Resolve a relative path to the application root (#665)

Summary:
This is necessary for #643. If we’re serving `/prototype/index.html`, we
need to to use `..` to refer to the root of the site. This patch adds
`rootFromPath`, which performs the relevant transformation. (The
implementation is trivial, but figuring out exactly what the
specification should be was not!)

Test Plan:
Unit tests added; `yarn test` suffices.

wchargin-branch: rootFromPath

src/app/assets.js

@@ -40,3 +40,26 @@ export class Assets {
     return normalize(`${this._getRoot()}/${path}`);
   }
 }
+
+/**
+ * Given an absolute path `p`, return a relative path `r` such that a
+ * web page at pathname `p` should use `r` to refer to the root of the
+ * application. The result will only contain components `.` and `..`.
+ *
+ * Examples:
+ *
+ *   - "/foo/" maps to "..";
+ *   - "/foo/bar" also maps to "..";
+ *   - "/foo/bar/" maps to "../..";
+ *   - "/" maps to ".".
+ *
+ * If the argument does not start with "/", an error will be thrown.
+ */
+export function rootFromPath(path: string) {
+  const normalized = normalize(path);
+  if (normalized[0] !== "/") {
+    throw new Error("expected absolute path: " + JSON.stringify(path));
+  }
+  const levels = (normalized.match(/\//g) || []).length;
+  return normalize(new Array(levels - 1).fill("..").join("/"));
+}

src/app/assets.test.js

@@ -1,6 +1,6 @@
 // @flow
 
-import {Assets} from "./assets";
+import {Assets, rootFromPath} from "./assets";
 
 describe("app/assets", () => {
   describe("Assets", () => {
@@ -161,4 +161,62 @@ describe("app/assets", () => {
       });
     });
   });
+
+  describe("rootFromPath", () => {
+    it("throws on the empty path", () => {
+      expect(() => rootFromPath("")).toThrow('expected absolute path: ""');
+    });
+    it('throws on an implicitly relative path ("wat")', () => {
+      expect(() => rootFromPath("wat")).toThrow(
+        'expected absolute path: "wat"'
+      );
+    });
+    it('throws on an explicitly relative path ("./wat")', () => {
+      expect(() => rootFromPath("./wat")).toThrow(
+        'expected absolute path: "./wat"'
+      );
+    });
+    describe('returns "." for a path at root', () => {
+      it('with no file component ("/")', () => {
+        expect(rootFromPath("/")).toEqual(".");
+      });
+      it('with a file component ("/index.html")', () => {
+        expect(rootFromPath("/index.html")).toEqual(".");
+      });
+      it('with superfluous slashes ("///")', () => {
+        expect(rootFromPath("///")).toEqual(".");
+      });
+      it('with indirection, like "/foo/../"', () => {
+        expect(rootFromPath("/foo/../")).toEqual(".");
+      });
+    });
+    describe('returns ".." for a path one level deep', () => {
+      it('with no file component ("/foo/")', () => {
+        expect(rootFromPath("/foo/")).toEqual("..");
+      });
+      it('with a file component ("/foo/index.html")', () => {
+        expect(rootFromPath("/foo/index.html")).toEqual("..");
+      });
+      it('with superfluous slashes ("//foo//")', () => {
+        expect(rootFromPath("//foo//")).toEqual("..");
+      });
+      it('with indirection, like "/foo/bar/../"', () => {
+        expect(rootFromPath("/foo/bar/../")).toEqual("..");
+      });
+    });
+    describe('returns "../.." for a path two levels deep', () => {
+      it('with no file component ("/foo/bar/")', () => {
+        expect(rootFromPath("/foo/bar/")).toEqual("../..");
+      });
+      it('with a file component ("/foo/bar/index.html")', () => {
+        expect(rootFromPath("/foo/bar/index.html")).toEqual("../..");
+      });
+      it('with superfluous slashes ("//foo//bar//")', () => {
+        expect(rootFromPath("//foo//bar//")).toEqual("../..");
+      });
+      it('with indirection, like "/foo/bar/baz/../"', () => {
+        expect(rootFromPath("/foo/bar/baz/../")).toEqual("../..");
+      });
+    });
+  });
 });