mock system interactions to fix flaky environment tests (#129)

CHANGELOG.md

@@ -26,6 +26,7 @@ and this project attempts to adhere to [Semantic Versioning](https://semver.org/
 
 ### Changed
 
+- **Internal (djls-project)**: Added `system` module to improve reliability of environment discovery tests.
 - **Internal**: Moved task queueing functionality to `djls-server` crate, renamed from `Worker` to `Queue`, and simplified API.
 - **Internal**: Improved Python environment handling, including refactored activation logic.
 - **Internal**: Centralized Python linking build logic into a shared `djls-dev` crate to reduce duplication.

crates/djls-project/src/lib.rs

@@ -1,12 +1,11 @@
+mod system;
 mod templatetags;
 
 pub use templatetags::TemplateTags;
 
 use pyo3::prelude::*;
-use std::env;
 use std::fmt;
 use std::path::{Path, PathBuf};
-use which::which;
 
 #[derive(Debug)]
 pub struct DjangoProject {
@@ -92,19 +91,16 @@ impl PythonEnvironment {
     fn new(project_path: &Path, venv_path: Option<&str>) -> Option<Self> {
         if let Some(path) = venv_path {
             let prefix = PathBuf::from(path);
-            // If an explicit path is provided and it's a valid venv, use it immediately.
             if let Some(env) = Self::from_venv_prefix(&prefix) {
                 return Some(env);
             }
-            // Explicit path was provided but was invalid. Continue searching.
+            // Invalid explicit path, continue searching...
         }
 
-        if let Ok(virtual_env) = env::var("VIRTUAL_ENV") {
+        if let Ok(virtual_env) = system::env_var("VIRTUAL_ENV") {
-            if !virtual_env.is_empty() {
+            let prefix = PathBuf::from(virtual_env);
-                let prefix = PathBuf::from(virtual_env);
+            if let Some(env) = Self::from_venv_prefix(&prefix) {
-                if let Some(env) = Self::from_venv_prefix(&prefix) {
+                return Some(env);
-                    return Some(env);
-                }
             }
         }
 
@@ -126,7 +122,6 @@ impl PythonEnvironment {
         #[cfg(windows)]
         let python_path = prefix.join("Scripts").join("python.exe");
 
-        // Check if the *prefix* and the *binary* exist.
         if !prefix.is_dir() || !python_path.exists() {
             return None;
         }
@@ -137,10 +132,9 @@ impl PythonEnvironment {
         let bin_dir = prefix.join("Scripts");
 
         let mut sys_path = Vec::new();
-        sys_path.push(bin_dir); // Add bin/ or Scripts/
+        sys_path.push(bin_dir);
 
         if let Some(site_packages) = Self::find_site_packages(prefix) {
-            // Check existence inside the if let, as find_site_packages might return a path that doesn't exist
             if site_packages.is_dir() {
                 sys_path.push(site_packages);
             }
@@ -167,12 +161,10 @@ impl PythonEnvironment {
     }
 
     fn from_system_python() -> Option<Self> {
-        let python_path = match which("python") {
+        let python_path = match system::find_executable("python") {
             Ok(p) => p,
             Err(_) => return None,
         };
-        // which() might return a path inside a bin/Scripts dir, or directly the executable
-        // We need the prefix, which is usually two levels up from the executable in standard layouts
         let bin_dir = python_path.parent()?;
         let prefix = bin_dir.parent()?;
 
