Throw an error if writing a file where there is already a directory

Fixes #119

Author: elegaanz

src/CacheFS.js

@@ -1,5 +1,5 @@
 const path = require("./path.js");
-const { EEXIST, ENOENT, ENOTDIR, ENOTEMPTY } = require("./errors.js");
+const { EEXIST, ENOENT, ENOTDIR, ENOTEMPTY, EISDIR } = require("./errors.js");
 
 const STAT = 0;
 
@@ -164,13 +164,22 @@ module.exports = class CacheFS {
   }
   writeStat(filepath, size, { mode }) {
     let ino;
+    let oldStat;
     try {
-      let oldStat = this.stat(filepath);
+      oldStat = this.stat(filepath);
+    } catch (err) {}
+
+    if (oldStat !== undefined) {
+      if (oldStat.type === 'dir') {
+        throw new EISDIR();
+      }
+  
       if (mode == null) {
         mode = oldStat.mode;
       }
       ino = oldStat.ino;
-    } catch (err) {}
+    }
+
     if (mode == null) {
       mode = 0o666;
     }

src/__tests__/fs.promises.spec.js

@@ -85,6 +85,19 @@ describe("fs.promises module", () => {
         });
       });
     });
+    it("write file in place of an existing directory throws", done => {
+      fs.mkdir("/writeFile").finally(() => {
+        fs.writeFile("/writeFile", "HELLO")
+          .then(() => {
+            fail();
+            done();
+          })
+          .catch(err => {
+            expect(err).not.toBe(null);
+            done();
+          });
+      });
+    });
   });
 
   describe("readFile", () => {

src/__tests__/fs.spec.js

@@ -71,6 +71,14 @@ describe("fs module", () => {
         });
       });
     });
+    it("write file in place of an existing directory throws", done => {
+      fs.mkdir("/writeFile", err => {
+        fs.writeFile("/writeFile", "HELLO", err => {
+          expect(err).not.toBe(null);
+          done();
+        });
+      });
+    });
   });
 
   describe("readFile", () => {

src/errors.js

@@ -17,5 +17,6 @@ const ENOENT = Err("ENOENT");
 const ENOTDIR = Err("ENOTDIR");
 const ENOTEMPTY = Err("ENOTEMPTY");
 const ETIMEDOUT = Err("ETIMEDOUT");
+const EISDIR = Err("EISDIR");
 
-module.exports = { EEXIST, ENOENT, ENOTDIR, ENOTEMPTY, ETIMEDOUT };
+module.exports = { EEXIST, ENOENT, ENOTDIR, ENOTEMPTY, ETIMEDOUT, EISDIR };