Transform backend code from compiletime-injection to dependency-injection

The title might be a bit inaccurate, but I cannot think of something
better.

Before the change
=================

Before this change, we had a compiletime backend for the store. This
means that the actual filesystem operations were compiled into the store
either as real filesystem operations (in a normal debug or release
build) but as a in-memory variant in the 'test' case.
So tests did not hit the filesystem when running.
This gave us us the possibility to run tests concurrently with multiple
stores that did not interfere with eachother.

Problem
=======

This approach worked perfectly well until we started to test not the
store itself but crates that depend on the store implementation.
When running tests in a crate A that depends on the store, the store
itself was compiled with the filesystem-hitting-backend.
This was problematic, as tests could not be implemented without hitting
the filesystem.

After the change
================

After this change, the backend code is injected into the store via
dependency injection (the `Store::new()` function automatically uses the
filesystem-backend).
The store can be created with a the in-memory backend when running tests
now.

Implementation
==============

The implementation of this is rather stupid, despite the big diff in
this commit.

Lets have a look at the `Store` code changes first and then we'll
discuss the `file_abstraction` changes this commit introduces.

libimagstore::fs_abstraction
----------------------------

The filesystem was abstracted via a trait `FileAbstraction` which
contains the essential functions for working with the filesystem.

Two implementations are provided in the code:

    * FSFileAbstraction
    * InMemoryFileAbstraction

whereas the first actually works with the filesystem and the latter
works with an in-memory HashMap that is used as filesystem.

Further, the trait `FileAbstractionInstance` was introduced for
functions which are executed on actual instances of content from the
filesystem, which was previousely tied into the general abstraction
mechanism.

So, the `FileAbstraction` trait is for working with the filesystem, the
`FileAbstractionInstance` trait is for working with instances of content
from the filesystem (speak: actual Files).

The `FileAbstraction` trait requires a function to be implemented that
can be used to create a `FileAbstractionInstance` object from it.

In case of the `FSFileAbstractionInstance`, which is the implementation
of the `FileAbstractionInstance` for the actual filesystem-hitting code,
the underlying resource is managed like with the old code before.
The `InMemoryFileAbstractionInstance` implementation is corrosponding to
the `InMemoryFileAbstraction` implementation - for the in-memory
"filesystem".

The implementation of the `get_file_content()` function had to be
changed to return a `String` rather than a `&mut Read` because of
lifetime issues.
This change is store-internally and the API of the store itself was not
affected.

libimagstore::store::Store changes
----------------------------------

So, first we need to make sure that the store knows the actual backend.
Therefor, the `new()` method was renamed to `new_with_backend()` and got
another parameter: the backend implementation.
A new `new()` function was created for convenience and
backwards-compatibility with the rest of the imag codebase (and also
because nobody wants to pass the backend manually all the time).

As the backend is abstracted via a trait, and the store should not
change its interface, the new `Store` member was introduced inside a
`Box`.

All calls (`remove_file()`, `copy()`, `rename()` and `create_dir_all()`)
were refactored from `FileAbstraction::*` calls into `self.backend.*`
calls.

libimagstore::store::StoreEntry changes
---------------------------------------

The `StoreEntry` type is constructed in the store internally for holding
a `StoreId` object as well as some status and the file abstraction code.

This object is constructed via a `new()` function that got a new
parameter: a `&Box<FileAbstraction>` to the backend abstraction the
store uses.
This backend is now used to create a new `FileAbstractionInstance`
object for the `StoreEntry`.

Also the `StoreEntry::get_entry()` code had to be adapted to the new
`Entry::from_str()` function interface.

This commit message is partially added as comment in the code.

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

libimagstore/src/file_abstraction.rs

@@ -17,112 +17,197 @@
 // Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 //
 