@@ -194,7 +186,6 @@ impl PythonEnvironment {
 
     #[cfg(unix)]
     fn find_site_packages(prefix: &Path) -> Option<PathBuf> {
-        // Look for lib/pythonX.Y/site-packages
         let lib_dir = prefix.join("lib");
         if !lib_dir.is_dir() {
             return None;
@@ -203,8 +194,8 @@ impl PythonEnvironment {
             .ok()?
             .filter_map(Result::ok)
             .find(|e| {
-                e.file_type().is_ok_and(|ft| ft.is_dir()) && // Ensure it's a directory
+                e.file_type().is_ok_and(|ft| ft.is_dir())
-                e.file_name().to_string_lossy().starts_with("python")
+                    && e.file_name().to_string_lossy().starts_with("python")
             })
             .map(|e| e.path().join("site-packages"))
     }
@@ -231,10 +222,14 @@ impl fmt::Display for PythonEnvironment {
 mod tests {
     use super::*;
     use std::fs;
+    #[cfg(unix)]
+    use std::os::unix::fs::PermissionsExt;
     use tempfile::tempdir;
 
     mod env_discovery {
+        use super::system::mock::{self as sys_mock, MockGuard};
         use super::*;
+        use which::Error as WhichError;
 
         fn create_mock_venv(dir: &Path, version: Option<&str>) -> PathBuf {
             let prefix = dir.to_path_buf();
@@ -243,7 +238,7 @@ mod tests {
             {
                 let bin_dir = prefix.join("bin");
                 fs::create_dir_all(&bin_dir).unwrap();
-                fs::write(bin_dir.join("python"), "").unwrap(); // Create dummy executable
+                fs::write(bin_dir.join("python"), "").unwrap();
                 let lib_dir = prefix.join("lib");
                 fs::create_dir_all(&lib_dir).unwrap();
                 let py_version_dir = lib_dir.join(version.unwrap_or("python3.9"));
@@ -254,7 +249,7 @@ mod tests {
             {
                 let bin_dir = prefix.join("Scripts");
                 fs::create_dir_all(&bin_dir).unwrap();
-                fs::write(bin_dir.join("python.exe"), "").unwrap(); // Create dummy executable
+                fs::write(bin_dir.join("python.exe"), "").unwrap();
                 let lib_dir = prefix.join("Lib");
                 fs::create_dir_all(&lib_dir).unwrap();
                 fs::create_dir_all(lib_dir.join("site-packages")).unwrap();
@@ -263,32 +258,6 @@ mod tests {
             prefix
         }
 
-        struct VirtualEnvGuard<'a> {
-            key: &'a str,
-            original_value: Option<String>,
-        }
-
-        impl<'a> VirtualEnvGuard<'a> {
-            fn set(key: &'a str, value: &str) -> Self {
-                let original_value = env::var(key).ok();
-                env::set_var(key, value);
-                Self {
-                    key,
-                    original_value,
-                }
-            }
-        }
-
-        impl Drop for VirtualEnvGuard<'_> {
-            fn drop(&mut self) {
-                if let Some(ref val) = self.original_value {
-                    env::set_var(self.key, val);
-                } else {
-                    env::remove_var(self.key);
-                }
-            }
-        }
-
         #[test]
         fn test_explicit_venv_path_found() {
             let project_dir = tempdir().unwrap();
@@ -324,11 +293,9 @@ mod tests {
             let project_dir = tempdir().unwrap();
             let project_venv_prefix = create_mock_venv(&project_dir.path().join(".venv"), None);
 
-            // Set VIRTUAL_ENV to something known to be invalid, rather than clearing.
+            let _guard = MockGuard;
-            // This prevents the test runner's VIRTUAL_ENV (e.g., from Nox) from interfering.
+            // Ensure VIRTUAL_ENV is not set (returns VarError::NotPresent)
-            let invalid_virtual_env_path = project_dir.path().join("non_existent_virtual_env");
+            sys_mock::remove_env_var("VIRTUAL_ENV");
-            let _guard =
-                VirtualEnvGuard::set("VIRTUAL_ENV", invalid_virtual_env_path.to_str().unwrap());
 
             // Provide an invalid explicit path
             let invalid_path = project_dir.path().join("non_existent_venv");
@@ -346,7 +313,9 @@ mod tests {
             let venv_dir = tempdir().unwrap();
             let venv_prefix = create_mock_venv(venv_dir.path(), None);
 
