Revert "wip: drop the file and keep the early-init logging"

This reverts commit 3b1a1e3c12.

magic-nix-cache/src/api.rs

@@ -57,6 +57,12 @@ async fn workflow_finish(
         }
     }
 
+    // NOTE(cole-h): see `init_logging`
+    let logfile = std::env::temp_dir().join("magic-nix-cache-tracing.log");
+    let logfile_contents = std::fs::read_to_string(logfile)?;
+    println!("Every log line throughout the lifetime of the program:");
+    println!("\n{logfile_contents}\n");
+
     let reply = WorkflowFinishResponse {};
 
     //state.metrics.num_new_paths.set(num_new_paths);

magic-nix-cache/src/main.rs

@@ -447,7 +447,7 @@ async fn main() -> Result<()> {
     }
 }
 
-fn init_logging() -> Result<()> {
+fn init_logging() -> Result<tracing_appender::non_blocking::WorkerGuard> {
     let filter = EnvFilter::try_from_default_env().unwrap_or_else(|_| {
         #[cfg(debug_assertions)]
         return EnvFilter::new("info")
@@ -462,12 +462,23 @@ fn init_logging() -> Result<()> {
         .with_writer(std::io::stderr)
         .pretty();
 
+    let logfile = std::env::temp_dir().join("magic-nix-cache-tracing.log");
+    let file = std::fs::OpenOptions::new()
+        .create(true)
+        .append(true)
+        .open(logfile)?;
+    let (nonblocking, guard) = tracing_appender::non_blocking(file);
+    let file_layer = tracing_subscriber::fmt::layer()
+        .with_writer(nonblocking)
+        .pretty();
+
     tracing_subscriber::registry()
         .with(filter)
         .with(stderr_layer)
+        .with(file_layer)
         .init();
 
-    Ok(())
+    Ok(guard)
 }
 
 #[cfg(debug_assertions)]