-pub use self::fs::FileAbstraction;
+//! The filesystem abstraction code
+//!
+//! # Problem
+//!
+//! First, we had a compiletime backend for the store. This means that the actual filesystem
+//! operations were compiled into the store either as real filesystem operations (in a normal debug
+//! or release build) but as a in-memory variant in the 'test' case.
+//! So tests did not hit the filesystem when running.
+//! This gave us us the possibility to run tests concurrently with multiple
+//! stores that did not interfere with eachother.
+//!
+//! This approach worked perfectly well until we started to test not the
+//! store itself but crates that depend on the store implementation.
+//! When running tests in a crate that depends on the store, the store
+//! itself was compiled with the filesystem-hitting-backend.
+//! This was problematic, as tests could not be implemented without hitting
+//! the filesystem.
+//!
+//! Hence we implemented this.
+//!
+//! # Implementation
+//!
+//! The filesystem is abstracted via a trait `FileAbstraction` which
+//! contains the essential functions for working with the filesystem.
+//!
+//! Two implementations are provided in the code:
+//!
+//! * FSFileAbstraction
+//! * InMemoryFileAbstraction
+//!
+//! whereas the first actually works with the filesystem and the latter
+//! works with an in-memory HashMap that is used as filesystem.
+//!
+//! Further, the trait `FileAbstractionInstance` was introduced for
+//! functions which are executed on actual instances of content from the
+//! filesystem, which was previousely tied into the general abstraction
+//! mechanism.
+//!
+//! So, the `FileAbstraction` trait is for working with the filesystem, the
+//! `FileAbstractionInstance` trait is for working with instances of content
+//! from the filesystem (speak: actual Files).
+//!
+//! In case of the `FSFileAbstractionInstance`, which is the implementation
+//! of the `FileAbstractionInstance` for the actual filesystem-hitting code,
+//! the underlying resource is managed like with the old code before.
+//! The `InMemoryFileAbstractionInstance` implementation is corrosponding to
+//! the `InMemoryFileAbstraction` implementation - for the in-memory
+//! "filesystem".
+//!
+//! The implementation of the `get_file_content()` function had to be
+//! changed to return a `String` rather than a `&mut Read` because of
+//! lifetime issues.
+//! This change is store-internally and the API of the store itself was not
+//! affected.
+//!
+
+use std::path::PathBuf;
+use std::fmt::Debug;
+
+use error::StoreError as SE;
+
+pub use self::fs::FSFileAbstraction;
+pub use self::fs::FSFileAbstractionInstance;
+pub use self::inmemory::InMemoryFileAbstraction;
+pub use self::inmemory::InMemoryFileAbstractionInstance;
 
 // TODO:
 // This whole thing can be written better with a trait based mechanism that is embedded into the
 // store. However it would mean rewriting most things to be generic which can be a pain in the ass.
 
-#[cfg(test)]
+/// An abstraction trait over filesystem actions
-mod fs {
+pub trait FileAbstraction : Debug {
-    use error::StoreError as SE;
+    fn remove_file(&self, path: &PathBuf) -> Result<(), SE>;
-    use error::StoreErrorKind as SEK;
+    fn copy(&self, from: &PathBuf, to: &PathBuf) -> Result<(), SE>;
-    use std::io::Cursor;
+    fn rename(&self, from: &PathBuf, to: &PathBuf) -> Result<(), SE>;
-    use std::path::PathBuf;
+    fn create_dir_all(&self, _: &PathBuf) -> Result<(), SE>;
-    use std::collections::HashMap;
-    use std::sync::Mutex;
 
