fix: close shebang descriptors after read failures

lib/util/readShebang.js

@@ -13,8 +13,13 @@ function readShebang(command) {
     try {
         fd = fs.openSync(command, 'r');
         fs.readSync(fd, buffer, 0, size, 0);
-        fs.closeSync(fd);
-    } catch (e) { /* Empty */ }
+    } catch (e) { /* Empty */ } finally {
+        if (fd !== undefined) {
+            try {
+                fs.closeSync(fd);
+            } catch (e) { /* Empty */ }
+        }
+    }
 
     // Attempt to extract shebang (null is returned if not a shebang)
     return shebangCommand(buffer.toString());

test/readShebang.test.js

@@ -0,0 +1,51 @@
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+const readShebang = require('../lib/util/readShebang');
+
+describe('readShebang', () => {
+    afterEach(() => jest.restoreAllMocks());
+
+    it('closes an opened descriptor when reading fails, including descriptor zero', () => {
+        jest.spyOn(fs, 'openSync').mockReturnValue(0);
+        jest.spyOn(fs, 'readSync').mockImplementation(() => {
+            throw new Error('read failed');
+        });
+        const close = jest.spyOn(fs, 'closeSync').mockImplementation(() => {});
+
+        expect(readShebang('command')).toBeNull();
+        expect(close).toHaveBeenCalledTimes(1);
+        expect(close).toHaveBeenCalledWith(0);
+    });
+
+    it('does not close a descriptor when opening fails', () => {
+        jest.spyOn(fs, 'openSync').mockImplementation(() => {
+            throw new Error('open failed');
+        });
+        const close = jest.spyOn(fs, 'closeSync').mockImplementation(() => {});
+
+        expect(readShebang('command')).toBeNull();
+        expect(close).not.toHaveBeenCalled();
+    });
+
+    it('preserves the fallback when closing also fails', () => {
+        jest.spyOn(fs, 'openSync').mockReturnValue(0);
+        jest.spyOn(fs, 'readSync').mockImplementation(() => {
+            throw new Error('read failed');
+        });
+        jest.spyOn(fs, 'closeSync').mockImplementation(() => {
+            throw new Error('close failed');
+        });
+
+        expect(readShebang('command')).toBeNull();
+    });
+
+    it('still reads a valid shebang', () => {
+        const close = jest.spyOn(fs, 'closeSync');
+        const command = path.join(__dirname, 'fixtures/shebang');
+
+        expect(readShebang(command)).toBe('node');
+        expect(close).toHaveBeenCalledTimes(1);
+    });
+});