diff --git a/.github/workflows/git.yml b/.github/workflows/git.yml
index b10b14b23..84100539d 100644
--- a/.github/workflows/git.yml
+++ b/.github/workflows/git.yml
@@ -38,3 +38,8 @@ jobs:
  
     - name: Test
       run: make test-coverage
+
+    - name: Build go-git with CGO disabled
+      run: go build ./...
+      env:
+        CGO_ENABLED: 0
diff --git a/go.mod b/go.mod
index 5903873de..9672762ea 100644
--- a/go.mod
+++ b/go.mod
@@ -14,7 +14,7 @@ require (
 	github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99
 	github.com/jessevdk/go-flags v1.5.0
 	github.com/kevinburke/ssh_config v1.2.0
-	github.com/pjbgf/sha1cd v0.2.1
+	github.com/pjbgf/sha1cd v0.2.3
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/sergi/go-diff v1.1.0
 	github.com/skeema/knownhosts v1.1.0
diff --git a/go.sum b/go.sum
index f632d700f..b859ce808 100644
--- a/go.sum
+++ b/go.sum
@@ -45,8 +45,8 @@ github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
 github.com/matryer/is v1.2.0 h1:92UTHpy8CDwaJ08GqLDzhhuixiBUUD1p3AU6PHddz4A=
 github.com/matryer/is v1.2.0/go.mod h1:2fLPjFQM9rhQ15aVEtbuwhJinnOqrmgXPNdZsdwlWXA=
 github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e/go.mod h1:zD1mROLANZcx1PVRCS0qkT7pwLkGfwJo4zjcN/Tysno=
-github.com/pjbgf/sha1cd v0.2.1 h1:g8hkbV5Cpj24YVSZ4MCAnecIBwsg0bQrvP6xkPUVit4=
-github.com/pjbgf/sha1cd v0.2.1/go.mod h1:HOK9QrgzdHpbc2Kzip0Q1yi3M2MFGPADtR6HjG65m5M=
+github.com/pjbgf/sha1cd v0.2.3 h1:uKQP/7QOzNtKYH7UTohZLcjF5/55EnTw0jO/Ru4jZwI=
+github.com/pjbgf/sha1cd v0.2.3/go.mod h1:HOK9QrgzdHpbc2Kzip0Q1yi3M2MFGPADtR6HjG65m5M=
 github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
 github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
diff --git a/plumbing/format/idxfile/idxfile.go b/plumbing/format/idxfile/idxfile.go
index 45bb594c1..6cc2e96a7 100644
--- a/plumbing/format/idxfile/idxfile.go
+++ b/plumbing/format/idxfile/idxfile.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"io"
 	"sort"
+	"sync"
 
 	encbin "encoding/binary"
 
@@ -58,6 +59,7 @@ type MemoryIndex struct {
 
 	offsetHash       map[int64]plumbing.Hash
 	offsetHashIsFull bool
+	offsetHashMutex  sync.RWMutex
 }
 
 var _ Index = (*MemoryIndex)(nil)
@@ -130,7 +132,9 @@ func (idx *MemoryIndex) FindOffset(h plumbing.Hash) (int64, error) {
 		if idx.offsetHash == nil {
 			idx.offsetHash = make(map[int64]plumbing.Hash)
 		}
+		idx.offsetHashMutex.Lock()
 		idx.offsetHash[int64(offset)] = h
+		idx.offsetHashMutex.Unlock()
 	}
 
 	return int64(offset), nil
@@ -173,7 +177,10 @@ func (idx *MemoryIndex) FindHash(o int64) (plumbing.Hash, error) {
 	var ok bool
 
 	if idx.offsetHash != nil {
-		if hash, ok = idx.offsetHash[o]; ok {
+		idx.offsetHashMutex.RLock()
+		hash, ok = idx.offsetHash[o]
+		idx.offsetHashMutex.RUnlock()
+		if ok {
 			return hash, nil
 		}
 	}
@@ -184,7 +191,9 @@ func (idx *MemoryIndex) FindHash(o int64) (plumbing.Hash, error) {
 			return plumbing.ZeroHash, err
 		}
 
+		idx.offsetHashMutex.RLock()
 		hash, ok = idx.offsetHash[o]
+		idx.offsetHashMutex.RUnlock()
 	}
 
 	if !ok {
@@ -206,6 +215,7 @@ func (idx *MemoryIndex) genOffsetHash() error {
 
 	var hash plumbing.Hash
 	i := uint32(0)
+	idx.offsetHashMutex.Lock()
 	for firstLevel, fanoutValue := range idx.Fanout {
 		mappedFirstLevel := idx.FanoutMapping[firstLevel]
 		for secondLevel := uint32(0); i < fanoutValue; i++ {
@@ -215,6 +225,7 @@ func (idx *MemoryIndex) genOffsetHash() error {
 			secondLevel++
 		}
 	}
+	idx.offsetHashMutex.Unlock()
 
 	return nil
 }
diff --git a/plumbing/format/packfile/parser.go b/plumbing/format/packfile/parser.go
index 22e8810d1..8ded21a61 100644
--- a/plumbing/format/packfile/parser.go
+++ b/plumbing/format/packfile/parser.go
@@ -237,6 +237,15 @@ func (p *Parser) indexObjects() error {
 				return err
 			}
 
+			// move children of placeholder parent into actual parent in case this was
+			// a non-external delta reference
+			if placeholder, ok := p.oiByHash[sha1]; ok {
+				ota.Children = placeholder.Children
+				for _, c := range ota.Children {
+					c.Parent = ota
+				}
+			}
+
 			ota.SHA1 = sha1
 			p.oiByHash[ota.SHA1] = ota
 		}