fix(snix/castore/import): don't allocate parent repeatedly

We use hashbrown directly (instead of through std::collections::HashMap)
so that we can use HashMap::entry_ref, which only allocates if the entry
is not yet occupied.

This implicitly switches our hash to a less long-term DoS-resistant one
(foldhash rather than SipHash), but we don't usually face HashDoS risks
when ingesting archives.

Change-Id: I3e7fc2cd08d96380cd9fd62bfcfe6cd24698bc9a
Reviewed-on: https://cl.snix.dev/c/snix/+/30277
Reviewed-by: Florian Klink <flokli@flokli.de>
Tested-by: besadii

snix/Cargo.lock

@@ -4186,6 +4186,7 @@ dependencies = [
  "fastcdc",
  "fuse-backend-rs",
  "futures",
+ "hashbrown 0.15.2",
  "hex-literal",
  "hyper-util",
  "libc",

snix/Cargo.nix

@@ -13640,6 +13640,10 @@ rec {
             name = "futures";
             packageId = "futures";
           }
+          {
+            name = "hashbrown";
+            packageId = "hashbrown 0.15.2";
+          }
           {
             name = "hyper-util";
             packageId = "hyper-util";

snix/castore/Cargo.toml

@@ -55,6 +55,7 @@ vmm-sys-util = { workspace = true, optional = true }
 virtio-bindings = { workspace = true, optional = true }
 wu-manber.workspace = true
 auto_impl = "1.2.0"
+hashbrown = "0.15.2"
 
 [build-dependencies]
 prost-build.workspace = true

snix/castore/src/import/mod.rs

@@ -10,7 +10,7 @@ use crate::{B3Digest, Directory, Node};
 use futures::{Stream, StreamExt};
 use tracing::Level;
 
-use std::collections::HashMap;
+use hashbrown::HashMap;
 use tracing::instrument;
 
 mod error;
@@ -131,7 +131,7 @@ where
 
             // record node in parent directory, creating a new [Directory] if not there yet.
             directories
-                .entry(parent.to_owned())
+                .entry_ref(parent)
                 .or_default()
                 .add(name, node)
                 .map_err(|e| {