Fix non thread-safe Iteration around go-patricia

* Unlike other methods in truncindex, Iterate was not locking before
using the Trie, making it potentially race e.g. Delete could result in
setting a child to nil, while Iterate dereferenced that node
while walking the Trie.

Signed-off-by: Petar Petrov <pppepito86@gmail.com>

pkg/truncindex/truncindex.go

@@ -125,8 +125,13 @@ func (idx *TruncIndex) Get(s string) (string, error) {
 	return "", ErrNotExist
 }
 
-// Iterate iterates over all stored IDs, and passes each of them to the given handler.
+// Iterate iterates over all stored IDs and passes each of them to the given
+// handler. Take care that the handler method does not call any public
+// method on truncindex as the internal locking is not reentrant/recursive
+// and will result in deadlock.
 func (idx *TruncIndex) Iterate(handler func(id string)) {
+	idx.Lock()
+	defer idx.Unlock()
 	idx.trie.Visit(func(prefix patricia.Prefix, item patricia.Item) error {
 		handler(string(prefix))
 		return nil

pkg/truncindex/truncindex_test.go

@@ -3,6 +3,7 @@ package truncindex
 import (
 	"math/rand"
 	"testing"
+	"time"
 
 	"github.com/docker/docker/pkg/stringid"
 )
@@ -98,6 +99,7 @@ func TestTruncIndex(t *testing.T) {
 	assertIndexGet(t, index, id, id, false)
 
 	assertIndexIterate(t)
+	assertIndexIterateDoNotPanic(t)
 }
 
 func assertIndexIterate(t *testing.T) {
@@ -121,6 +123,28 @@ func assertIndexIterate(t *testing.T) {
 	})
 }
 
+func assertIndexIterateDoNotPanic(t *testing.T) {
+	ids := []string{
+		"19b36c2c326ccc11e726eee6ee78a0baf166ef96",
+		"28b36c2c326ccc11e726eee6ee78a0baf166ef96",
+	}
+
+	index := NewTruncIndex(ids)
+	iterationStarted := make(chan bool, 1)
+
+	go func() {
+		<-iterationStarted
+		index.Delete("19b36c2c326ccc11e726eee6ee78a0baf166ef96")
+	}()
+
+	index.Iterate(func(targetId string) {
+		if targetId == "19b36c2c326ccc11e726eee6ee78a0baf166ef96" {
+			iterationStarted <- true
+			time.Sleep(100 * time.Millisecond)
+		}
+	})
+}
+
 func assertIndexGet(t *testing.T, index *TruncIndex, input, expectedResult string, expectError bool) {
 	if result, err := index.Get(input); err != nil && !expectError {
 		t.Fatalf("Unexpected error getting '%s': %s", input, err)