Bug 1643395 - Allow configuring FOG server to localhost via pref r=janerik

Differential Revision: https://phabricator.services.mozilla.com/D78347

modules/libpref/init/StaticPrefList.yaml

@@ -8751,6 +8751,12 @@
   value: 300000
   mirror: always
 
+- name: telemetry.fog.test.localhost_port
+  type: RelaxedAtomicUint32
+  value: 0
+  mirror: always
+  rust: true
+
 #---------------------------------------------------------------------------
 # Prefs starting with "test."
 #---------------------------------------------------------------------------

toolkit/components/glean/docs/preferences.md

@@ -1,7 +1,15 @@
 # Preferences
 
+## Test-only Preferences
+
 `telemetry.fog.temporary_and_just_for_testing.data_path`
 
 This controls the data path Glean is initialized with (`nightly` only).
 It is intended for manual testing only. It will be removed once FOG gets a fixed data path.
 By default it is not set. Change requires restart.
+
+`telemetry.fog.test.localhost_port`
+
+If set to a value `port` which is greater than 0, pings will be sent to
+`http://localhost:port` instead of `https://incoming.telemetry.mozilla.org`.
+Defaults to 0.

toolkit/components/glean/docs/testing.md

@@ -3,6 +3,12 @@
 Given the multiple API languages, processes, and dependencies,
 testing FOG is a matter of choosing the right tool for the situation.
 
+## Logging
+
+An often-overlooked first line of testing is "what do the logs say?".
+To turn on logging for FOG, run Firefox with `RUST_LOG="glean,fog"`.
+This will also turn on logging for `glean_core` which is often illuminating.
+
 ## Rust
 
 Not all of our Rust code can be tested in a single fashion, unfortunately.

toolkit/components/glean/src/api.rs

@@ -148,7 +148,14 @@ fn register_uploader() {
         );
         let result: std::result::Result<UploadResult, viaduct::Error> = (move || {
             const SERVER: &str = "https://incoming.telemetry.mozilla.org";
-            let url = Url::parse(SERVER)?.join(&ping_request.path)?;
+            let mut server = String::from(SERVER);
+            let localhost_port = static_prefs::pref!("telemetry.fog.test.localhost_port");
+            if localhost_port > 0 {
+                server = format!("http://localhost:{}", localhost_port);
+            }
+            let url = Url::parse(&server)?.join(&ping_request.path)?;
+            log::info!("FOG Ping uploader uploading to {:?}", url);
+
             let mut req = Request::post(url).body(ping_request.body.clone());
             for (&header_key, header_value) in ping_request.headers.iter() {
                 req = req.header(header_key, header_value)?;