Cap a report at 32 KB on both sides of the upload

The client would send up to 256 KB and the sink answered a settling 400 above
64 KB, so the two disagreed about what a valid report is, and the client
deletes what it is told is invalid. Unreachable in practice — the module table
and backtrace are both bounded, which puts the ceiling near 10 KB — but the
two constants have to agree for the disagreement not to matter.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crash-telemetry/test.mjs

@@ -43,7 +43,7 @@ assert.equal(await sent.body.get("files[0]").text(), REPORT);
 
 // junk: client should delete these, so they must be 400
 assert.equal((await post("hello")).status, 400);
-assert.equal((await post("x".repeat(64 * 1024 + 1))).status, 400);
+assert.equal((await post("x".repeat(32 * 1024 + 1))).status, 400);
 
 // throttled: 429, and nothing reaches Discord. reportIsSettled() leaves 429
 // unsettled, so the client keeps the report for next launch.

crash-telemetry/worker.js

@@ -13,7 +13,11 @@
 // So never answer a settling 4xx for a failure on our side: that throws the report
 // away. 429 is the one 4xx that is safe, because the client does not settle it.
 
-const MAX_BYTES = 64 * 1024; // reports run 2-8 KB; anything near this is not ours
+// Reports run 2-8 KB, and the fixed bounds in writeExceptionBacktrace (128
+// modules, 64 frames) cap them near 10 KB. Kept equal to kMaxReportBytes in the
+// client: anything the client is willing to send must not meet a settling 400
+// here, or the upload is what destroys the report.
+const MAX_BYTES = 32 * 1024;
 
 // Every field below is lifted out of the uploaded file, and the endpoint is public
 // and unauthenticated: treat all of it as attacker-chosen, not just the handle.

sgp/crash_telemetry.cpp

@@ -37,8 +37,10 @@ void writeConsent(bool yes) {
 	CloseHandle(h);
 }
 
-// A report bigger than this is not one of ours; never put it on the wire.
-const DWORD kMaxReportBytes = 256 * 1024;
+// A report bigger than this is not one of ours; never put it on the wire. Kept
+// equal to MAX_BYTES in the sink, which answers a settling 400 above it: sending
+// what the sink will not take is how an upload destroys the report it carries.
+const DWORD kMaxReportBytes = 32 * 1024;
 
 // POST one report file to url. Returns the HTTP status, or 0 if the request never
 // completed (no connection, DNS failure, timeout) — see reportIsSettled().