-            let _guard = VirtualEnvGuard::set("VIRTUAL_ENV", venv_prefix.to_str().unwrap());
+            let _guard = MockGuard;
+            // Mock VIRTUAL_ENV to point to the mock venv
+            sys_mock::set_env_var("VIRTUAL_ENV", venv_prefix.to_str().unwrap().to_string());
 
             let env = PythonEnvironment::new(project_dir.path(), None)
                 .expect("Should find environment via VIRTUAL_ENV");
@@ -363,18 +332,20 @@ mod tests {
         fn test_explicit_path_overrides_virtual_env() {
             let project_dir = tempdir().unwrap();
             let venv1_dir = tempdir().unwrap();
-            let venv1_prefix = create_mock_venv(venv1_dir.path(), None); // Set by VIRTUAL_ENV
+            let venv1_prefix = create_mock_venv(venv1_dir.path(), None); // Mocked by VIRTUAL_ENV
             let venv2_dir = tempdir().unwrap();
-            let venv2_prefix = create_mock_venv(venv2_dir.path(), None); // Set by explicit path
+            let venv2_prefix = create_mock_venv(venv2_dir.path(), None); // Provided explicitly
 
-            let _guard = VirtualEnvGuard::set("VIRTUAL_ENV", venv1_prefix.to_str().unwrap());
+            let _guard = MockGuard;
+            // Mock VIRTUAL_ENV to point to venv1
+            sys_mock::set_env_var("VIRTUAL_ENV", venv1_prefix.to_str().unwrap().to_string());
 
-            let env = PythonEnvironment::new(
+            // Call with explicit path to venv2
-                project_dir.path(),
+            let env =
-                Some(venv2_prefix.to_str().unwrap()), // Explicit path
+                PythonEnvironment::new(project_dir.path(), Some(venv2_prefix.to_str().unwrap()))
-            )
+                    .expect("Should find environment via explicit path");
-            .expect("Should find environment via explicit path");
 
+            // Explicit path (venv2) should take precedence
             assert_eq!(
                 env.sys_prefix, venv2_prefix,
                 "Explicit path should take precedence"
@@ -386,10 +357,9 @@ mod tests {
             let project_dir = tempdir().unwrap();
             let venv_prefix = create_mock_venv(&project_dir.path().join(".venv"), None);
 
-            // Set VIRTUAL_ENV to something known to be invalid to ensure it's ignored.
+            let _guard = MockGuard;
-            let invalid_virtual_env_path = project_dir.path().join("non_existent_venv_proj_found");
+            // Ensure VIRTUAL_ENV is not set
-            let _guard =
+            sys_mock::remove_env_var("VIRTUAL_ENV");
-                VirtualEnvGuard::set("VIRTUAL_ENV", invalid_virtual_env_path.to_str().unwrap());
 
             let env = PythonEnvironment::new(project_dir.path(), None)
                 .expect("Should find environment in project .venv");
@@ -400,48 +370,88 @@ mod tests {
         #[test]
         fn test_project_venv_priority() {
             let project_dir = tempdir().unwrap();
-            // Create multiple potential venvs
             let dot_venv_prefix = create_mock_venv(&project_dir.path().join(".venv"), None);
-            let _venv_prefix = create_mock_venv(&project_dir.path().join("venv"), None); // Should be ignored if .venv found first
+            let _venv_prefix = create_mock_venv(&project_dir.path().join("venv"), None);
 
-            // Set VIRTUAL_ENV to something known to be invalid to ensure it's ignored.
+            let _guard = MockGuard;
-            let invalid_virtual_env_path = project_dir.path().join("non_existent_venv_priority");
+            // Ensure VIRTUAL_ENV is not set
-            let _guard =
+            sys_mock::remove_env_var("VIRTUAL_ENV");
-                VirtualEnvGuard::set("VIRTUAL_ENV", invalid_virtual_env_path.to_str().unwrap());
 
             let env =
                 PythonEnvironment::new(project_dir.path(), None).expect("Should find environment");
 
