Check if nixtract produced any output

This avoids a problem where `genealogos` would produce an empty sbom.

We check for both the status code and the output. The latter should not be needed, but `nixtract` does not yet produce non-zero status codes.

genealogos/src/backend/nixtract.rs

@@ -116,6 +116,18 @@ impl crate::backend::BackendTrait for Nixtract {
         let output = command.output()?;
         let stdout = String::from_utf8_lossy(&output.stdout);
 
+        // Check if the command was successful
+        if !output.status.success() {
+            return Err(crate::Error::NixtractCommand(format!(
+                "nixtract exited with status code {}",
+                output.status
+            )));
+        } else if output.stdout.is_empty() {
+            return Err(crate::Error::NixtractCommand(
+                "nixtract produced no output".to_string(),
+            ));
+        }
+
         // Split stdout into lines
         let lines = stdout.lines();
 

genealogos/src/error.rs

@@ -9,7 +9,7 @@ pub enum Error {
     #[error("Errors constructing CycloneDX output: {0}")]
     CycloneDX(#[source] Box<dyn std::error::Error + Send + Sync + 'static>),
 
-    #[error("Nixtract exited with a non-zero exit code: {0}")]
+    #[error("Nixtract failed: {0}")]
     NixtractCommand(String),
 
     #[error("Genealogos encountered an IO error: {0}")]