fix: return error when bgzf_open fails to open a file (#444)

* Before `rust_htslib::bgzf:Reader` and `rust_htslib::bgzf:Writer`
    would never fail when opening a file that could not be opened
    and it would allow writing to `rust_htslib::bgzf:Writer` without
    returning any error messages.

Co-authored-by: Johannes Köster <johannes.koester@uni-due.de>

src/bgzf/mod.rs

@@ -87,7 +87,13 @@ impl Reader {
         let mode = ffi::CString::new("r").unwrap();
         let cpath = ffi::CString::new(path).unwrap();
         let inner = unsafe { htslib::bgzf_open(cpath.as_ptr(), mode.as_ptr()) };
-        Ok(Self { inner })
+        if inner != std::ptr::null_mut() {
+            Ok(Self { inner })
+        } else {
+            Err(Error::FileOpen {
+                path: String::from_utf8(path.to_vec()).unwrap(),
+            })
+        }
     }
 
     /// Set the thread pool to use for parallel decompression.
@@ -211,7 +217,13 @@ impl Writer {
         let mode = Self::get_open_mode(level)?;
         let cpath = ffi::CString::new(path).unwrap();
         let inner = unsafe { htslib::bgzf_open(cpath.as_ptr(), mode.as_ptr()) };
-        Ok(Self { inner, tpool: None })
+        if inner != std::ptr::null_mut() {
+            Ok(Self { inner, tpool: None })
+        } else {
+            Err(Error::FileOpen {
+                path: String::from_utf8(path.to_vec()).unwrap(),
+            })
+        }
     }
 
     /// Internal function to convert compression level to "mode"

src/errors.rs

@@ -10,6 +10,8 @@ pub enum Error {
     // General errors
     #[error("file not found: {path}")]
     FileNotFound { path: PathBuf },
+    #[error("file could not be opened: {path}")]
+    FileOpen { path: String },
     #[error("invalid (non-unicode) characters in path")]
     NonUnicodePath,
     #[error("failed to fetch region")]