-    use libimagerror::into::IntoError;
+    fn new_instance(&self, p: PathBuf) -> Box<FileAbstractionInstance>;
-
+}
-    use error::MapErrInto;
+
-
+/// An abstraction trait over actions on files
-    lazy_static! {
+pub trait FileAbstractionInstance : Debug {
-        static ref MAP: Mutex<HashMap<PathBuf, Cursor<Vec<u8>>>> = {
+    fn get_file_content(&mut self) -> Result<String, SE>;
-            Mutex::new(HashMap::new())
+    fn write_file_content(&mut self, buf: &[u8]) -> Result<(), SE>;
-        };
-    }
-
-    /// `FileAbstraction` type, this is the Test version!
-    ///
-    /// A lazy file is either absent, but a path to it is available, or it is present.
-    #[derive(Debug)]
-    pub enum FileAbstraction {
-        Absent(PathBuf),
-    }
-
-    impl FileAbstraction {
-
-        /**
-         * Get the mutable file behind a FileAbstraction object
-         */
-        pub fn get_file_content(&mut self) -> Result<Cursor<Vec<u8>>, SE> {
-            debug!("Getting lazy file: {:?}", self);
-            match *self {
-                FileAbstraction::Absent(ref f) => {
-                    let map = try!(MAP.lock().map_err_into(SEK::LockPoisoned));
-                    return map.get(f).cloned().ok_or(SEK::FileNotFound.into_error());
-                },
-            };
-        }
-
-        pub fn write_file_content(&mut self, buf: &[u8]) -> Result<(), SE> {
-            match *self {
-                FileAbstraction::Absent(ref f) => {
-                    let mut map = try!(MAP.lock().map_err_into(SEK::LockPoisoned));
-                    if let Some(ref mut cur) = map.get_mut(f) {
-                        let mut vec = cur.get_mut();
-                        vec.clear();
-                        vec.extend_from_slice(buf);
-                        return Ok(());
-                    }
-                    let vec = Vec::from(buf);
-                    map.insert(f.clone(), Cursor::new(vec));
-                    return Ok(());
-                },
-            };
-        }
-
-        pub fn remove_file(path: &PathBuf) -> Result<(), SE> {
-            try!(MAP.lock().map_err_into(SEK::LockPoisoned))
-                .remove(path)
-                .map(|_| ())
-                .ok_or(SEK::FileNotFound.into_error())
-        }
-
-        pub fn copy(from: &PathBuf, to: &PathBuf) -> Result<(), SE> {
-            let mut map = try!(MAP.lock().map_err_into(SEK::LockPoisoned));
-            let a = try!(map.get(from).cloned().ok_or(SEK::FileNotFound.into_error()));
-            map.insert(to.clone(), a);
-            Ok(())
-        }
-
-        pub fn rename(from: &PathBuf, to: &PathBuf) -> Result<(), SE> {
-            let mut map = try!(MAP.lock().map_err_into(SEK::LockPoisoned));
-            let a = try!(map.get(from).cloned().ok_or(SEK::FileNotFound.into_error()));
-            map.insert(to.clone(), a);
-            Ok(())
-        }
-
-        pub fn create_dir_all(_: &PathBuf) -> Result<(), SE> {
-            Ok(())
-        }
-    }
 }
 
-#[cfg(not(test))]
 mod fs {
-    use error::{MapErrInto, StoreError as SE, StoreErrorKind as SEK};
+    use std::fs::{File, OpenOptions, create_dir_all, remove_file, copy, rename};
     use std::io::{Seek, SeekFrom, Read};
     use std::path::{Path, PathBuf};
-    use std::fs::{File, OpenOptions, create_dir_all, remove_file, copy, rename};
 
-    /// `FileAbstraction` type
+    use error::{MapErrInto, StoreError as SE, StoreErrorKind as SEK};
+
+    use super::FileAbstraction;
+    use super::FileAbstractionInstance;
+
+    #[derive(Debug)]
+    pub enum FSFileAbstractionInstance {
+        Absent(PathBuf),
+        File(File, PathBuf)
+    }
+
+    impl FileAbstractionInstance for FSFileAbstractionInstance {
+
+        /**
+         * Get the content behind this file
+         */
+        fn get_file_content(&mut self) -> Result<String, SE> {
+            debug!("Getting lazy file: {:?}", self);
+            let (file, path) = match *self {
+                FSFileAbstractionInstance::File(ref mut f, _) => return {
+                    // We seek to the beginning of the file since we expect each
+                    // access to the file to be in a different context
+                    try!(f.seek(SeekFrom::Start(0))
+                        .map_err_into(SEK::FileNotSeeked));
+
+                    let mut s = String::new();
+                    f.read_to_string(&mut s);
+                    Ok(s)
+                },
+                FSFileAbstractionInstance::Absent(ref p) =>
+                    (try!(open_file(p).map_err_into(SEK::FileNotFound)), p.clone()),
+            };
+            *self = FSFileAbstractionInstance::File(file, path);
+            if let FSFileAbstractionInstance::File(ref mut f, _) = *self {
+                let mut s = String::new();
+                f.read_to_string(&mut s);
+                return Ok(s);
+            }
+            unreachable!()
+        }
+
+        /**
+         * Write the content of this file
+         */
+        fn write_file_content(&mut self, buf: &[u8]) -> Result<(), SE> {
+            use std::io::Write;
+            let (file, path) = match *self {
+                FSFileAbstractionInstance::File(ref mut f, _) => return {
+                    // We seek to the beginning of the file since we expect each
+                    // access to the file to be in a different context
+                    try!(f.seek(SeekFrom::Start(0))
+                        .map_err_into(SEK::FileNotCreated));
+                    f.write_all(buf).map_err_into(SEK::FileNotWritten)
+                },
+                FSFileAbstractionInstance::Absent(ref p) =>
+                    (try!(create_file(p).map_err_into(SEK::FileNotCreated)), p.clone()),
+            };
+            *self = FSFileAbstractionInstance::File(file, path);
+            if let FSFileAbstractionInstance::File(ref mut f, _) = *self {
+                return f.write_all(buf).map_err_into(SEK::FileNotWritten);
+            }
+            unreachable!();
+        }
+
+    }
+
+    /// `FSFileAbstraction` state type
     ///
     /// A lazy file is either absent, but a path to it is available, or it is present.
     #[derive(Debug)]
-    pub enum FileAbstraction {
+    pub struct FSFileAbstraction {
-        Absent(PathBuf),
+    }
-        File(File, PathBuf)
+
+    impl FSFileAbstraction {
+        pub fn new() -> FSFileAbstraction {
+            FSFileAbstraction { }
+        }
+    }
+
+    impl FileAbstraction for FSFileAbstraction {
+
+        fn remove_file(&self, path: &PathBuf) -> Result<(), SE> {
+            remove_file(path).map_err_into(SEK::FileNotRemoved)
+        }
+
+        fn copy(&self, from: &PathBuf, to: &PathBuf) -> Result<(), SE> {
+            copy(from, to).map_err_into(SEK::FileNotCopied).map(|_| ())
+        }
+
+        fn rename(&self, from: &PathBuf, to: &PathBuf) -> Result<(), SE> {
+            rename(from, to).map_err_into(SEK::FileNotRenamed)
+        }
+
+        fn create_dir_all(&self, path: &PathBuf) -> Result<(), SE> {
+            create_dir_all(path).map_err_into(SEK::DirNotCreated)
+        }
+
+        fn new_instance(&self, p: PathBuf) -> Box<FileAbstractionInstance> {
+            Box::new(FSFileAbstractionInstance::Absent(p))
+        }
     }
 
