tests: use one thread per core in concurrency tests

The tests have been failing in GitHub's CI quite frequently. It's
about time I try to do something about it. Let's see if this helps.

Cargo.lock

@@ -582,6 +582,7 @@ dependencies = [
  "git2",
  "hex",
  "maplit",
+ "num_cpus",
  "pest",
  "pest_derive",
  "protobuf",

lib/Cargo.toml

@@ -38,4 +38,5 @@ whoami = "1.0.1"
 zstd = "0.6.0"
 
 [dev-dependencies]
+num_cpus = "1.13.0"
 test-case = "1.0.0"

lib/src/lock.rs

@@ -65,6 +65,7 @@ impl Drop for FileLock {
 
 #[cfg(test)]
 mod tests {
+    use std::cmp::max;
     use std::{env, thread};
 
     use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
@@ -94,8 +95,9 @@ mod tests {
             .open(data_path.clone())
             .unwrap();
         data_file.write_u32::<LittleEndian>(0).unwrap();
+        let num_threads = max(num_cpus::get(), 4);
         let mut threads = vec![];
-        for _ in 0..100 {
+        for _ in 0..num_threads {
             let data_path = data_path.clone();
             let lock_path = lock_path.clone();
             let handle = thread::spawn(move || {
@@ -116,6 +118,6 @@ mod tests {
         }
         let mut data_file = OpenOptions::new().read(true).open(data_path).unwrap();
         let value = data_file.read_u32::<LittleEndian>().unwrap();
-        assert_eq!(value, 100);
+        assert_eq!(value, num_threads as u32);
     }
 }

lib/tests/test_commit_concurrent.rs

@@ -12,13 +12,14 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::cmp::max;
 use std::thread;
 
 use jujube_lib::repo::ReadonlyRepo;
 use jujube_lib::{dag_walk, testutils};
 use test_case::test_case;
 
-fn count_non_merge_operations(repo: &ReadonlyRepo) -> u32 {
+fn count_non_merge_operations(repo: &ReadonlyRepo) -> usize {
     let op_store = repo.op_store();
     let op_id = repo.op_id().clone();
     let mut num_ops = 0;
@@ -45,8 +46,9 @@ fn test_commit_parallel(use_git: bool) {
     let settings = testutils::user_settings();
     let (_temp_dir, repo) = testutils::init_repo(&settings, use_git);
 
+    let num_threads = max(num_cpus::get(), 4);
     let mut threads = vec![];
-    for _ in 0..100 {
+    for _ in 0..num_threads {
         let settings = settings.clone();
         let repo = repo.clone();
         let handle = thread::spawn(move || {
@@ -61,11 +63,11 @@ fn test_commit_parallel(use_git: bool) {
     let repo = repo.reload().unwrap();
     // One commit per thread plus the commit from the initial checkout on top of the
     // root commit
-    assert_eq!(repo.view().heads().len(), 101);
+    assert_eq!(repo.view().heads().len(), num_threads + 1);
 
     // One operation for initializing the repo (containing the root id and the
     // initial working copy commit).
-    assert_eq!(count_non_merge_operations(&repo), 101);
+    assert_eq!(count_non_merge_operations(&repo), num_threads + 1);
 }
 
 #[cfg(unix)]
@@ -77,8 +79,9 @@ fn test_commit_parallel_instances(use_git: bool) {
     let settings = testutils::user_settings();
     let (_temp_dir, repo) = testutils::init_repo(&settings, use_git);
 
+    let num_threads = max(num_cpus::get(), 4);
     let mut threads = vec![];
-    for _ in 0..100 {
+    for _ in 0..num_threads {
         let settings = settings.clone();
         let repo = ReadonlyRepo::load(&settings, repo.working_copy_path().clone()).unwrap();
         let handle = thread::spawn(move || {
@@ -93,9 +96,9 @@ fn test_commit_parallel_instances(use_git: bool) {
     // One commit per thread plus the commit from the initial checkout on top of the
     // root commit
     let repo = ReadonlyRepo::load(&settings, repo.working_copy_path().clone()).unwrap();
-    assert_eq!(repo.view().heads().len(), 101);
+    assert_eq!(repo.view().heads().len(), num_threads + 1);
 
     // One operation for initializing the repo (containing the root id and the
     // initial working copy commit).
-    assert_eq!(count_non_merge_operations(&repo), 101);
+    assert_eq!(count_non_merge_operations(&repo), num_threads + 1);
 }

lib/tests/test_working_copy_concurrent.rs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::cmp::max;
 use std::collections::HashSet;
 use std::thread;
 
@@ -104,8 +105,9 @@ fn test_checkout_parallel(use_git: bool) {
     let (_temp_dir, repo) = testutils::init_repo(&settings, use_git);
     let store = repo.store();
 
+    let num_threads = max(num_cpus::get(), 4);
     let mut commit_ids = vec![];
-    for i in 0..100 {
+    for i in 0..num_threads {
         let path = FileRepoPath::from(format!("file{}", i).as_str());
         let tree = testutils::create_tree(&repo, &[(&path, "contents")]);
         let commit = CommitBuilder::for_new_commit(&settings, store, tree.id().clone())