Central as Beta simulation:
https://treeherder.mozilla.org/#/jobs?repo=try&group_state=expanded&resultStatus=pending%2Crunning%2Csuccess%2Ctestfailed%2Cbusted%2Cexception%2Crunnable&revision=f1c365e9bf21220130335842a7008f057a3eafb0&searchStr=%28dt&selectedJob=277656942

Failure log:
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=277656942&repo=try&lineNumber=14058

[task 2019-11-22T14:27:31.632Z] 14:27:31 INFO - Waiting for state change: paused
[task 2019-11-22T14:27:31.632Z] 14:27:31 INFO - Finished waiting for state change: paused
[task 2019-11-22T14:27:31.632Z] 14:27:31 INFO - Finished waiting for state change: paused
[task 2019-11-22T14:27:31.632Z] 14:27:31 INFO - Waiting for state change: fetched frames
[task 2019-11-22T14:27:31.633Z] 14:27:31 INFO - Finished waiting for state change: fetched frames
[task 2019-11-22T14:27:31.633Z] 14:27:31 INFO - Waiting for state change: selected source
[task 2019-11-22T14:27:31.633Z] 14:27:31 INFO - Finished waiting for state change: selected source
[task 2019-11-22T14:27:31.633Z] 14:27:31 INFO - resume and wait for fib.c
[task 2019-11-22T14:27:31.633Z] 14:27:31 INFO - Resuming from 29
[task 2019-11-22T14:27:31.633Z] 14:27:31 INFO - Waiting on sources: doc-wasm-sourcemaps.html, fib.c
[task 2019-11-22T14:27:31.633Z] 14:27:31 INFO - Finished waiting on sources: doc-wasm-sourcemaps.html, fib.c
[task 2019-11-22T14:27:31.633Z] 14:27:31 INFO - Set breakpoint and reload the page.
[task 2019-11-22T14:27:31.633Z] 14:27:31 INFO - TEST-PASS | devtools/client/debugger/test/mochitest/browser_dbg-wasm-sourcemaps.js | Original sources exist -
[task 2019-11-22T14:27:31.634Z] 14:27:31 INFO - Buffered messages logged at 14:27:31
[task 2019-11-22T14:27:31.634Z] 14:27:31 INFO - Waiting for state change: selected source
[task 2019-11-22T14:27:31.634Z] 14:27:31 INFO - Finished waiting for state change: selected source
[task 2019-11-22T14:27:31.634Z] 14:27:31 INFO - TEST-PASS | devtools/client/debugger/test/mochitest/browser_dbg-wasm-sourcemaps.js | a new breakpoint was created -
[task 2019-11-22T14:27:31.634Z] 14:27:31 INFO - reload.
[task 2019-11-22T14:27:31.634Z] 14:27:31 INFO - Waiting for state change: paused
[task 2019-11-22T14:27:31.634Z] 14:27:31 INFO - Finished waiting for state change: paused
[task 2019-11-22T14:27:31.634Z] 14:27:31 INFO - Finished waiting for state change: paused
[task 2019-11-22T14:27:31.635Z] 14:27:31 INFO - Waiting for state change: fetched frames
[task 2019-11-22T14:27:31.635Z] 14:27:31 INFO - Finished waiting for state change: fetched frames
[task 2019-11-22T14:27:31.635Z] 14:27:31 INFO - Waiting for state change: selected source
[task 2019-11-22T14:27:31.635Z] 14:27:31 INFO - Finished waiting for state change: selected source
[task 2019-11-22T14:27:31.635Z] 14:27:31 INFO - Buffered messages finished
[task 2019-11-22T14:27:31.635Z] 14:27:31 INFO - TEST-UNEXPECTED-FAIL | devtools/client/debugger/test/mochitest/browser_dbg-wasm-sourcemaps.js | Uncaught exception - at chrome://mochitests/content/browser/devtools/client/debugger/test/mochitest/helpers.js:651 - Error: Unable to find source: fib.c
[task 2019-11-22T14:27:31.635Z] 14:27:31 INFO - Stack trace:
[task 2019-11-22T14:27:31.635Z] 14:27:31 INFO - findSource@chrome://mochitests/content/browser/devtools/client/debugger/test/mochitest/helpers.js:651:11
[task 2019-11-22T14:27:31.635Z] 14:27:31 INFO - selectSource@chrome://mochitests/content/browser/devtools/client/debugger/test/mochitest/helpers.js:724:28
[task 2019-11-22T14:27:31.635Z] 14:27:31 INFO - @chrome://mochitests/content/browser/devtools/client/debugger/test/mochitest/browser_dbg-wasm-sourcemaps.js:32:9
[task 2019-11-22T14:27:31.636Z] 14:27:31 INFO - Leaving test bound
[task 2019-11-22T14:27:31.636Z] 14:27:31 INFO - GECKO(2056) | [ACTION] ADD_SOURCES - {"type":"ADD_SOURCES","cx":{"navigateCounter":2},"sources":[{"id":"sourceURL-wasm:http://example.com/browser/devtools/client/debugger/test/mochitest/examples/doc-wasm-sourcemaps.html%20line%2022%20%3E%20WebAssembly.instantiate:079e9ac0b5ceb36a/originalSource-dc767bcc8289d4b243729308e8a60342","url":"wasm-src:///fib.c","relativeUrl":"wasm-src:///fib.c","isPrettyPrinted":false,"isWasm":false,"isBlackBoxed":false,"introductionUrl":null,"isExtension":false,"extensionName":null}]}
