improve readme and error messages

README.md

@@ -8,6 +8,28 @@ It can be used to simplify snapshot handling in an automated pipeline.
 
 ## Usage
 
+### Credentials
+
+Coldsnap uses the same credential mechanisms as the `aws cli`.
+For example, if you have credentials in `~/.aws/credentials`, these will be used.
+You can specify the name of the profile to be used by adding `--profile profile-name`.
+
+You can also define environment variables, for example:
+
+```
+$ export AWS_ACCESS_KEY_ID=EXAMPLEAKIAIOSFODNN7
+$ export AWS_SECRET_ACCESS_KEY=EXAMPLEKEYwJalrXUtnFEMI/K7MDENG/bPxRfiCY
+$ export AWS_DEFAULT_REGION=us-west-2
+```
+
+If the name of a profile is provided, then it will be used.
+If not, then the default behavior of the AWS Rust SDK credential provider will be used.
+[Here] is the description of the default behavior.
+
+[Here]: https://docs.rs/aws-config/latest/aws_config/default_provider/credentials/struct.DefaultCredentialsChain.html
+
+### Upload
+
 Upload a local file into an EBS snapshot:
 
 ```
@@ -26,6 +48,8 @@ Alternately, you can use `coldsnap wait`, which offers more flexibility in terms
 $ coldsnap wait snap-1234
 ```
 
+### Download
+
 Download an EBS snapshot into a local file:
 
 ```

src/download.rs

@@ -619,7 +619,7 @@ mod error {
         #[snafu(display("Missing temporary file"))]
         MissingTempFile {},
 
-        #[snafu(display("Failed to list snapshot blocks '{}': {}", snapshot_id, source))]
+        #[snafu(display("Failed to list snapshot blocks '{snapshot_id}': {source}", source = crate::error_stack(source, 2)))]
         ListSnapshotBlocks {
             snapshot_id: String,
             source: aws_sdk_ebs::error::SdkError<ListSnapshotBlocksError>,

src/lib.rs

@@ -69,3 +69,21 @@ pub use upload::SnapshotUploader;
 
 pub use wait::Error as WaitError;
 pub use wait::{SnapshotWaiter, WaitParams};
+
+/// Errors from the AWS Rust SDK crate often swallow relevant information when they are printed
+/// using `Display`. This leads to errors that do not have enough information for the user to know
+/// what went wrong. This function `Display` prints an error and recursively adds up to n levels of
+/// underlying errors to that printed message.
+pub(crate) fn error_stack(e: &dyn std::error::Error, n: u16) -> String {
+    let mut current_error = e;
+    let mut s = format!("{}", e);
+
+    for _ in 0..n {
+        current_error = match current_error.source() {
+            None => return s,
+            Some(next_error) => next_error,
+        };
+        s += &format!(": {}", current_error)
+    }
+    s
+}

src/upload.rs

@@ -470,7 +470,7 @@ mod error {
             source: std::io::Error,
         },
 
-        #[snafu(display("Failed to start snapshot: {}", source))]
+        #[snafu(display("Failed to start snapshot: {source}", source = crate::error_stack(&source, 2)))]
         StartSnapshot {
             source: aws_sdk_ebs::error::SdkError<StartSnapshotError>,
         },