Use locks to prevent concurrent accesses to the same Git repo (#304)

Ensures that if we need to access the same Git repo twice in a
resolution, we only have one handler to that repo at a time. (Otherwise,
`git2` panics.)

crates/puffin-cli/tests/pip_compile.rs

@@ -804,6 +804,43 @@ fn compile_git_refs_https_dependency() -> Result<()> {
     Ok(())
 }
 
+/// Resolve two packages from a `requirements.in` file with the same Git HTTPS dependency.
+#[test]
+fn compile_git_concurrent_access() -> Result<()> {
+    let temp_dir = assert_fs::TempDir::new()?;
+    let cache_dir = assert_fs::TempDir::new()?;
+    let venv = temp_dir.child(".venv");
+
+    Command::new(get_cargo_bin(BIN_NAME))
+        .arg("venv")
+        .arg(venv.as_os_str())
+        .arg("--cache-dir")
+        .arg(cache_dir.path())
+        .current_dir(&temp_dir)
+        .assert()
+        .success();
+    venv.assert(predicates::path::is_dir());
+
+    let requirements_in = temp_dir.child("requirements.in");
+    requirements_in.touch()?;
+    requirements_in
+        .write_str("flask @ git+https://github.com/pallets/flask.git@2.0.0\ndask @ git+https://github.com/pallets/flask.git@3.0.0")?;
+
+    insta::with_settings!({
+        filters => INSTA_FILTERS.to_vec()
+    }, {
+        assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+            .arg("pip-compile")
+            .arg("requirements.in")
+            .arg("--cache-dir")
+            .arg(cache_dir.path())
+            .env("VIRTUAL_ENV", venv.as_os_str())
+            .current_dir(&temp_dir));
+    });
+
+    Ok(())
+}
+
 /// Request Flask, but include a URL dependency for Werkzeug, which should avoid adding a
 /// duplicate dependency from `PyPI`.
 #[test]

crates/puffin-cli/tests/snapshots/pip_compile__compile_git_concurrent_access.snap

@@ -0,0 +1,43 @@
+---
+source: crates/puffin-cli/tests/pip_compile.rs
+info:
+  program: puffin
+  args:
+    - pip-compile
+    - requirements.in
+    - "--cache-dir"
+    - /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmp6j5dBk
+  env:
+    VIRTUAL_ENV: /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmpYyHlPp/.venv
+---
+success: true
+exit_code: 0
+----- stdout -----
+# This file was autogenerated by Puffin v0.0.1 via the following command:
+#    [BIN_PATH] pip-compile requirements.in --cache-dir [CACHE_DIR]
+click==8.1.7
+    # via
+    #   dask
+    #   flask
+dask @ git+https://github.com/pallets/flask.git@735a4701d6d5e848241e7d7535db898efb62d400
+flask @ git+https://github.com/pallets/flask.git@2f0c62f5e6e290843f03c1fa70817c7a3c7fd661
+itsdangerous==2.1.2
+    # via
+    #   dask
+    #   flask
+jinja2==3.1.2
+    # via
+    #   dask
+    #   flask
+markupsafe==2.1.3
+    # via
+    #   jinja2
+    #   werkzeug
+werkzeug==3.0.1
+    # via
+    #   dask
+    #   flask
+
+----- stderr -----
+Resolved 7 packages in [TIME]
+