Allow opening a database using a file handle instead of a path

[adamreichold]

This enables using this library with e.g. cap-std which avoids path-based access in favour of a capability-based approach.

I am not sure about the name, but open while probably most fitting is already taken for the path-based approach. Alternatives I considered were from_file, access, mount and open_or_init.

I added another commit to ensure that the build is Clippy-clean before submitting the PR. Please let me if know if you like me to drop that from the PR as it is admittedly unrelated. (Bumping fastrand resolves a few warnings about unnecessary mutability in the LMDB benchmark.)

[adamreichold]

Having read the implementation of Builder::open, I wonder if the following diff

diff --git a/src/db.rs b/src/db.rs
index 586bd98..57f10cb 100644
--- a/src/db.rs
+++ b/src/db.rs
@@ -12,7 +12,6 @@ use crate::{
 use crate::{ReadTransaction, Result, WriteTransaction};
 use std::fmt::{Display, Formatter};
 use std::fs::{File, OpenOptions};
-use std::io;
 use std::io::ErrorKind;
 use std::marker::PhantomData;
 use std::ops::RangeFull;
@@ -769,20 +768,19 @@ impl Builder {
 
     /// Opens an existing redb database.
     pub fn open(&self, path: impl AsRef<Path>) -> Result<Database, DatabaseError> {
-        if !path.as_ref().exists() {
-            Err(StorageError::Io(ErrorKind::NotFound.into()).into())
-        } else if File::open(path.as_ref())?.metadata()?.len() > 0 {
-            let file = OpenOptions::new().read(true).write(true).open(path)?;
-            Database::new(
-                file,
-                self.page_size,
-                None,
-                self.read_cache_size_bytes,
-                self.write_cache_size_bytes,
-            )
-        } else {
-            Err(StorageError::Io(io::Error::from(ErrorKind::InvalidData)).into())
+        let file = OpenOptions::new().read(true).write(true).open(path)?;
+
+        if file.metadata()?.len() == 0 {
+            return Err(StorageError::Io(ErrorKind::InvalidData.into()).into());
         }
+
+        Database::new(
+            file,
+            self.page_size,
+            None,
+            self.read_cache_size_bytes,
+            self.write_cache_size_bytes,
+        )
     }
 
     /// Open an existing or create a new database in the given `file`.

could be used to avoid the extra system calls to stat and open?

[cberner]

ya, that diff seems like it would work. As long as there's a test for a missing and an empty file it should be safe to change it

[cberner]

Couple minor comments. Otherwise this seems good to me!

[adamreichold]

ya, that diff seems like it would work. As long as there's a test for a missing and an empty file it should be safe to change it

I added test cases for these two error paths as a separate commit before the refactoring.

[cberner]

Merged. Thanks!