try to recreate multi-threaded panic, without success. (#536)

It's supposed to happen when multiple threads, maybe, trigger the
loading of packs at the same time. There are 15 packs to load
here but apparently that doesn't manage to trigger it.

git-odb/Cargo.toml

@@ -48,6 +48,8 @@ git-actor = { path = "../git-actor" }
 pretty_assertions = "1.0.0"
 filetime = "0.2.15"
 maplit = "1.0.2"
+num_cpus = "1.13.1"
+crossbeam-channel = "0.5.6"
 
 [package.metadata.docs.rs]
 features = ["document-features", "serde1"]

git-odb/tests/fixtures/make_repo_multi_index.sh

@@ -1,6 +1,8 @@
 #!/bin/bash
 set -eu -o pipefail
 
+omit_multi_index=${1:-no}
+
 git init -q
 git config commit.gpgsign false
 
@@ -26,6 +28,7 @@ for round in $(seq $rounds); do
   write_files "${dirs[$dir_index]}" $num_files "$round"
   git add .
   git commit -qm "$round $num_files"
+  git repack
 done
 
 echo hello world > referee
@@ -34,6 +37,6 @@ git commit -qm "to be forgotten"
 git tag -m "a tag object" referrer
 git reset --hard HEAD~1
 
-# speed up all access by creating a pack
-git gc --aggressive
-git multi-pack-index write
+if [ "$omit_multi_index" == "no" ]; then
+  git multi-pack-index write
+fi

git-odb/tests/odb/regression/mod.rs

@@ -4,15 +4,58 @@ mod repo_with_small_packs {
     use crate::odb::{fixture_path, hex_to_id};
 
     #[test]
-    fn all_packed_objects_can_be_found() {
-        let store = git_odb::at(fixture_path("repos/small-packs.git/objects")).unwrap();
+    fn all_packed_objects_can_be_found() -> crate::Result {
+        let store = git_odb::at(fixture_path("repos/small-packs.git/objects"))?;
         let mut buf = Vec::new();
         assert!(
             store
-                .try_find(hex_to_id("ecc68100297fff843a7eef8df0d0fb80c1c8bac5"), &mut buf)
-                .unwrap()
+                .try_find(hex_to_id("ecc68100297fff843a7eef8df0d0fb80c1c8bac5"), &mut buf)?
                 .is_some(),
             "object is present and available"
         );
+        Ok(())
+    }
+
+    #[test]
+    #[cfg(feature = "internal-testing-git-features-parallel")]
+    fn multi_threaded_access_will_not_panic() {
+        for arg in ["no", "without-multi-index"] {
+            let base = git_testtools::scripted_fixture_repo_read_only_with_args("make_repo_multi_index.sh", Some(arg))
+                .unwrap()
+                .join(".git")
+                .join("objects");
+            let store = git_odb::at(base).unwrap();
+            let (tx, barrier) = crossbeam_channel::unbounded::<()>();
+            let handles = (0..num_cpus::get()).map(|tid| {
+                std::thread::spawn({
+                    let store = store.clone();
+                    let barrier = barrier.clone();
+                    move || {
+                        barrier.recv().ok();
+                        let mut buf = Vec::new();
+                        let mut count = 0;
+                        for id in store.iter().unwrap() {
+                            let id = id.unwrap();
+                            assert!(
+                                store.try_find(id, &mut buf).is_ok(),
+                                "Thread {} could not find {}",
+                                tid,
+                                id
+                            );
+                            count += 1;
+                        }
+                        count
+                    }
+                })
+            });
+
+            std::thread::sleep(std::time::Duration::from_millis(50));
+            drop(tx);
+            let expected = store.iter().unwrap().count();
+            for handle in handles {
+                let actual = handle.join().expect("no panic");
+                assert_eq!(actual, expected);
+            }
+        }
     }
 }

git-odb/tests/odb/store/dynamic.rs

@@ -62,7 +62,7 @@ fn multi_index_access() -> crate::Result {
         assert!(handle.find(oid, &mut buf).is_ok());
         count += 1;
     }
-    assert_eq!(count, 868);
+    assert_eq!(count, 1732);
 
     assert_eq!(
         handle.store_ref().metrics(),
@@ -71,8 +71,8 @@ fn multi_index_access() -> crate::Result {
             num_refreshes: 1,
             open_reachable_indices: 1,
             known_reachable_indices: 1,
-            open_reachable_packs: 1,
-            known_packs: 1,
+            open_reachable_packs: 15,
+            known_packs: 15,
             unused_slots: 31,
             loose_dbs: 1,
             unreachable_indices: 0,
@@ -91,8 +91,8 @@ fn multi_index_access() -> crate::Result {
             num_refreshes: 2,
             open_reachable_indices: 1,
             known_reachable_indices: 1,
-            open_reachable_packs: 1,
-            known_packs: 1,
+            open_reachable_packs: 15,
+            known_packs: 15,
             unused_slots: 31,
             loose_dbs: 1,
             unreachable_indices: 0,
@@ -115,7 +115,7 @@ fn multi_index_access() -> crate::Result {
             open_reachable_indices: 1,
             known_reachable_indices: 1,
             open_reachable_packs: 0,
-            known_packs: 1,
+            known_packs: 15,
             unused_slots: 31,
             loose_dbs: 1,
             unreachable_indices: 0,
@@ -147,7 +147,7 @@ fn multi_index_keep_open() -> crate::Result {
             open_reachable_indices: 1,
             known_reachable_indices: 1,
             open_reachable_packs: 0,
-            known_packs: 1,
+            known_packs: 15,
             unused_slots: 31,
             loose_dbs: 1,
             unreachable_indices: 0,
@@ -176,7 +176,7 @@ fn multi_index_keep_open() -> crate::Result {
             open_reachable_indices: 1,
             known_reachable_indices: 1,
             open_reachable_packs: 0, /*no pack is open anymore at least as seen from the index*/
-            known_packs: 1,
+            known_packs: 15,
             unused_slots: 30,
             loose_dbs: 1,
             unreachable_indices: 1,