Closes #235 - adds a new error type (#398)

error message suggests possible causes

Addressed code review 1 thread + smaller heap size

src/error.rs

@@ -9,6 +9,7 @@ use schema;
 use serde_json;
 use std::path::PathBuf;
 use std::sync::PoisonError;
+use core::LOCKFILE_FILEPATH;
 
 /// The library's failure based error enum
 #[derive(Debug, Fail)]
@@ -19,6 +20,9 @@ pub enum TantivyError {
     /// File already exists, this is a problem when we try to write into a new file.
     #[fail(display = "file already exists: '{:?}'", _0)]
     FileAlreadyExists(PathBuf),
+    /// Lockfile already exists
+    #[fail(display = "Lockfile '{:?}' already exists. Possible causes: another IndexWriter instance or panic during previous lock drop.", _0)]
+    LockFileAlreadyExists(PathBuf),
     /// IO Error.
     #[fail(display = "an IO error occurred: '{}'", _0)]
     IOError(#[cause] IOError),
@@ -95,7 +99,12 @@ impl From<OpenWriteError> for TantivyError {
     fn from(error: OpenWriteError) -> TantivyError {
         match error {
             OpenWriteError::FileAlreadyExists(filepath) => {
-                TantivyError::FileAlreadyExists(filepath)
+                let lockfile_fname = LOCKFILE_FILEPATH.to_str().unwrap();
+                if filepath.ends_with(lockfile_fname) {
+                    TantivyError::LockFileAlreadyExists(filepath)
+                } else {
+                    TantivyError::FileAlreadyExists(filepath)
+                }
             }
             OpenWriteError::IOError(io_error) => TantivyError::IOError(io_error),
         }.into()

src/indexer/index_writer.rs

@@ -657,11 +657,27 @@ mod tests {
         let index = Index::create_in_ram(schema_builder.build());
         let _index_writer = index.writer(40_000_000).unwrap();
         match index.writer(40_000_000) {
-            Err(TantivyError::FileAlreadyExists(_)) => {}
+            Err(TantivyError::LockFileAlreadyExists(_)) => {}
             _ => panic!("Expected FileAlreadyExists error"),
         }
     }
 
+    #[test]
+    fn test_lockfile_already_exists_error_msg() {
+        let schema_builder = schema::SchemaBuilder::default();
+        let index = Index::create_in_ram(schema_builder.build());
+        let _index_writer = index.writer_with_num_threads(1, 3_000_000).unwrap();
+        match index.writer_with_num_threads(1, 3_000_000) {
+            Err(err) => {
+                let err_msg = err.to_string();
+                assert!(err_msg.contains("Lockfile"));
+                assert!(err_msg.contains("already exists"));
+                assert!(err_msg.contains("Possible causes:"))
+            },
+            _ => panic!("Expected LockfileAlreadyExists error"),
+        }
+    }
+
     #[test]
     fn test_set_merge_policy() {
         let schema_builder = schema::SchemaBuilder::default();