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/namespaced_test.go

@@ -9,10 +9,12 @@ package db
 import (
 	"testing"
 	"time"
+
+	"github.com/syncthing/syncthing/lib/db/backend"
 )
 
 func TestNamespacedInt(t *testing.T) {
-	ldb := OpenMemory()
+	ldb := NewLowlevel(backend.OpenMemory())
 
 	n1 := NewNamespacedKV(ldb, "foo")
 	n2 := NewNamespacedKV(ldb, "bar")
@@ -23,7 +25,9 @@ func TestNamespacedInt(t *testing.T) {
 		t.Errorf("Incorrect return v %v != 0 || ok %v != false", v, ok)
 	}
 
-	n1.PutInt64("test", 42)
+	if err := n1.PutInt64("test", 42); err != nil {
+		t.Fatal(err)
+	}
 
 	// It should now exist in n1
 
@@ -37,7 +41,9 @@ func TestNamespacedInt(t *testing.T) {
 		t.Errorf("Incorrect return v %v != 0 || ok %v != false", v, ok)
 	}
 
-	n1.Delete("test")
+	if err := n1.Delete("test"); err != nil {
+		t.Fatal(err)
+	}
 
 	// It should no longer exist
 
@@ -47,7 +53,7 @@ func TestNamespacedInt(t *testing.T) {
 }
 
 func TestNamespacedTime(t *testing.T) {
-	ldb := OpenMemory()
+	ldb := NewLowlevel(backend.OpenMemory())
 
 	n1 := NewNamespacedKV(ldb, "foo")
 
@@ -56,7 +62,9 @@ func TestNamespacedTime(t *testing.T) {
 	}
 
 	now := time.Now()
-	n1.PutTime("test", now)
+	if err := n1.PutTime("test", now); err != nil {
+		t.Fatal(err)
+	}
 
 	if v, ok := n1.Time("test"); !v.Equal(now) || !ok {
 		t.Errorf("Incorrect return v %v != %v || ok %v != true", v, now, ok)
@@ -64,7 +72,7 @@ func TestNamespacedTime(t *testing.T) {
 }
 
 func TestNamespacedString(t *testing.T) {
-	ldb := OpenMemory()
+	ldb := NewLowlevel(backend.OpenMemory())
 
 	n1 := NewNamespacedKV(ldb, "foo")
 
@@ -72,7 +80,9 @@ func TestNamespacedString(t *testing.T) {
 		t.Errorf("Incorrect return v %q != \"\" || ok %v != false", v, ok)
 	}
 
-	n1.PutString("test", "yo")
+	if err := n1.PutString("test", "yo"); err != nil {
+		t.Fatal(err)
+	}
 
 	if v, ok := n1.String("test"); v != "yo" || !ok {
 		t.Errorf("Incorrect return v %q != \"yo\" || ok %v != true", v, ok)
@@ -80,13 +90,19 @@ func TestNamespacedString(t *testing.T) {
 }
 
 func TestNamespacedReset(t *testing.T) {
-	ldb := OpenMemory()
+	ldb := NewLowlevel(backend.OpenMemory())
 
 	n1 := NewNamespacedKV(ldb, "foo")
 
-	n1.PutString("test1", "yo1")
-	n1.PutString("test2", "yo2")
-	n1.PutString("test3", "yo3")
+	if err := n1.PutString("test1", "yo1"); err != nil {
+		t.Fatal(err)
+	}
+	if err := n1.PutString("test2", "yo2"); err != nil {
+		t.Fatal(err)
+	}
+	if err := n1.PutString("test3", "yo3"); err != nil {
+		t.Fatal(err)
+	}
 
 	if v, ok := n1.String("test1"); v != "yo1" || !ok {
 		t.Errorf("Incorrect return v %q != \"yo1\" || ok %v != true", v, ok)
@@ -98,7 +114,7 @@ func TestNamespacedReset(t *testing.T) {
 		t.Errorf("Incorrect return v %q != \"yo3\" || ok %v != true", v, ok)
 	}
 
-	n1.Reset()
+	reset(n1)
 
 	if v, ok := n1.String("test1"); v != "" || ok {
 		t.Errorf("Incorrect return v %q != \"\" || ok %v != false", v, ok)
@@ -110,3 +126,22 @@ func TestNamespacedReset(t *testing.T) {
 		t.Errorf("Incorrect return v %q != \"\" || ok %v != false", v, ok)
 	}
 }
+
+// reset removes all entries in this namespace.
+func reset(n *NamespacedKV) {
+	tr, err := n.db.NewWriteTransaction()
+	if err != nil {
+		return
+	}
+	defer tr.Release()
+
+	it, err := tr.NewPrefixIterator(n.prefix)
+	if err != nil {
+		return
+	}
+	for it.Next() {
+		_ = tr.Delete(it.Key())
+	}
+	it.Release()
+	_ = tr.Commit()
+}