[task 2019-11-22T14:27:31.636Z] 14:27:31 INFO - GECKO(2056) | [ACTION] LOAD_SOURCE_TEXT [start] - {"type":"LOAD_SOURCE_TEXT","sourceId":"sourceURL-wasm:http://example.com/browser/devtools/client/debugger/test/mochitest/examples/doc-wasm-sourcemaps.html%20line%2022%20%3E%20WebAssembly.instantiate:079e9ac0b5ceb36a/originalSource-dc767bcc8289d4b243729308e8a60342","epoch":3,"seqId":"15","status":"start"}
[task 2019-11-22T14:27:31.636Z] 14:27:31 INFO - GECKO(2056) | [ACTION] SET_EXPANDED_STATE - {"type":"SET_EXPANDED_STATE","expanded":{}}
[task 2019-11-22T14:27:31.636Z] 14:27:31 INFO - GECKO(2056) | [ACTION] SET_EXPANDED_STATE - {"type":"SET_EXPANDED_STATE","expanded":{}}
[task 2019-11-22T14:27:31.636Z] 14:27:31 INFO - GECKO(2056) | [ACTION] LOAD_SOURCE_TEXT [start] - {"type":"LOAD_SOURCE_TEXT","sourceId":"sourceURL-wasm:http://example.com/browser/devtools/client/debugger/test/mochitest/examples/doc-wasm-sourcemaps.html%20line%2022%20%3E%20WebAssembly.instantiate:079e9ac0b5ceb36a","epoch":3,"seqId":"16","status":"start"}
[task 2019-11-22T14:27:31.636Z] 14:27:31 INFO - GECKO(2056) | [ACTION] SET_VIEWPORT - {"type":"SET_VIEWPORT","viewport":{"start":{"line":1,"column":0},"end":{"line":52,"column":205}}}
[task 2019-11-22T14:27:32.409Z] 14:27:32 INFO - GECKO(2056) | console.warn: "sourceContents failed: Error: Connection closed, pending request to server0.conn114.child1/source39, type source failed\n\nRequest stack:\nrequest@resource://devtools/shared/protocol/Front.js:214:14\ngenerateRequestMethods/</frontProto[name]@resource://devtools/shared/protocol/Front/FrontClassWithSpec.js:49:19\nsource@resource://devtools/shared/fronts/source.js:60:34\nsourceContents@resource://devtools/client/debugger/src/client/firefox/commands.js:150:25\nloadSource@resource://devtools/client/debugger/src/actions/sources/loadSourceText.js:82:31\nloadSourceTextPromise@resource://devtools/client/debugger/src/actions/sources/loadSourceText.js:108:25\naction@resource://devtools/client/debugger/src/actions/sources/loadSourceText.js:190:41\nmemoizeableAction/</</<@resource://devtools/client/debugger/src/utils/memoizableAction.js:50:19\nmemoizeableAction/</<@resource://devtools/client/debugger/src/utils/memoizableAction.js:56:11\nthunk/</</<@resource://devtools/client/debugger/src/actions/utils/middleware/thunk.js:29:45\ndispatch@resource://devtools/client/shared/vendor/redux.js:755:18\ncheckPendingBreakpoints/<@resource://devtools/client/debugger/src/actions/sources/newSources.js:189:11\nthunk/</</<@resource://devtools/client/debugger/src/actions/utils/middleware/thunk.js:29:45\ndispatch@resource://devtools/client/shared/vendor/redux.js:755:18\nnewGeneratedSources/</<@resource://devtools/client/debugger/src/actions/sources/newSources.js:384:17\n"
[task 2019-11-22T14:27:32.409Z] 14:27:32 INFO - Removing tab.
[task 2019-11-22T14:27:32.409Z] 14:27:32 INFO - Waiting for event: 'TabClose' on [object XULElement].
[task 2019-11-22T14:27:32.409Z] 14:27:32 INFO - Got event: 'TabClose' on [object XULElement].
[task 2019-11-22T14:27:32.419Z] 14:27:32 INFO - Tab removed and finished closing
[task 2019-11-22T14:27:32.449Z] 14:27:32 INFO - TEST-PASS | devtools/client/debugger/test/mochitest/browser_dbg-wasm-sourcemaps.js | The main process DebuggerServer has no pending connection when the test ends -
[task 2019-11-22T14:27:32.499Z] 14:27:32 INFO - GECKO(2056) | MEMORY STAT | vsize 2111427MB | vsizeMaxContiguous 65226066MB | residentFast 803MB | heapAllocated 226MB
[task 2019-11-22T14:27:32.499Z] 14:27:32 INFO - TEST-OK | devtools/client/debugger/test/mochitest/browser_dbg-wasm-sourcemaps.js | took 2667ms
[task 2019-11-22T14:27:32.509Z] 14:27:32 INFO - checking window state
[task 2019-11-22T14:27:32.519Z] 14:27:32 INFO - TEST-START | devtools/client/debugger/test/mochitest/browser_dbg-watchpoints.js