-            // Asserts it finds .venv because it's checked first in the loop
+            // Should find .venv because it's checked first in the loop
             assert_eq!(env.sys_prefix, dot_venv_prefix);
         }
 
         #[test]
-        #[ignore = "Relies on system python being available and having standard layout"]
         fn test_system_python_fallback() {
             let project_dir = tempdir().unwrap();
 
-            // Set VIRTUAL_ENV to something known to be invalid to ensure it's ignored.
+            let _guard = MockGuard;
-            let invalid_virtual_env_path =
+            // Ensure VIRTUAL_ENV is not set
-                project_dir.path().join("non_existent_venv_sys_fallback");
+            sys_mock::remove_env_var("VIRTUAL_ENV");
-            let _guard =
+
-                VirtualEnvGuard::set("VIRTUAL_ENV", invalid_virtual_env_path.to_str().unwrap());
+            let mock_sys_python_dir = tempdir().unwrap();
-            // We don't create any venvs in project_dir
+            let mock_sys_python_prefix = mock_sys_python_dir.path();
+
+            #[cfg(unix)]
+            let (bin_subdir, python_exe, site_packages_rel_path) = (
+                "bin",
+                "python",
+                Path::new("lib").join("python3.9").join("site-packages"),
+            );
+            #[cfg(windows)]
+            let (bin_subdir, python_exe, site_packages_rel_path) = (
+                "Scripts",
+                "python.exe",
+                Path::new("Lib").join("site-packages"),
+            );
+
+            let bin_dir = mock_sys_python_prefix.join(bin_subdir);
+            fs::create_dir_all(&bin_dir).unwrap();
+            let python_path = bin_dir.join(python_exe);
+            fs::write(&python_path, "").unwrap();
+
+            #[cfg(unix)]
+            {
+                let mut perms = fs::metadata(&python_path).unwrap().permissions();
+                perms.set_mode(0o755);
+                fs::set_permissions(&python_path, perms).unwrap();
+            }
+
+            let site_packages_path = mock_sys_python_prefix.join(site_packages_rel_path);
+            fs::create_dir_all(&site_packages_path).unwrap();
+
+            sys_mock::set_exec_path("python", python_path.clone());
 
-            // This test assumes `which python` works and points to a standard layout
             let system_env = PythonEnvironment::new(project_dir.path(), None);
 
+            // Assert it found the mock system python via the mocked finder
             assert!(
                 system_env.is_some(),
-                "Should fall back to system python if available"
+                "Should fall back to the mock system python"
             );
 
             if let Some(env) = system_env {
-                // Basic checks - exact paths depend heavily on the test environment
+                assert_eq!(
-                assert!(env.python_path.exists());
+                    env.python_path, python_path,
-                assert!(env.sys_prefix.exists());
+                    "Python path should match mock"
-                assert!(!env.sys_path.is_empty());
+                );
-                assert!(env.sys_path[0].exists()); // Should contain the bin/Scripts dir
+                assert_eq!(
+                    env.sys_prefix, mock_sys_python_prefix,
+                    "Sys prefix should match mock prefix"
+                );
+                assert!(
+                    env.sys_path.contains(&bin_dir),
+                    "Sys path should contain mock bin dir"
+                );
+                assert!(
+                    env.sys_path.contains(&site_packages_path),
+                    "Sys path should contain mock site-packages"
+                );
+            } else {
+                panic!("Expected to find environment, but got None");
             }
         }
 
@@ -449,39 +459,19 @@ mod tests {
         fn test_no_python_found() {
             let project_dir = tempdir().unwrap();
 
-            // Ensure no explicit path, no project venvs, and set VIRTUAL_ENV to invalid.
+            let _guard = MockGuard; // Setup guard to clear mocks
-            let invalid_virtual_env_path = project_dir.path().join("non_existent_venv_no_python");
-            let _guard =
-                VirtualEnvGuard::set("VIRTUAL_ENV", invalid_virtual_env_path.to_str().unwrap());
 
