add testing for runInBackground (#877)

Author: eleanorjboyd

src/common/childProcess.apis.ts

@@ -0,0 +1,28 @@
+import * as cp from 'child_process';
+
+/**
+ * Spawns a new process using the specified command and arguments.
+ * This function abstracts cp.spawn to make it easier to mock in tests.
+ *
+ * When stdio: 'pipe' is used, returns ChildProcessWithoutNullStreams.
+ * Otherwise returns the standard ChildProcess.
+ */
+
+// Overload for stdio: 'pipe' - guarantees non-null streams
+export function spawnProcess(
+    command: string,
+    args: string[],
+    options: cp.SpawnOptions & { stdio: 'pipe' },
+): cp.ChildProcessWithoutNullStreams;
+
+// Overload for general case
+export function spawnProcess(command: string, args: string[], options?: cp.SpawnOptions): cp.ChildProcess;
+
+// Implementation - delegates to cp.spawn to preserve its typing magic
+export function spawnProcess(
+    command: string,
+    args: string[],
+    options?: cp.SpawnOptions,
+): cp.ChildProcess | cp.ChildProcessWithoutNullStreams {
+    return cp.spawn(command, args, options ?? {});
+}

src/features/execution/runInBackground.ts

@@ -1,5 +1,5 @@
-import * as cp from 'child_process';
 import { PythonBackgroundRunOptions, PythonEnvironment, PythonProcess } from '../../api';
+import { spawnProcess } from '../../common/childProcess.apis';
 import { traceError, traceInfo, traceWarn } from '../../common/logging';
 import { quoteStringIfNecessary } from './execUtils';
 
@@ -39,7 +39,11 @@ export async function runInBackground(
         traceWarn(`Error checking if executable exists: ${err instanceof Error ? err.message : String(err)}`);
     }
 
-    const proc = cp.spawn(executable, allArgs, { stdio: 'pipe', cwd: options.cwd, env: options.env });
+    const proc = spawnProcess(executable, allArgs, {
+        stdio: 'pipe',
+        cwd: options.cwd,
+        env: options.env,
+    });
 
     return {
         pid: proc.pid,