neon_local: background_process: launch all processes in repo dir (or datadir) (#8058)

Before this PR, storage controller and broker would run in the
PWD of neon_local, i.e., most likely the checkout of neon.git.

With this PR, the shared infrastructure for background processes
sets the PWD.

Benefits:
* easy listing of processes in a repo dir using `lsof`, see added
  comment in the code
* coredumps go in the right directory (next to the process)
* generally matching common expectations, I think

Changes:
* set the working directory in `background_process` module
* drive-by: fix reliance of storage_controller on NEON_REPO_DIR being
set by neon_local for the local compute hook to work correctly

storage_controller/src/compute_hook.rs

@@ -283,7 +283,13 @@ impl ComputeHook {
         // all calls to this function
         let _locked = self.neon_local_lock.lock().await;
 
-        let env = match LocalEnv::load_config() {
+        let Some(repo_dir) = self.config.neon_local_repo_dir.as_deref() else {
+            tracing::warn!(
+                "neon_local_repo_dir not set, likely a bug in neon_local; skipping compute update"
+            );
+            return Ok(());
+        };
+        let env = match LocalEnv::load_config(repo_dir) {
             Ok(e) => e,
             Err(e) => {
                 tracing::warn!("Couldn't load neon_local config, skipping compute update ({e})");

storage_controller/src/main.rs

@@ -4,6 +4,7 @@ use clap::Parser;
 use diesel::Connection;
 use metrics::launch_timestamp::LaunchTimestamp;
 use metrics::BuildInfo;
+use std::path::PathBuf;
 use std::sync::Arc;
 use storage_controller::http::make_router;
 use storage_controller::metrics::preinitialize_metrics;
@@ -77,6 +78,12 @@ struct Cli {
     /// How long to wait for the initial database connection to be available.
     #[arg(long, default_value = "5s")]
     db_connect_timeout: humantime::Duration,
+
+    /// `neon_local` sets this to the path of the neon_local repo dir.
+    /// Only relevant for testing.
+    // TODO: make `cfg(feature = "testing")`
+    #[arg(long)]
+    neon_local_repo_dir: Option<PathBuf>,
 }
 
 enum StrictMode {
@@ -260,6 +267,7 @@ async fn async_main() -> anyhow::Result<()> {
             .reconciler_concurrency
             .unwrap_or(RECONCILER_CONCURRENCY_DEFAULT),
         split_threshold: args.split_threshold,
+        neon_local_repo_dir: args.neon_local_repo_dir,
     };
 
     // After loading secrets & config, but before starting anything else, apply database migrations

storage_controller/src/service.rs

@@ -2,6 +2,7 @@ use std::{
     borrow::Cow,
     cmp::Ordering,
     collections::{BTreeMap, HashMap, HashSet},
+    path::PathBuf,
     str::FromStr,
     sync::Arc,
     time::{Duration, Instant},
@@ -236,6 +237,9 @@ pub struct Config {
     /// How large must a shard grow in bytes before we split it?
     /// None disables auto-splitting.
     pub split_threshold: Option<u64>,
+
+    // TODO: make this cfg(feature  = "testing")
+    pub neon_local_repo_dir: Option<PathBuf>,
 }
 
 impl From<DatabaseError> for ApiError {