-            // To *ensure* system fallback fails, we'd need to manipulate PATH,
+            // Ensure VIRTUAL_ENV is not set
-            // which is tricky and platform-dependent. Instead, we test the scenario
+            sys_mock::remove_env_var("VIRTUAL_ENV");
-            // where `from_system_python` *would* be called but returns None.
-            // We can simulate this by ensuring `which("python")` fails.
-            // For this unit test, let's assume a scenario where all checks fail.
-            // A more direct test would mock `which`, but that adds complexity.
 
-            // Let's simulate the *call* path assuming `from_system_python` returns None.
+            // Ensure find_executable returns an error
-            // We can't easily force `which` to fail here without PATH manipulation.
+            sys_mock::set_exec_error("python", WhichError::CannotFindBinaryPath);
-            // So, this test mainly verifies that if all preceding steps fail,
-            // the result of `from_system_python` (which *could* be None) is returned.
-            // If system python *is* found, this test might incorrectly pass if not ignored.
-            // A better approach might be needed if strict testing of "None" is required.
 
-            // For now, let's assume a setup where system python isn't found by `which`.
-            // This test is inherently flaky if system python *is* on the PATH.
-            // Consider ignoring it or using mocking for `which` in a real-world scenario.
-
-            // If system python IS found, this test doesn't truly test the "None" case.
-            // If system python IS NOT found, it tests the final `None` return.
             let env = PythonEnvironment::new(project_dir.path(), None);
 
-            // This assertion depends on whether system python is actually found or not.
+            assert!(
-            // assert!(env.is_none(), "Expected no environment to be found");
+                env.is_none(),
-            // Given the difficulty, let's skip asserting None directly unless we mock `which`.
+                "Expected no environment to be found when all discovery methods fail"
-            println!(
-                "Test 'test_no_python_found' ran. Result depends on system state: {:?}",
-                env
             );
         }
 
@@ -495,7 +485,6 @@ mod tests {
             fs::write(bin_dir.join("python"), "").unwrap();
             let lib_dir = prefix.join("lib");
             fs::create_dir_all(&lib_dir).unwrap();
-            // Create two python version dirs, ensure it picks one
             let py_version_dir1 = lib_dir.join("python3.8");
             fs::create_dir_all(&py_version_dir1).unwrap();
             fs::create_dir_all(py_version_dir1.join("site-packages")).unwrap();
@@ -505,14 +494,11 @@ mod tests {
 
             let env = PythonEnvironment::from_venv_prefix(prefix).unwrap();
 
-            // It should find *a* site-packages dir. The exact one depends on read_dir order.
             let found_site_packages = env.sys_path.iter().any(|p| p.ends_with("site-packages"));
             assert!(
                 found_site_packages,
                 "Should have found a site-packages directory"
             );
-
-            // Ensure it contains the bin dir as well
             assert!(env.sys_path.contains(&prefix.join("bin")));
         }
 
@@ -527,7 +513,7 @@ mod tests {
             let lib_dir = prefix.join("Lib");
             fs::create_dir_all(&lib_dir).unwrap();
             let site_packages = lib_dir.join("site-packages");
-            fs::create_dir_all(&site_packages).unwrap(); // Create the actual dir
+            fs::create_dir_all(&site_packages).unwrap();
 
             let env = PythonEnvironment::from_venv_prefix(prefix).unwrap();
 
@@ -541,7 +527,6 @@ mod tests {
         #[test]
         fn test_from_venv_prefix_returns_none_if_dir_missing() {
             let dir = tempdir().unwrap();
-            // Don't create the venv structure
             let result = PythonEnvironment::from_venv_prefix(dir.path());
             assert!(result.is_none());
         }
@@ -550,7 +535,6 @@ mod tests {
         fn test_from_venv_prefix_returns_none_if_binary_missing() {
             let dir = tempdir().unwrap();
             let prefix = dir.path();
-            // Create prefix dir but not the binary
             fs::create_dir_all(prefix).unwrap();
 