     fn open_file<A: AsRef<Path>>(p: A) -> ::std::io::Result<File> {
@@ -139,87 +224,173 @@ mod fs {
         OpenOptions::new().write(true).read(true).create(true).open(p)
     }
 
-    impl FileAbstraction {
+}
+
+mod inmemory {
+    use error::StoreError as SE;
+    use error::StoreErrorKind as SEK;
+    use std::io::Read;
+    use std::io::Cursor;
+    use std::path::PathBuf;
+    use std::collections::HashMap;
+    use std::sync::Mutex;
+    use std::cell::RefCell;
+    use std::sync::Arc;
+
+    use libimagerror::into::IntoError;
+
+    use super::FileAbstraction;
+    use super::FileAbstractionInstance;
+
+    type Backend = Arc<Mutex<RefCell<HashMap<PathBuf, Cursor<Vec<u8>>>>>>;
+
+    /// `FileAbstraction` type, this is the Test version!
+    ///
+    /// A lazy file is either absent, but a path to it is available, or it is present.
+    #[derive(Debug)]
+    pub struct InMemoryFileAbstractionInstance {
+        fs_abstraction: Backend,
+        absent_path: PathBuf,
+    }
+
+    impl InMemoryFileAbstractionInstance {
+
+        pub fn new(fs: Backend, pb: PathBuf) -> InMemoryFileAbstractionInstance {
+            InMemoryFileAbstractionInstance {
+                fs_abstraction: fs,
+                absent_path: pb
+            }
+        }
+
+    }
+
+    impl FileAbstractionInstance for InMemoryFileAbstractionInstance {
 
