Implement database abstraction, error checking (ref #5907) (#6107)

This PR does two things, because one lead to the other:

- Move the leveldb specific stuff into a small "backend" package that
defines a backend interface and the leveldb implementation. This allows,
potentially, in the future, switching the db implementation so another
KV store should we wish to do so.

- Add proper error handling all along the way. The db and backend
packages are now errcheck clean. However, I drew the line at modifying
the FileSet API in order to keep this manageable and not continue
refactoring all of the rest of Syncthing. As such, the FileSet methods
still panic on database errors, except for the "database is closed"
error which is instead handled by silently returning as quickly as
possible, with the assumption that we're anyway "on the way out".

lib/db/smallindex_test.go

@@ -6,11 +6,15 @@
 
 package db
 
-import "testing"
+import (
+	"testing"
+
+	"github.com/syncthing/syncthing/lib/db/backend"
+)
 
 func TestSmallIndex(t *testing.T) {
-	db := OpenMemory()
-	idx := newSmallIndex(db.DB, []byte{12, 34})
+	db := NewLowlevel(backend.OpenMemory())
+	idx := newSmallIndex(db, []byte{12, 34})
 
 	// ID zero should be unallocated
 	if val, ok := idx.Val(0); ok || val != nil {
@@ -18,7 +22,9 @@ func TestSmallIndex(t *testing.T) {
 	}
 
 	// A new key should get ID zero
-	if id := idx.ID([]byte("hello")); id != 0 {
+	if id, err := idx.ID([]byte("hello")); err != nil {
+		t.Fatal(err)
+	} else if id != 0 {
 		t.Fatal("Expected 0, not", id)
 	}
 	// Looking up ID zero should work
@@ -30,23 +36,29 @@ func TestSmallIndex(t *testing.T) {
 	idx.Delete([]byte("hello"))
 
 	// Next ID should be one
-	if id := idx.ID([]byte("key2")); id != 1 {
+	if id, err := idx.ID([]byte("key2")); err != nil {
+		t.Fatal(err)
+	} else if id != 1 {
 		t.Fatal("Expected 1, not", id)
 	}
 
 	// Now lets create a new index instance based on what's actually serialized to the database.
-	idx = newSmallIndex(db.DB, []byte{12, 34})
+	idx = newSmallIndex(db, []byte{12, 34})
 
 	// Status should be about the same as before.
 	if val, ok := idx.Val(0); ok || val != nil {
 		t.Fatal("Unexpected return for deleted ID 0")
 	}
-	if id := idx.ID([]byte("key2")); id != 1 {
+	if id, err := idx.ID([]byte("key2")); err != nil {
+		t.Fatal(err)
+	} else if id != 1 {
 		t.Fatal("Expected 1, not", id)
 	}
 
 	// Setting "hello" again should get us ID 2, not 0 as it was originally.
-	if id := idx.ID([]byte("hello")); id != 2 {
+	if id, err := idx.ID([]byte("hello")); err != nil {
+		t.Fatal(err)
+	} else if id != 2 {
 		t.Fatal("Expected 2, not", id)
 	}
 }