             #[cfg(unix)]
@@ -574,7 +558,6 @@ mod tests {
 
         fn create_test_env(sys_paths: Vec<PathBuf>) -> PythonEnvironment {
             PythonEnvironment {
-                // Dummy values for fields not directly used by activate
                 python_path: PathBuf::from("dummy/bin/python"),
                 sys_prefix: PathBuf::from("dummy"),
                 sys_path: sys_paths,
@@ -605,8 +588,6 @@ mod tests {
                     initial_len + 2,
                     "Should have added 2 paths"
                 );
-
-                // Check that the *exact* paths were appended in the correct order
                 assert_eq!(
                     final_sys_path.get(initial_len).unwrap(),
                     path1.to_str().expect("Path 1 should be valid UTF-8")
@@ -644,7 +625,6 @@ mod tests {
         #[test]
         fn test_activate_with_non_existent_paths() -> PyResult<()> {
             let temp_dir = tempdir().unwrap();
-            // These paths do not actually exist on the filesystem
             let path1 = temp_dir.path().join("non_existent_dir");
             let path2 = temp_dir.path().join("another_missing/path");
 
@@ -687,11 +667,9 @@ mod tests {
             let valid_path = temp_dir.path().join("valid_dir");
             fs::create_dir(&valid_path).unwrap();
 
-            // Create a PathBuf from invalid UTF-8 bytes
             let invalid_bytes = b"invalid_\xff_utf8";
             let os_str = OsStr::from_bytes(invalid_bytes);
             let non_utf8_path = PathBuf::from(os_str);
-            // Sanity check: ensure this path *cannot* be converted to str
             assert!(
                 non_utf8_path.to_str().is_none(),
                 "Path should not be convertible to UTF-8 str"
@@ -708,7 +686,6 @@ mod tests {
                 test_env.activate(py)?;
 
                 let final_sys_path = get_sys_path(py)?;
-                // Should have added only the valid path
                 assert_eq!(
                     final_sys_path.len(),
                     initial_len + 1,
@@ -719,7 +696,6 @@ mod tests {
                     valid_path.to_str().unwrap()
                 );
 
-                // Check that the invalid path string representation is NOT present
                 let invalid_path_lossy = non_utf8_path.to_string_lossy();
                 assert!(
                     !final_sys_path
@@ -733,17 +709,14 @@ mod tests {
         }
 
         #[test]
-        #[cfg(windows)] // Test specific behavior for invalid UTF-16/WTF-8 on Windows
+        #[cfg(windows)]
         fn test_activate_skips_non_utf8_paths_windows() -> PyResult<()> {
             use std::ffi::OsString;
             use std::os::windows::ffi::OsStringExt;
 
             let temp_dir = tempdir().unwrap();
             let valid_path = temp_dir.path().join("valid_dir");
-            // No need to create dir, just need the PathBuf
 
-            // Create an OsString from invalid UTF-16 (a lone surrogate)
-            // D800 is a high surrogate, not valid unless paired with a low surrogate.
             let invalid_wide: Vec<u16> = vec![
                 'i' as u16, 'n' as u16, 'v' as u16, 'a' as u16, 'l' as u16, 'i' as u16, 'd' as u16,
                 '_' as u16, 0xD800, '_' as u16, 'w' as u16, 'i' as u16, 'd' as u16, 'e' as u16,
@@ -751,7 +724,6 @@ mod tests {
             let os_string = OsString::from_wide(&invalid_wide);
             let non_utf8_path = PathBuf::from(os_string);
 