         /**
-         * Get the content behind this file
+         * Get the mutable file behind a InMemoryFileAbstraction object
          */
-        pub fn get_file_content(&mut self) -> Result<&mut Read, SE> {
+        fn get_file_content(&mut self) -> Result<String, SE> {
             debug!("Getting lazy file: {:?}", self);
-            let (file, path) = match *self {
+
-                FileAbstraction::File(ref mut f, _) => return {
+            let p = self.absent_path.clone();
-                    // We seek to the beginning of the file since we expect each
+            match self.fs_abstraction.lock() {
-                    // access to the file to be in a different context
+                Ok(mut mtx) => {
-                    try!(f.seek(SeekFrom::Start(0))
+                    mtx.get_mut()
-                        .map_err_into(SEK::FileNotSeeked));
+                        .get_mut(&p)
-                    Ok(f)
+                        .ok_or(SEK::FileNotFound.into_error())
+                        .map(|t| {
+                            let mut s = String::new();
+                            t.read_to_string(&mut s);
+                            s
+                        })
+                }
+
+                Err(_) => Err(SEK::LockError.into_error())
+            }
+        }
+
+        fn write_file_content(&mut self, buf: &[u8]) -> Result<(), SE> {
+            match *self {
+                InMemoryFileAbstractionInstance { ref absent_path, .. } => {
+                    let mut mtx = self.fs_abstraction.lock().expect("Locking Mutex failed");
+                    let mut backend = mtx.get_mut();
+
+                    if let Some(ref mut cur) = backend.get_mut(absent_path) {
+                        let mut vec = cur.get_mut();
+                        vec.clear();
+                        vec.extend_from_slice(buf);
+                        return Ok(());
+                    }
+                    let vec = Vec::from(buf);
+                    backend.insert(absent_path.clone(), Cursor::new(vec));
+                    return Ok(());
                 },
-                FileAbstraction::Absent(ref p) => (try!(open_file(p).map_err_into(SEK::FileNotFound)),
-                                            p.clone()),
             };
-            *self = FileAbstraction::File(file, path);
-            if let FileAbstraction::File(ref mut f, _) = *self {
-                return Ok(f);
         }
-            unreachable!()
     }
 
-        /**
+    #[derive(Debug)]
-         * Write the content of this file
+    pub struct InMemoryFileAbstraction {
-         */
+        virtual_filesystem: Backend,
-        pub fn write_file_content(&mut self, buf: &[u8]) -> Result<(), SE> {
-            use std::io::Write;
-            let (file, path) = match *self {
-                FileAbstraction::File(ref mut f, _) => return {
-                    // We seek to the beginning of the file since we expect each
-                    // access to the file to be in a different context
-                    try!(f.seek(SeekFrom::Start(0))
-                        .map_err_into(SEK::FileNotCreated));
-                    f.write_all(buf).map_err_into(SEK::FileNotWritten)
-                },
-                FileAbstraction::Absent(ref p) => (try!(create_file(p).map_err_into(SEK::FileNotCreated)),
-                                            p.clone()),
-            };
-            *self = FileAbstraction::File(file, path);
-            if let FileAbstraction::File(ref mut f, _) = *self {
-                return f.write_all(buf).map_err_into(SEK::FileNotWritten);
-            }
-            unreachable!();
     }
 
-        pub fn remove_file(path: &PathBuf) -> Result<(), SE> {
+    impl InMemoryFileAbstraction {
-            remove_file(path).map_err_into(SEK::FileNotRemoved)
+
+        pub fn new() -> InMemoryFileAbstraction {
+            InMemoryFileAbstraction {
+                virtual_filesystem: Arc::new(Mutex::new(RefCell::new(HashMap::new()))),
+            }
         }
 
-        pub fn copy(from: &PathBuf, to: &PathBuf) -> Result<(), SE> {
+        pub fn backend(&self) -> &Backend {
-            copy(from, to).map_err_into(SEK::FileNotCopied).map(|_| ())
+            &self.virtual_filesystem
         }
 
-        pub fn rename(from: &PathBuf, to: &PathBuf) -> Result<(), SE> {
-            rename(from, to).map_err_into(SEK::FileNotRenamed)
     }
 
