Support suspending background thread

Useful for locally testing query timings with sqlx debug logging.

src/build.rs

@@ -52,6 +52,13 @@ impl Display for Toolchain {
 }
 
 pub async fn background_builder(db: Db) -> Result<()> {
+    if concurrent_jobs() == 0 {
+        info!("Suspending background thread since DOES_IT_BUILD_PARALLEL_JOBS=0");
+        loop {
+            tokio::time::sleep(Duration::from_secs(3600)).await;
+        }
+    }
+
     loop {
         if let Err(err) = background_builder_inner(&db).await {
             error!("error in background builder: {err}");
@@ -192,21 +199,12 @@ pub async fn build_every_target_for_toolchain(
         .await
         .wrap_err("failed to get targets")?;
 
-    let concurrent = std::env::var("DOES_IT_BUILD_PARALLEL_JOBS")
-        .map(|jobs| jobs.parse().unwrap())
-        .unwrap_or_else(|_| {
-            std::thread::available_parallelism()
-                .unwrap_or(NonZeroUsize::new(2).unwrap())
-                .get()
-                / 2
-        });
-
     let results = futures::stream::iter(
         targets
             .iter()
             .map(|target| build_single_target(db, nightly, target, mode)),
     )
-    .buffer_unordered(concurrent)
+    .buffer_unordered(concurrent_jobs())
     .collect::<Vec<Result<()>>>()
     .await;
     for result in results {
@@ -354,3 +352,14 @@ async fn build_target(
         rustflags,
     })
 }
+
+fn concurrent_jobs() -> usize {
+    std::env::var("DOES_IT_BUILD_PARALLEL_JOBS")
+        .map(|jobs| jobs.parse().unwrap())
+        .unwrap_or_else(|_| {
+            std::thread::available_parallelism()
+                .unwrap_or(NonZeroUsize::new(2).unwrap())
+                .get()
+                / 2
+        })
+}