prevent panic after mmap failure

db.go

@@ -247,10 +247,14 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 		},
 	}
 
-	// Memory map the data file.
-	if err := db.mmap(options.InitialMmapSize); err != nil {
-		_ = db.close()
-		return nil, err
+	// Memory-map the data file as a byte slice.
+	if err := mmap(db, size); err != nil {
+		// mmap failed; the system may have run out of space. Fallback to
+		// mapping the bare minimum needed for the current db size.
+		if err2 := mmap(db, db.datasz); err2 != nil {
+			panic(fmt.Sprintf("failed to revert db size after failed mmap: %v", err2))
+		}
+		return MmapError(err.Error())
 	}
 
 	if db.readOnly {

errors.go

@@ -69,3 +69,10 @@ var (
 	// non-bucket key on an existing bucket key.
 	ErrIncompatibleValue = errors.New("incompatible value")
 )
+
+// MmapError represents an error resulting from a failed mmap call. Typically,
+// this error means that no further database writes will be possible. The most
+// common cause is insufficient disk space.
+type MmapError string
+
+func (e MmapError) Error() string { return string(e) }