-        pub fn create_dir_all(path: &PathBuf) -> Result<(), SE> {
+    impl FileAbstraction for InMemoryFileAbstraction {
-            create_dir_all(path).map_err_into(SEK::DirNotCreated)
+
+        fn remove_file(&self, path: &PathBuf) -> Result<(), SE> {
+            debug!("Removing: {:?}", path);
+            self.backend()
+                .lock()
+                .expect("Locking Mutex failed")
+                .get_mut()
+                .remove(path)
+                .map(|_| ())
+                .ok_or(SEK::FileNotFound.into_error())
+        }
+
+        fn copy(&self, from: &PathBuf, to: &PathBuf) -> Result<(), SE> {
+            debug!("Copying : {:?} -> {:?}", from, to);
+            let mut mtx = self.backend().lock().expect("Locking Mutex failed");
+            let mut backend = mtx.get_mut();
+
+            let a = try!(backend.get(from).cloned().ok_or(SEK::FileNotFound.into_error()));
+            backend.insert(to.clone(), a);
+            debug!("Copying: {:?} -> {:?} worked", from, to);
+            Ok(())
+        }
+
+        fn rename(&self, from: &PathBuf, to: &PathBuf) -> Result<(), SE> {
+            debug!("Renaming: {:?} -> {:?}", from, to);
+            let mut mtx = self.backend().lock().expect("Locking Mutex failed");
+            let mut backend = mtx.get_mut();
+
+            let a = try!(backend.get(from).cloned().ok_or(SEK::FileNotFound.into_error()));
+            backend.insert(to.clone(), a);
+            debug!("Renaming: {:?} -> {:?} worked", from, to);
+            Ok(())
+        }
+
+        fn create_dir_all(&self, _: &PathBuf) -> Result<(), SE> {
+            Ok(())
+        }
+
+        fn new_instance(&self, p: PathBuf) -> Box<FileAbstractionInstance> {
+            Box::new(InMemoryFileAbstractionInstance::new(self.backend().clone(), p))
         }
     }
+
 }
 
 #[cfg(test)]
 mod test {
-    use super::FileAbstraction;
+    use super::FileAbstractionInstance;
-    use std::io::Read;
+    use super::inmemory::InMemoryFileAbstraction;
+    use super::inmemory::InMemoryFileAbstractionInstance;
     use std::path::PathBuf;
 
     #[test]
     fn lazy_file() {
+        let fs = InMemoryFileAbstraction::new();
+
         let mut path = PathBuf::from("/tests");
         path.set_file_name("test1");
-        let mut lf = FileAbstraction::Absent(path);
+        let mut lf = InMemoryFileAbstractionInstance::new(fs.backend().clone(), path);
         lf.write_file_content(b"Hello World").unwrap();
-        let mut bah = Vec::new();
+        let bah = lf.get_file_content().unwrap();
-        lf.get_file_content().unwrap().read_to_end(&mut bah).unwrap();
+        assert_eq!(bah, "Hello World");
-        assert_eq!(bah, b"Hello World");
     }
 
 }

libimagstore/src/store.rs

@@ -42,6 +42,8 @@ use error::{StoreError as SE, StoreErrorKind as SEK};
 use error::MapErrInto;
 use storeid::{IntoStoreId, StoreId, StoreIdIterator};
 use file_abstraction::FileAbstraction;
+use file_abstraction::FileAbstractionInstance;
+use file_abstraction::FSFileAbstraction;
 use toml_ext::*;
 
 use libimagerror::into::IntoError;
@@ -65,7 +67,7 @@ enum StoreEntryStatus {
 #[derive(Debug)]
 struct StoreEntry {
     id: StoreId,
-    file: FileAbstraction,
+    file: Box<FileAbstractionInstance>,
     status: StoreEntryStatus,
 }
 
@@ -132,7 +134,7 @@ impl Iterator for Walk {
 
 impl StoreEntry {
 
-    fn new(id: StoreId) -> Result<StoreEntry> {
+    fn new(id: StoreId, backend: &Box<FileAbstraction>) -> Result<StoreEntry> {
         let pb = try!(id.clone().into_pathbuf());
 
         #[cfg(feature = "fs-lock")]
@@ -144,7 +146,7 @@ impl StoreEntry {
 
         Ok(StoreEntry {
             id: id,
-            file: FileAbstraction::Absent(pb),
+            file: backend.new_instance(pb),
             status: StoreEntryStatus::Present,
         })
     }
@@ -160,7 +162,7 @@ impl StoreEntry {
         if !self.is_borrowed() {
             self.file
                 .get_file_content()
-                .and_then(|mut file| Entry::from_reader(id.clone(), &mut file))
+                .and_then(|content| Entry::from_str(id.clone(), &content))
                 .or_else(|err| if err.err_type() == SEK::FileNotFound {
                     Ok(Entry::new(id.clone()))
                 } else {
@@ -213,6 +215,11 @@ pub struct Store {
     /// Could be optimized for a threadsafe HashMap
     ///
     entries: Arc<RwLock<HashMap<StoreId, StoreEntry>>>,
+
+    /// The backend to use
+    ///
+    /// This provides the filesystem-operation functions (or pretends to)
+    backend: Box<FileAbstraction>,
 }
 
