Fix proxy redirect for base path without slash

Signed-off-by: Tim Hansen <timbonicus@gmail.com>

.changeset/wild-wasps-turn.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-proxy-backend': patch
+---
+
+Fixed proxy requests to the base URL of routes without a trailing slash redirecting to the `target` with the full path appended.

plugins/proxy-backend/src/service/router.test.ts

@@ -74,7 +74,7 @@ describe('buildMiddleware', () => {
     expect(filter('', { method: 'PATCH', headers: {} })).toBe(true);
     expect(filter('', { method: 'DELETE', headers: {} })).toBe(true);
 
-    expect(fullConfig.pathRewrite).toEqual({ '^/proxy/test/': '/' });
+    expect(fullConfig.pathRewrite).toEqual({ '^/proxy/test/?': '/' });
     expect(fullConfig.changeOrigin).toBe(true);
     expect(fullConfig.logProvider!(logger)).toBe(logger);
   });
@@ -94,7 +94,7 @@ describe('buildMiddleware', () => {
     expect(filter('', { method: 'PATCH', headers: {} })).toBe(true);
     expect(filter('', { method: 'DELETE', headers: {} })).toBe(true);
 
-    expect(fullConfig.pathRewrite).toEqual({ '^/proxy/test/': '/' });
+    expect(fullConfig.pathRewrite).toEqual({ '^/proxy/test/?': '/' });
     expect(fullConfig.changeOrigin).toBe(true);
     expect(fullConfig.logProvider!(logger)).toBe(logger);
   });
@@ -114,7 +114,7 @@ describe('buildMiddleware', () => {
     expect(filter('', { method: 'PATCH', headers: {} })).toBe(true);
     expect(filter('', { method: 'DELETE', headers: {} })).toBe(true);
 
-    expect(fullConfig.pathRewrite).toEqual({ '^/proxy/test/': '/' });
+    expect(fullConfig.pathRewrite).toEqual({ '^/proxy/test/?': '/' });
     expect(fullConfig.changeOrigin).toBe(true);
     expect(fullConfig.logProvider!(logger)).toBe(logger);
   });
@@ -137,7 +137,7 @@ describe('buildMiddleware', () => {
     expect(filter('', { method: 'PATCH', headers: {} })).toBe(false);
     expect(filter('', { method: 'DELETE', headers: {} })).toBe(true);
 
-    expect(fullConfig.pathRewrite).toEqual({ '^/proxy/test/': '/' });
+    expect(fullConfig.pathRewrite).toEqual({ '^/proxy/test/?': '/' });
     expect(fullConfig.changeOrigin).toBe(true);
     expect(fullConfig.logProvider!(logger)).toBe(logger);
   });

plugins/proxy-backend/src/service/router.ts

@@ -95,8 +95,12 @@ export function buildMiddleware(
       routeWithSlash = routeWithSlash.substring(1);
     }
 
+    // The ? makes the slash optional for the rewrite, so that a base path without an ending slash
+    // will also be matched (e.g. '/sample' and then requesting just '/api/proxy/sample' without an
+    // ending slash). Otherwise the target gets called with the full '/api/proxy/sample' path
+    // appended.
     fullConfig.pathRewrite = {
-      [`^${pathPrefix}${routeWithSlash}`]: '/',
+      [`^${pathPrefix}${routeWithSlash}?`]: '/',
     };
   }