Jason,
This looks to be caused by the changes in Bug 1594861, could you please take a look at it?

Hi Brian,
can you please look at this test failure?

It's easily reproducible on my machine. I am seeing the following error:

Unexpected Results
------------------
devtools/client/debugger/test/mochitest/browser_dbg-wasm-sourcemaps.js
  FAIL Uncaught exception - at chrome://mochitests/content/browser/devtools/client/debugger/test/mochitest/helpers.js:651 - Error: Unable to find source: fib.c
Stack trace:
findSource@chrome://mochitests/content/browser/devtools/client/debugger/test/mochitest/helpers.js:651:11
selectSource@chrome://mochitests/content/browser/devtools/client/debugger/test/mochitest/helpers.js:724:28
@chrome://mochitests/content/browser/devtools/client/debugger/test/mochitest/browser_dbg-wasm-sourcemaps.js:32:9
Async*Tester_execTest/<@chrome://mochikit/content/browser-test.js:1069:34
Tester_execTest@chrome://mochikit/content/browser-test.js:1104:11
nextTest/<@chrome://mochikit/content/browser-test.js:932:14
SimpleTest.waitForFocus/waitForFocusInner/focusedOrLoaded/<@chrome://mochikit/content/tests/SimpleTest/SimpleTest.js:805:67

It looks like racing issue. Perhaps the test isn't waiting for the source in the Sources tree?

The following wait seems to help, which indicates that ...

--- a/devtools/client/debugger/test/mochitest/browser_dbg-wasm-sourcemaps.js
+++ b/devtools/client/debugger/test/mochitest/browser_dbg-wasm-sourcemaps.js
@@ -24,16 +24,19 @@ add_task(async function() {
   await addBreakpoint(dbg, "fib.c", 10);

   info("reload.");
   reload(dbg);

   // The same debugger statement as above, but using at for
   // workaround to break at original source (see below) and not generated.
   await waitForPaused(dbg);
+
+  // Waiting for the source seems to help.
+  await waitForTime(1000);
   await selectSource(dbg, "fib.c");

   info("resume");
   resume(dbg);
   await waitForPaused(dbg, "fib.c");

   const frames = findAllElements(dbg, "frames");
   const firstFrameTitle = frames[0].querySelector(".title").textContent;

Thanks!
Honza

Does it pass with javascript.options.wasm_gc set to false? https://searchfox.org/mozilla-central/rev/cce8b90aece0f42e5025e45282de16066eeaa662/modules/libpref/init/all.js#1072

It doesn't fail on every platform but on Windows DevEdition and beta permanently and at least on Linux DevEdition intermittently.