 impl Store {
@@ -240,6 +247,17 @@ impl Store {
     ///   - StorePathCreate(_) if creating the store directory failed
     ///   - StorePathExists() if location exists but is a file
     pub fn new(location: PathBuf, store_config: Option<Value>) -> Result<Store> {
+        let backend = Box::new(FSFileAbstraction::new());
+        Store::new_with_backend(location, store_config, backend)
+    }
+
+    /// Create a Store object as descripbed in `Store::new()` documentation, but with an alternative
+    /// backend implementation.
+    ///
+    /// Do not use directly, only for testing purposes.
+    pub fn new_with_backend(location: PathBuf,
+                            store_config: Option<Value>,
+                            backend: Box<FileAbstraction>) -> Result<Store> {
         use configuration::*;
 
         debug!("Validating Store configuration");
@@ -256,7 +274,7 @@ impl Store {
                     .map_err_into(SEK::IoError);
             }
 
-            try!(FileAbstraction::create_dir_all(&location)
+            try!(backend.create_dir_all(&location)
                  .map_err_into(SEK::StorePathCreate)
                  .map_dbg_err_str("Failed"));
         } else if location.is_file() {
@@ -268,6 +286,7 @@ impl Store {
             location: location.clone(),
             configuration: store_config,
             entries: Arc::new(RwLock::new(HashMap::new())),
+            backend: backend,
         };
 
         debug!("Store building succeeded");
@@ -367,7 +386,7 @@ impl Store {
             }
             hsmap.insert(id.clone(), {
                 debug!("Creating: '{}'", id);
-                let mut se = try!(StoreEntry::new(id.clone()));
+                let mut se = try!(StoreEntry::new(id.clone(), &self.backend));
                 se.status = StoreEntryStatus::Borrowed;
                 se
             });
@@ -400,7 +419,7 @@ impl Store {
                 .write()
                 .map_err(|_| SE::new(SEK::LockPoisoned, None))
                 .and_then(|mut es| {
-                    let new_se = try!(StoreEntry::new(id.clone()));
+                    let new_se = try!(StoreEntry::new(id.clone(), &self.backend));
                     let mut se = es.entry(id.clone()).or_insert(new_se);
                     let entry = se.get_entry();
                     se.status = StoreEntryStatus::Borrowed;
@@ -557,7 +576,7 @@ impl Store {
             return Err(SE::new(SEK::IdLocked, None)).map_err_into(SEK::RetrieveCopyCallError);
         }
 
-        try!(StoreEntry::new(id)).get_entry()
+        try!(StoreEntry::new(id, &self.backend)).get_entry()
     }
 
     /// Delete an entry
@@ -596,7 +615,7 @@ impl Store {
             // remove the entry first, then the file
             entries.remove(&id);
             let pb = try!(id.clone().with_base(self.path().clone()).into_pathbuf());
-            if let Err(e) = FileAbstraction::remove_file(&pb) {
+            if let Err(e) = self.backend.remove_file(&pb) {
                 return Err(SEK::FileError.into_error_with_cause(Box::new(e)))
                     .map_err_into(SEK::DeleteCallError);
             }
@@ -638,11 +657,11 @@ impl Store {
 
         let old_id_as_path = try!(old_id.clone().with_base(self.path().clone()).into_pathbuf());
         let new_id_as_path = try!(new_id.clone().with_base(self.path().clone()).into_pathbuf());
-        FileAbstraction::copy(&old_id_as_path, &new_id_as_path)
+        self.backend.copy(&old_id_as_path, &new_id_as_path)
             .and_then(|_| {
                 if remove_old {
                     debug!("Removing old '{:?}'", old_id_as_path);
-                    FileAbstraction::remove_file(&old_id_as_path)
+                    self.backend.remove_file(&old_id_as_path)
                 } else {
                     Ok(())
                 }
@@ -710,7 +729,7 @@ impl Store {
             let old_id_pb = try!(old_id.clone().with_base(self.path().clone()).into_pathbuf());
             let new_id_pb = try!(new_id.clone().with_base(self.path().clone()).into_pathbuf());
 
-            match FileAbstraction::rename(&old_id_pb, &new_id_pb) {
+            match self.backend.rename(&old_id_pb, &new_id_pb) {
                 Err(e) => return Err(SEK::EntryRenameError.into_error_with_cause(Box::new(e))),
                 Ok(_) => {
                     debug!("Rename worked on filesystem");