Add explicit error for missing exec.d paths (#387)

heroku · Mar 30, 2022 · 70fcf6a · 70fcf6a
1 parent 9144aba
commit 70fcf6a
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diff --git a/libcnb/CHANGELOG.md b/libcnb/CHANGELOG.md
@@ -5,6 +5,7 @@
 - Allow compilation of libcnb.rs buildpacks on Windows. Please note that this does not imply Windows container support, it's meant to allow running unit tests without cross-compiling. ([#368](https://github.com/Malax/libcnb.rs/pull/368))
 - Expose `runtime::libcnb_runtime_detect`, `runtime::libcnb_runtime_build` and their related types for advanced use-cases. Buildpack authors should not use these. ([#375](https://github.com/Malax/libcnb.rs/pull/375)).
 - Only create layer `env`, `env.build` and `env.launch` directories when environment variables are being set within them ([#385](https://github.com/Malax/libcnb.rs/pull/385)).
+- Add `WriteLayerError::MissingExecDFile` error to ease debugging when an exec.d path is missing ([#387](https://github.com/Malax/libcnb.rs/pull/387)).
 
 ## [0.6.0] 2022-02-28
 

diff --git a/libcnb/src/layer/handling.rs b/libcnb/src/layer/handling.rs
@@ -213,6 +213,9 @@ pub enum WriteLayerError {
 
     #[error("Error while writing layer content metadata TOML: {0}")]
     TomlFileError(#[from] TomlFileError),
+
+    #[error("Cannot find exec.d file for copying: {0}")]
+    MissingExecDFile(PathBuf),
 }
 
 #[derive(Debug)]
@@ -262,7 +265,17 @@ fn write_layer<M: Serialize, P: AsRef<Path>>(
                 fs::create_dir_all(&exec_d_dir)?;
 
                 for (name, path) in exec_d_programs {
-                    fs::copy(path, exec_d_dir.join(name))?;
+                    // We could just try to copy the file here and let the call-site deal with the
+                    // IO errors when the path does not exist. We're using an explicit error variant
+                    // for a missing exec.d binary makes it easier to debug issues with packaging
+                    // since the usage of exec.d binaries often relies on implicit packaging the
+                    // buildpack author might not be aware of.
+                    Some(&path)
+                        .filter(|path| path.exists())
+                        .ok_or_else(|| WriteLayerError::MissingExecDFile(path.clone()))
+                        .and_then(|path| {
+                            fs::copy(path, exec_d_dir.join(name)).map_err(WriteLayerError::IoError)
+                        })?;
                 }
             }
         }
@@ -447,6 +460,42 @@ mod tests {
         );
     }
 
+    #[test]
+    fn write_nonexisting_layer_with_nonexisting_exec_d_path() {
+        let layer_name = layer_name!("foo");
+        let temp_dir = tempdir().unwrap();
+        let layers_dir = temp_dir.path();
+
+        let execd_file = PathBuf::from("/this/path/should/not/exist/exec_d_binary");
+        let write_layer_error = super::write_layer(
+            &layers_dir,
+            &layer_name,
+            &LayerEnv::new(),
+            &LayerContentMetadata {
+                types: Some(LayerTypes {
+                    launch: true,
+                    build: true,
+                    cache: false,
+                }),
+                metadata: GenericMetadata::default(),
+            },
+            ExecDPrograms::Overwrite(HashMap::from([(String::from("foo"), execd_file.clone())])),
+        )
+        .unwrap_err();
+
+        match write_layer_error {
+            WriteLayerError::MissingExecDFile(path) => {
+                assert_eq!(path, execd_file);
+            }
+            other => {
+                panic!(
+                    "Expected WriteLayerError::MissingExecDFile, but got {:?}",
+                    other
+                );
+            }
+        };
+    }
+
     #[test]
     fn write_existing_layer() {
         let layer_name = layer_name!("foo");