[BRC-1425] Plumb through and set the requisite GUCs when starting the compute instance (#12732)

## Problem

We need the set the following Postgres GUCs to the correct value before
starting Postgres in the compute instance:

```
databricks.workspace_url
databricks.enable_databricks_identity_login
databricks.enable_sql_restrictions
```

## Summary of changes

Plumbed through `workspace_url` and other GUC settings via
`DatabricksSettings` in `ComputeSpec`. The spec is sent to the compute
instance when it starts up and the GUCs are written to `postgresql.conf`
before the postgres process is launched.

---------

Co-authored-by: Jarupat Jisarojito <jarupat.jisarojito@databricks.com>
Co-authored-by: William Huang <william.huang@databricks.com>

compute_tools/src/compute.rs

@@ -413,6 +413,52 @@ struct StartVmMonitorResult {
     vm_monitor: Option<JoinHandle<Result<()>>>,
 }
 
+/// Databricks-specific environment variables to be passed to the `postgres` sub-process.
+pub struct DatabricksEnvVars {
+    /// The Databricks "endpoint ID" of the compute instance. Used by `postgres` to check
+    /// the token scopes of internal auth tokens.
+    pub endpoint_id: String,
+    /// Hostname of the Databricks workspace URL this compute instance belongs to.
+    /// Used by postgres to verify Databricks PAT tokens.
+    pub workspace_host: String,
+}
+
+impl DatabricksEnvVars {
+    pub fn new(compute_spec: &ComputeSpec, compute_id: Option<&String>) -> Self {
+        // compute_id is a string format of "{endpoint_id}/{compute_idx}"
+        // endpoint_id is a uuid. We only need to pass down endpoint_id to postgres.
+        // Panics if compute_id is not set or not in the expected format.
+        let endpoint_id = compute_id.unwrap().split('/').next().unwrap().to_string();
+        let workspace_host = compute_spec
+            .databricks_settings
+            .as_ref()
+            .map(|s| s.databricks_workspace_host.clone())
+            .unwrap_or("".to_string());
+        Self {
+            endpoint_id,
+            workspace_host,
+        }
+    }
+
+    /// Constants for the names of Databricks-specific postgres environment variables.
+    const DATABRICKS_ENDPOINT_ID_ENVVAR: &'static str = "DATABRICKS_ENDPOINT_ID";
+    const DATABRICKS_WORKSPACE_HOST_ENVVAR: &'static str = "DATABRICKS_WORKSPACE_HOST";
+
+    /// Convert DatabricksEnvVars to a list of string pairs that can be passed as env vars. Consumes `self`.
+    pub fn to_env_var_list(self) -> Vec<(String, String)> {
+        vec![
+            (
+                Self::DATABRICKS_ENDPOINT_ID_ENVVAR.to_string(),
+                self.endpoint_id.clone(),
+            ),
+            (
+                Self::DATABRICKS_WORKSPACE_HOST_ENVVAR.to_string(),
+                self.workspace_host.clone(),
+            ),
+        ]
+    }
+}
+
 impl ComputeNode {
     pub fn new(params: ComputeNodeParams, config: ComputeConfig) -> Result<Self> {
         let connstr = params.connstr.as_str();
@@ -1581,14 +1627,31 @@ impl ComputeNode {
     pub fn start_postgres(&self, storage_auth_token: Option<String>) -> Result<PostgresHandle> {
         let pgdata_path = Path::new(&self.params.pgdata);
 
+        let env_vars: Vec<(String, String)> = if self.params.lakebase_mode {
+            let databricks_env_vars = {
+                let state = self.state.lock().unwrap();
+                let spec = &state.pspec.as_ref().unwrap().spec;
+                DatabricksEnvVars::new(spec, Some(&self.params.compute_id))
+            };
+
+            info!(
+                "Starting Postgres for databricks endpoint id: {}",
+                &databricks_env_vars.endpoint_id
+            );
+
+            let mut env_vars = databricks_env_vars.to_env_var_list();
+            env_vars.extend(storage_auth_token.map(|t| ("NEON_AUTH_TOKEN".to_string(), t)));
+            env_vars
+        } else if let Some(storage_auth_token) = &storage_auth_token {
+            vec![("NEON_AUTH_TOKEN".to_owned(), storage_auth_token.to_owned())]
+        } else {
+            vec![]
+        };
+
         // Run postgres as a child process.
         let mut pg = maybe_cgexec(&self.params.pgbin)
             .args(["-D", &self.params.pgdata])
-            .envs(if let Some(storage_auth_token) = &storage_auth_token {
-                vec![("NEON_AUTH_TOKEN", storage_auth_token)]
-            } else {
-                vec![]
-            })
+            .envs(env_vars)
             .stderr(Stdio::piped())
             .spawn()
             .expect("cannot start postgres process");