Tiny build-time optimization for config handling (#2526)

This change reworks build_notifications from re-loading the task config
from an environment variable, and then parsing as toml, once for every
task. Instead, we now just get and parse the config once, and grab what
we want from the pre-parsed table.

This was noticed incidentally while debugging #2524

Author: jamesmunns

build/util/src/lib.rs

@@ -198,20 +198,31 @@ pub fn task_extern_regions<T: DeserializeOwned>() -> Result<IndexMap<String, T>>
     Ok(t)
 }
 
+/// Pulls the entire config for all tasks
+pub fn all_tasks_full_config<T: DeserializeOwned>()
+-> Result<IndexMap<String, toml_task::Task<T>>> {
+    toml_from_env::<IndexMap<String, toml_task::Task<_>>>(
+        "HUBRIS_ALL_TASK_CONFIGS",
+    )?
+    .ok_or_else(|| anyhow!("HUBRIS_ALL_TASK_CONFIGS is not defined"))
+}
+
 /// Pulls the full task configuration block of a different task
 pub fn other_task_full_config<T: DeserializeOwned>(
     name: &str,
 ) -> Result<toml_task::Task<T>> {
-    let mut t = toml_from_env::<IndexMap<String, toml_task::Task<_>>>(
-        "HUBRIS_ALL_TASK_CONFIGS",
-    )?
-    .ok_or_else(|| anyhow!("HUBRIS_ALL_TASK_CONFIGS is not defined"))?;
+    let mut t = all_tasks_full_config::<T>()?;
     let out = t
         .remove(name)
         .ok_or_else(|| anyhow!("Could not find {name} in tasks"))?;
     Ok(out)
 }
 
+pub fn all_tasks_full_config_toml()
+-> Result<IndexMap<String, toml_task::Task<ordered_toml::Value>>> {
+    all_tasks_full_config()
+}
+
 pub fn other_task_full_config_toml(
     name: &str,
 ) -> Result<toml_task::Task<ordered_toml::Value>> {
@@ -325,11 +336,15 @@ pub fn build_notifications() -> Result<()> {
 
     write_task_notifications(&mut out, &full_task_config.notifications)?;
 
-    for task in env_var("HUBRIS_TASKS")
-        .expect("missing HUBRIS_TASKS")
-        .split(',')
-    {
-        let full_task_config = other_task_full_config_toml(task)?;
+    // Rather than re-parse the toml on every task iter, we just get the full
+    // task toml, and pluck out each task as we need it.
+    let all_task_configs = all_tasks_full_config_toml()?;
+    let all_tasks = env_var("HUBRIS_TASKS").expect("missing HUBRIS_TASKS");
+
+    for task in all_tasks.split(',') {
+        let full_task_config = all_task_configs
+            .get(task)
+            .ok_or_else(|| anyhow!("Could not find {task} in tasks"))?;
         writeln!(&mut out, "pub mod {task} {{")?;
         write_task_notifications(&mut out, &full_task_config.notifications)?;
         writeln!(&mut out, "}}")?;