fix: throw EEXIST when opening file that already exists with 'wx' flag (

#853)

* fix: throw EEXIST if open exisiting file with exclude flag

* fix: check exclude flag before creating link in openFile

src/__tests__/volume.test.ts

@@ -400,6 +400,13 @@ describe('volume', () => {
           done();
         });
       });
+      it('Error with exclude flag if file exists', (done) => {
+        vol.writeFileSync('/existing-file.txt', 'foo');
+        vol.open('/existing-file.txt', 'wx', (err) => {
+          expect(err).toHaveProperty('code', 'EEXIST');
+          done();
+        });
+      });
       it('Invalid path correct error code thrown synchronously', done => {
         try {
           (vol as any).open(123, 'r', (err, fd) => {
@@ -599,6 +606,14 @@ describe('volume', () => {
           done();
         });
       });
+      it('Create a file at root (/writeFile2.json) with exclude flag', done => {
+        vol.writeFile('/writeFile2.json', data, { flag: "wx" }, err => {
+          expect(err).toBe(null);
+          const str = tryGetChildNode(vol.root, 'writeFile2.json').getString();
+          expect(str).toBe(data);
+          done();
+        });
+      });
       it('Throws error when no callback provided', () => {
         try {
           vol.writeFile('/asdf.txt', 'asdf', 'utf8', undefined as any);

src/volume.ts

@@ -1005,6 +1005,8 @@ export class Volume {
     const steps = filenameToSteps(filename);
     let link: Link | null = resolveSymlinks ? this.getResolvedLink(steps) : this.getLink(steps);
 
+    if (link && flagsNum & O_EXCL) throw createError(EEXIST, 'open', filename);
+
     // Try creating a new file, if it does not exist.
     if (!link && flagsNum & O_CREAT) {
       // const dirLink: Link = this.getLinkParent(steps);