-            // Sanity check: ensure this path *cannot* be converted to a valid UTF-8 str
             assert!(
                 non_utf8_path.to_str().is_none(),
                 "Path with lone surrogate should not be convertible to UTF-8 str"
@@ -768,7 +740,6 @@ mod tests {
                 test_env.activate(py)?;
 
                 let final_sys_path = get_sys_path(py)?;
-                // Should have added only the valid path
                 assert_eq!(
                     final_sys_path.len(),
                     initial_len + 1,
@@ -779,7 +750,6 @@ mod tests {
                     valid_path.to_str().unwrap()
                 );
 
-                // Check that the invalid path string representation is NOT present
                 let invalid_path_lossy = non_utf8_path.to_string_lossy();
                 assert!(
                     !final_sys_path

crates/djls-project/src/system.rs

@@ -0,0 +1,196 @@
+use std::env::VarError;
+use std::path::PathBuf;
+use which::Error as WhichError;
+
+pub fn find_executable(name: &str) -> Result<PathBuf, WhichError> {
+    #[cfg(not(test))]
+    {
+        which::which(name)
+    }
+    #[cfg(test)]
+    {
+        mock::find_executable_mocked(name)
+    }
+}
+
+pub fn env_var(key: &str) -> Result<String, VarError> {
+    #[cfg(not(test))]
+    {
+        std::env::var(key)
+    }
+    #[cfg(test)]
+    {
+        mock::env_var_mocked(key)
+    }
+}
+
+#[cfg(test)]
+pub mod mock {
+    use super::*;
+    use std::cell::RefCell;
+    use std::collections::HashMap;
+    use std::thread_local;
+
+    thread_local! {
+        static MOCK_EXEC_RESULTS: RefCell<HashMap<String, Result<PathBuf, WhichError>>> = RefCell::new(HashMap::new());
+        static MOCK_ENV_RESULTS: RefCell<HashMap<String, Result<String, VarError>>> = RefCell::new(HashMap::new());
+    }
+
+    pub(super) fn find_executable_mocked(name: &str) -> Result<PathBuf, WhichError> {
+        MOCK_EXEC_RESULTS.with(|mocks| {
+            mocks
+                .borrow()
+                .get(name)
+                .cloned()
+                .unwrap_or(Err(WhichError::CannotFindBinaryPath))
+        })
+    }
+
+    pub(super) fn env_var_mocked(key: &str) -> Result<String, VarError> {
+        MOCK_ENV_RESULTS.with(|mocks| {
+            mocks
+                .borrow()
+                .get(key)
+                .cloned()
+                .unwrap_or(Err(VarError::NotPresent))
+        })
+    }
+
+    // RAII guard to clear all mocks automatically after each test.
+    pub struct MockGuard;
+    impl Drop for MockGuard {
+        fn drop(&mut self) {
+            MOCK_EXEC_RESULTS.with(|mocks| mocks.borrow_mut().clear());
+            MOCK_ENV_RESULTS.with(|mocks| mocks.borrow_mut().clear());
+        }
+    }
+
+    pub fn set_exec_path(name: &str, path: PathBuf) {
+        MOCK_EXEC_RESULTS.with(|mocks| {
+            mocks.borrow_mut().insert(name.to_string(), Ok(path));
+        });
+    }
+
+    pub fn set_exec_error(name: &str, error: WhichError) {
+        MOCK_EXEC_RESULTS.with(|mocks| {
+            mocks.borrow_mut().insert(name.to_string(), Err(error));
+        });
+    }
+
+    pub fn set_env_var(key: &str, value: String) {
+        MOCK_ENV_RESULTS.with(|mocks| {
+            mocks.borrow_mut().insert(key.to_string(), Ok(value));
+        });
+    }
+
+    // Simulates VarError::NotPresent
+    pub fn remove_env_var(key: &str) {
+        MOCK_ENV_RESULTS.with(|mocks| {
+            mocks
+                .borrow_mut()
+                .insert(key.to_string(), Err(VarError::NotPresent));
+        });
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::mock::{self as sys_mock, MockGuard};
