -27,7 +28,7 @@ use proxmox_async::broadcast_future::BroadcastFuture;
 use proxmox_http::client::HttpsConnector;
 use proxmox_http::uri::{build_authority, json_object_to_query};
 use proxmox_http::{ProxyConfig, RateLimiter};
-use proxmox_log::{error, info, warn};
+use proxmox_log::{debug, error, info, warn};
 
 use pbs_api_types::percent_encoding::DEFAULT_ENCODE_SET;
 use pbs_api_types::{Authid, RateLimitConfig, Userid};
@@ -216,10 +217,7 @@ pub struct HttpClient {
 
 /// Delete stored ticket data (logout)
 pub fn delete_ticket_info(prefix: &str, server: &str, username: &Userid) -> Result<(), Error> {
-    let base = BaseDirectories::with_prefix(prefix)?;
-
-    // usually /run/user/<uid>/...
-    let path = base.place_runtime_file("tickets")?;
+    let path = place_runtime_file(prefix, "tickets")?;
 
     let mode = nix::sys::stat::Mode::from_bits_truncate(0o0600);
 
@@ -305,10 +303,7 @@ fn store_ticket_info(
     ticket: &str,
     token: &str,
 ) -> Result<(), Error> {
-    let base = BaseDirectories::with_prefix(prefix)?;
-
-    // usually /run/user/<uid>/...
-    let path = base.place_runtime_file("tickets")?;
+    let path = place_runtime_file(prefix, "tickets")?;
 
     let mode = nix::sys::stat::Mode::from_bits_truncate(0o0600);
 
@@ -345,10 +340,9 @@ fn store_ticket_info(
 }
 
 fn load_ticket_info(prefix: &str, server: &str, userid: &Userid) -> Option<(String, String)> {
-    let base = BaseDirectories::with_prefix(prefix).ok()?;
-
-    // usually /run/user/<uid>/...
-    let path = base.place_runtime_file("tickets").ok()?;
+    let path = place_runtime_file(prefix, "tickets")
+        .inspect_err(|err| error!("could not place runtime file: {err:#}"))
+        .ok()?;
     let data = file_get_json(path, None).ok()?;
     let now = proxmox_time::epoch_i64();
     let ticket_lifetime = proxmox_auth_api::TICKET_LIFETIME - 60;
@@ -1181,3 +1175,36 @@ impl H2Client {
         Ok(request)
     }
 }
+
+// Returns an absolute path in `XDG_RUNTIME_DIR`` where a runtime file may be
+// stored. Leading directories in the returned path are pre-created; if that is
+// not possible, an error is returned.
+//
+// Similar to [BaseDirectories::place_runtime_file] but will fall back to either
+// `/run/{prefix}` or `/run/user/{uid}/{prefix}` if the `XDG_RUNTIME_DIR`
+// variable is not set.
+fn place_runtime_file(prefix: &str, basename: &str) -> Result<PathBuf, Error> {
+    let base =
+        BaseDirectories::with_prefix(prefix).with_context(|| "failed to get base directories")?;
+
+    let path = if base.has_runtime_directory() {
+        base.place_runtime_file(basename)
+            .with_context(|| format!("failed to place runtime file {basename}"))?
+    } else {
+        let uid = nix::unistd::Uid::current();
+        let path = if uid.is_root() {
+            PathBuf::from("/run/proxmox-backup/")
+        } else {
+            PathBuf::from(format!("/run/user/{uid}/proxmox-backup/"))
+        };
+        std::fs::create_dir_all(&path)?;
+        debug!(
+            "XDG_RUNTIME_DIR is not set, using {} as fallback",
+            path.display()
+        );
+        path.join(basename)
+    };
+    debug!("placing {basename} at {}", path.display());
+
+    Ok(path)
+}
-- 
2.39.5