git: close blob objects when finished

When we read a tree, commit, or tag from gitobj, the object is
automatically closed for us, since these objects are relatively small
and can be read and parsed all in memory.  However, blob objects may be
large, so we return a reader which can be used to read the data from the
object.

In order to properly clean up after ourselves, including closing any
object files which may open, we need to call Close on the blob after
reading it.  Otherwise, we'll eventually run out of file descriptors,
causing us to silently hang.  Restore the reset function that was
removed in e3fcde7 ("git: replace object scanner with one based on
gitobj", 2018-09-05) and ensure we close objects and reset the object
scanner between every call to Scan.

Author: bk2204

git/object_scanner.go

@@ -20,6 +20,8 @@ type object struct {
 	Size int64
 	// Type is the type of the object being held.
 	Type string
+	// object is the gitobj object being handled.
+	object gitobj.Object
 }
 
 // ObjectScanner is a scanner type that scans for Git objects reference-able in
@@ -68,6 +70,11 @@ func NewObjectScannerFrom(db *gitobj.ObjectDatabase) *ObjectScanner {
 // Scan() returns whether the scan was successful, or in other words, whether or
 // not the scanner can continue to progress.
 func (s *ObjectScanner) Scan(oid string) bool {
+	if err := s.reset(); err != nil {
+		s.err = err
+		return false
+	}
+
 	obj, err := s.scan(oid)
 	s.object = obj
 
@@ -88,6 +95,8 @@ func (s *ObjectScanner) Close() error {
 		return nil
 	}
 
+	s.reset()
+
 	return nil
 }
 
@@ -116,6 +125,21 @@ func (s *ObjectScanner) Type() string {
 // operation.
 func (s *ObjectScanner) Err() error { return s.err }
 
+func (s *ObjectScanner) reset() error {
+	if s.object != nil {
+		if c, ok := s.object.object.(interface {
+			Close() error
+		}); ok && c != nil {
+			if err := c.Close(); err != nil {
+				return err
+			}
+		}
+	}
+
+	s.object, s.err = nil, nil
+	return nil
+}
+
 type missingErr struct {
 	oid string
 }
@@ -162,5 +186,6 @@ func (s *ObjectScanner) scan(oid string) (*object, error) {
 		Oid:      oid,
 		Size:     size,
 		Type:     obj.Type().String(),
+		object:   obj,
 	}, nil
 }