chore: Add more information to backend errors

tooling/backend_interface/src/cli/contract.rs

@@ -2,6 +2,8 @@ use std::path::{Path, PathBuf};
 
 use crate::BackendError;
 
+use super::string_from_stderr;
+
 /// VerifyCommand will call the barretenberg binary
 /// to return a solidity library with the verification key
 /// that can be used to verify proofs on-chain.
@@ -31,9 +33,9 @@ impl ContractCommand {
 
         if output.status.success() {
             String::from_utf8(output.stdout)
-                .map_err(|error| BackendError::MalformedResponse(error.into_bytes()))
+                .map_err(|error| BackendError::InvalidUTF8Vector(error.into_bytes()))
         } else {
-            Err(BackendError::CommandFailed(output.stderr))
+            Err(BackendError::CommandFailed(string_from_stderr(&output.stderr)))
         }
     }
 }

tooling/backend_interface/src/cli/gates.rs

@@ -2,6 +2,8 @@ use std::path::{Path, PathBuf};
 
 use crate::BackendError;
 
+use super::string_from_stderr;
+
 /// GatesCommand will call the barretenberg binary
 /// to return the number of gates needed to create a proof
 /// for the given bytecode.
@@ -21,13 +23,16 @@ impl GatesCommand {
             .output()?;
 
         if !output.status.success() {
-            return Err(BackendError::CommandFailed(output.stderr));
+            return Err(BackendError::CommandFailed(string_from_stderr(&output.stderr)));
         }
         // Note: barretenberg includes the newline, so that subsequent prints to stdout
         // are not on the same line as the gates output.
 
-        let gates_bytes: [u8; 8] =
-            output.stdout.try_into().map_err(BackendError::MalformedResponse)?;
+        const EXPECTED_BYTES: usize = 8;
+        let gates_bytes: [u8; EXPECTED_BYTES] =
+            output.stdout.as_slice().try_into().map_err(|_| {
+                BackendError::UnexpectedNumberOfBytes(EXPECTED_BYTES, output.stdout.clone())
+            })?;
 
         // Convert bytes to u64 in little-endian format
         let value = u64::from_le_bytes(gates_bytes);

tooling/backend_interface/src/cli/info.rs

@@ -5,6 +5,8 @@ use std::path::{Path, PathBuf};
 
 use crate::{BackendError, BackendOpcodeSupport};
 
+use super::string_from_stderr;
+
 pub(crate) struct InfoCommand {
     pub(crate) crs_path: PathBuf,
 }
@@ -43,7 +45,7 @@ impl InfoCommand {
         let output = command.output()?;
 
         if !output.status.success() {
-            return Err(BackendError::CommandFailed(output.stderr));
+            return Err(BackendError::CommandFailed(string_from_stderr(&output.stderr)));
         }
 
         let backend_info: InfoResponse =

tooling/backend_interface/src/cli/mod.rs

@@ -22,9 +22,14 @@ fn no_command_provided_works() -> Result<(), crate::BackendError> {
 
     let output = std::process::Command::new(backend.binary_path()).output()?;
 
-    let stderr = String::from_utf8_lossy(&output.stderr);
+    let stderr = string_from_stderr(&output.stderr);
     // Assert help message is printed due to no command being provided.
     assert!(stderr.contains("Usage: mock_backend <COMMAND>"));
 
     Ok(())
 }
+
+// Converts a stderr byte array to a string (including invalid characters)
+fn string_from_stderr(stderr: &[u8]) -> String {
+    String::from_utf8_lossy(stderr).to_string()
+}

tooling/backend_interface/src/cli/prove.rs

@@ -2,6 +2,8 @@ use std::path::{Path, PathBuf};
 
 use crate::BackendError;
 
+use super::string_from_stderr;
+
 /// ProveCommand will call the barretenberg binary
 /// to create a proof, given the witness and the bytecode.
 ///
@@ -39,7 +41,7 @@ impl ProveCommand {
         if output.status.success() {
             Ok(output.stdout)
         } else {
-            Err(BackendError::CommandFailed(output.stderr))
+            Err(BackendError::CommandFailed(string_from_stderr(&output.stderr)))
         }
     }
 }

tooling/backend_interface/src/cli/write_vk.rs

@@ -1,5 +1,6 @@
 use std::path::{Path, PathBuf};
 
+use super::string_from_stderr;
 use crate::BackendError;
 
 /// WriteCommand will call the barretenberg binary
@@ -32,7 +33,7 @@ impl WriteVkCommand {
         if output.status.success() {
             Ok(())
         } else {
-            Err(BackendError::CommandFailed(output.stderr))
+            Err(BackendError::CommandFailed(string_from_stderr(&output.stderr)))
         }
     }
 }

tooling/backend_interface/src/lib.rs

@@ -40,11 +40,16 @@ pub enum BackendError {
     #[error("Backend binary does not exist")]
     MissingBinary,
 
-    #[error("The backend responded with malformed data: {0:?}")]
-    MalformedResponse(Vec<u8>),
+    #[error("The backend responded with a malformed UTF8 byte vector: {0:?}")]
+    InvalidUTF8Vector(Vec<u8>),
 
-    #[error("The backend encountered an error")]
-    CommandFailed(Vec<u8>),
+    #[error(
+        "The backend responded with a unexpected number of bytes. Expected: {0} but got {} ({1:?})", .1.len()
+    )]
+    UnexpectedNumberOfBytes(usize, Vec<u8>),
+
+    #[error("The backend encountered an error: {0:?}")]
+    CommandFailed(String),
 }
 
 #[derive(Debug)]