Modifying Autogenerated CAP Index Page Response Causes 502 Error Due to Content-Length Mismatch #54
Description
Description:
When the cds-launchpad-plugin modifies the CAP-generated index page (/), it results in a 502 error when accessed via the AppRouter. This happens because the response is altered without updating or removing the Content-Length header, causing an inconsistency between the declared and actual content size.
Steps to Reproduce:
- Install and enable
cds-launchpad-plugin. - Set up an SAP AppRouter in
app/router/with the following configuration (default port is5000):app/router/xs-app.json:{ "routes": [ { "source": "^/(.*)$", "target": "$1", "destination": "srv-api", "csrfProtection": true } ] }app/router/default-env.json:{ "destinations": [ { "name": "srv-api", "url": "http://localhost:4004", "forwardAuthToken": true } ] }
- Start the CAP application:
cds watch
- Start the AppRouter from the
app/router/directory:npm run start
- Access
http://localhost:5000/in a browser. - Observe that the request fails with a
502error.
Observed Behavior:
The Approuter logs indicate an issue with forwarding the modified response:
Apr 01, 2025 04:05:10 PM /app/router/node_modules/@sap/approuter/lib/middleware/error-handler.js [m8yklfgo] DEBUG: stack: VError: error while forwarding request to http://localhost:4004/: Parse Error: Expected HTTP/ correlation_id = 241cddd5-dbe7-47b0-a5d0-b9c2c196f0d2
at returnGateWayError (/app/router/node_modules/@sap/approuter/lib/middleware/request-handler.js:271:13)
at ClientRequest.<anonymous> (/app/router/node_modules/@sap/approuter/lib/middleware/request-handler.js:107:7)
at ClientRequest.emit (node:events:524:28)
at emitErrorEvent (node:_http_client:104:11)
at Socket.socketOnData (node:_http_client:567:5)
at Socket.emit (node:events:524:28)
at addChunk (node:internal/streams/readable:561:12)
at readableAddChunkPushByteMode (node:internal/streams/readable:512:3)
at Readable.push (node:internal/streams/readable:392:5)
at TCP.onStreamRead (node:internal/stream_base_commons:189:23)
#2.0#2025 04 01 16:05:10:931#+02:00#INFO#/request/incoming#####241cddd5-dbe7-47b0-a5d0-b9c2c196f0d2##########m8yklfgo#PLAIN##GET to / took 11 ms to complete with status code 502. 910 bytes sent by the client. 393 bytes sent back to the client#
Expected Behavior:
The root page should load correctly with the injected modifications.
Proposed Fix:
Remove the Content-Length header before modifying the response, allowing the server to recalculate it dynamically.
router.get("/", (req, res, next) => {
const originalEnd = res.end;
res.end = function (content: any, encoding?: BufferEncoding) {
if (typeof content === "string" && content.includes("<html>")) {
cdsLaunchpadLogger._debug && cdsLaunchpadLogger.debug("Injecting Launchpad link into CAP index page...");
content = content.replace(
/<h2> Web Applications: <\/h2>/,
`<h2><b><a href="${options.basePath}">Sandbox Launchpad</a></b></h2><h2>Web Applications: </h2>`
);
// Remove Content-Length header to avoid issues with @sap/approuter (removing it lets the server recalculate)
res.removeHeader("Content-Length");
} else {
cdsLaunchpadLogger.warn("Unexpected response format. Skipping modification of CAP index page.");
}
originalEnd.call(res, content, encoding);
};
next();
});