Convert the Io error type to have a string of context

magic-nix-cache/src/api.rs

@@ -112,7 +112,8 @@ async fn workflow_finish(
 
     // NOTE(cole-h): see `init_logging`
     if let Some(logfile) = &state.logfile {
-        let logfile_contents = std::fs::read_to_string(logfile)?;
+        let logfile_contents = std::fs::read_to_string(logfile)
+            .map_err(|e| crate::error::Error::Io(e, format!("Reading {}", logfile.display())))?;
         println!("Every log line throughout the lifetime of the program:");
         println!("\n{logfile_contents}\n");
     }

magic-nix-cache/src/error.rs

@@ -19,8 +19,8 @@ pub enum Error {
     #[error("Bad Request")]
     BadRequest,
 
-    #[error("I/O error: {0}")]
+    #[error("I/O error: {0}. Context: {1}")]
-    Io(#[from] std::io::Error),
+    Io(std::io::Error, String),
 
     #[error("GHA cache is disabled")]
     GHADisabled,

magic-nix-cache/src/util.rs

@@ -11,9 +11,19 @@ use crate::error::Result;
 pub async fn get_store_paths(store: &NixStore) -> Result<HashSet<PathBuf>> {
     // FIXME: use the Nix API.
     let store_dir = store.store_dir();
-    let mut listing = tokio::fs::read_dir(store_dir).await?;
+    let mut listing = tokio::fs::read_dir(store_dir).await.map_err(|e| {
+        crate::error::Error::Io(
+            e,
+            format!("Enumerating store paths in {}", store_dir.display()),
+        )
+    })?;
     let mut paths = HashSet::new();
-    while let Some(entry) = listing.next_entry().await? {
+    while let Some(entry) = listing.next_entry().await.map_err(|e| {
+        crate::error::Error::Io(
+            e,
+            format!("Reading existing store paths from {}", store_dir.display()),
+        )
+    })? {
         let file_name = entry.file_name();
         let file_name = Path::new(&file_name);