+    use super::*;
+    use std::env::VarError;
+    use std::path::PathBuf;
+    use which::Error as WhichError;
+
+    #[test]
+    fn test_exec_mock_path_retrieval() {
+        let _guard = MockGuard;
+        let expected_path = PathBuf::from("/mock/path/to/python");
+        sys_mock::set_exec_path("python", expected_path.clone());
+        let result = find_executable("python");
+        assert_eq!(result.unwrap(), expected_path);
+    }
+
+    #[test]
+    fn test_exec_mock_error_retrieval() {
+        let _guard = MockGuard;
+        sys_mock::set_exec_error("cargo", WhichError::CannotFindBinaryPath);
+        let result = find_executable("cargo");
+        assert!(matches!(result, Err(WhichError::CannotFindBinaryPath)));
+    }
+
+    #[test]
+    fn test_exec_mock_default_error_if_unmocked() {
+        let _guard = MockGuard;
+        let result = find_executable("git"); // Not mocked
+        assert!(matches!(result, Err(WhichError::CannotFindBinaryPath)));
+    }
+
+    #[test]
+    fn test_env_mock_set_var_retrieval() {
+        let _guard = MockGuard;
+        sys_mock::set_env_var("MY_VAR", "my_value".to_string());
+        let result = env_var("MY_VAR");
+        assert_eq!(result.unwrap(), "my_value");
+    }
+
+    #[test]
+    fn test_env_mock_remove_var_retrieval() {
+        let _guard = MockGuard;
+        // Set it first, then remove it via mock
+        sys_mock::set_env_var("TEMP_VAR", "temp_value".to_string());
+        sys_mock::remove_env_var("TEMP_VAR");
+        let result = env_var("TEMP_VAR");
+        assert!(matches!(result, Err(VarError::NotPresent)));
+    }
+
+    #[test]
+    fn test_env_mock_default_error_if_unmocked() {
+        let _guard = MockGuard;
+        let result = env_var("UNMOCKED_VAR"); // Not mocked
+        assert!(matches!(result, Err(VarError::NotPresent)));
+    }
+
+    #[test]
+    fn test_mock_guard_clears_all_mocks() {
+        let expected_exec_path = PathBuf::from("/tmp/myprog");
+        let expected_env_val = "test_value".to_string();
+
+        {
+            let _guard = MockGuard;
+            sys_mock::set_exec_path("myprog", expected_exec_path.clone());
+            sys_mock::set_env_var("MY_TEST_ENV", expected_env_val.clone());
+            // Guard drops here, clearing both mocks
+        }
+
+        // Verify mocks were cleared
+        let _guard = MockGuard;
+        let result_exec = find_executable("myprog");
+        assert!(matches!(result_exec, Err(WhichError::CannotFindBinaryPath)));
+        let result_env = env_var("MY_TEST_ENV");
+        assert!(matches!(result_env, Err(VarError::NotPresent)));
+    }
+
+    #[test]
+    fn test_mocks_are_separate_between_tests() {
+        let _guard = MockGuard; // Ensures clean state
+
+        // Check state from previous tests (should be cleared)
+        let result_python = find_executable("python");
+        assert!(matches!(
+            result_python,
+            Err(WhichError::CannotFindBinaryPath)
+        ));
+        let result_myvar = env_var("MY_VAR");
+        assert!(matches!(result_myvar, Err(VarError::NotPresent)));
+
+        // Set mocks specific to this test
+        let expected_path_node = PathBuf::from("/usr/bin/node");
+        sys_mock::set_exec_path("node", expected_path_node.clone());
+        sys_mock::set_env_var("NODE_ENV", "production".to_string());
+
+        let result_node = find_executable("node");
+        assert_eq!(result_node.unwrap(), expected_path_node);
+        let result_node_env = env_var("NODE_ENV");
+        assert_eq!(result_node_env.unwrap(), "production");
+    }
+}