Add the table of pending approvals

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/brokerd/src/approvals.rs

@@ -0,0 +1,76 @@
+//! The pending-approval table: whoever takes an entry out answers it, and everyone else finds it
+//! gone. In memory only.
+
+use crate::policy::{Ask, Decision};
+use proto::{DenyReason, PendingApproval, Timestamp};
+use std::collections::BTreeMap;
+use std::sync::mpsc::{Receiver, Sender, channel};
+use std::sync::{Mutex, MutexGuard};
+
+/// What the waiting thread is told. Boxed: clippy's `large_enum_variant` rejects it unboxed.
+#[derive(Debug)]
+pub enum Verdict {
+    Run(Box<Decision>),
+    Denied(DenyReason),
+}
+
+#[derive(Debug)]
+pub struct Entry {
+    pub info: PendingApproval,
+    pub ask: Ask,
+    pub reply: Sender<Verdict>,
+}
+
+#[derive(Debug, Default)]
+pub struct Table {
+    entries: Mutex<BTreeMap<u64, Entry>>,
+}
+
+impl Table {
+    pub fn new() -> Table {
+        Table {
+            entries: Mutex::new(BTreeMap::new()),
+        }
+    }
+
+    fn lock(&self) -> MutexGuard<'_, BTreeMap<u64, Entry>> {
+        self.entries
+            .lock()
+            .unwrap_or_else(|poisoned| poisoned.into_inner())
+    }
+
+    pub fn insert(&self, info: PendingApproval, ask: Ask) -> Receiver<Verdict> {
+        let (sender, receiver) = channel();
+        self.lock().insert(
+            info.approval,
+            Entry {
+                info,
+                ask,
+                reply: sender,
+            },
+        );
+        receiver
+    }
+
+    pub fn take(&self, id: u64) -> Option<Entry> {
+        self.lock().remove(&id)
+    }
+
+    pub fn take_expired(&self, now: Timestamp) -> Vec<Entry> {
+        // One lock: taking it once per id would let an approve slip in between.
+        let mut entries = self.lock();
+        let ids: Vec<u64> = entries
+            .values()
+            .filter(|entry| now >= entry.info.expires)
+            .map(|entry| entry.info.approval)
+            .collect();
+        ids.iter().filter_map(|id| entries.remove(id)).collect()
+    }
+
+    pub fn list(&self) -> Vec<PendingApproval> {
+        self.lock()
+            .values()
+            .map(|entry| entry.info.clone())
+            .collect()
+    }
+}