Merge pull request #31951 from m1093782566/m109-master-errs

Automatic merge from submit-queue

Fix errors.NewAggregate nil pointer panic

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**What this PR does / why we need it**:

Consider the following code block,

```
err := myfunc()
agg := errors.NewAggregate([]error{err})
fmt.Println("aggregate error is %v", agg)
```

If the `err` is **nil**, then it will cause a panic:

```
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x20 pc=0x481a61]

goroutine 6 [running]:
panic(0x5769c0, 0xc082002090)
        C:/Go/src/runtime/panic.go:464 +0x3f4
testing.tRunner.func1(0xc082074090)
        C:/Go/src/testing/testing.go:467 +0x199
panic(0x5769c0, 0xc082002090)
        C:/Go/src/runtime/panic.go:426 +0x4f7
k8s.io/kubernetes/pkg/util/errors.aggregate.Error(0xc082002590, 0x1, 0x1, 0x0, 0x0)
        D:/goproject/community/src/k8s.io/kubernetes/pkg/util/errors/errors.go:71 +0x91
k8s.io/kubernetes/pkg/util/errors.(*aggregate).Error(0xc08200c7e0, 0x0, 0x0)
        <autogenerated>:1 +0xb4
k8s.io/kubernetes/pkg/util/errors.TestAggregateWithNil(0xc082074090)
        D:/goproject/community/src/k8s.io/kubernetes/pkg/util/errors/errors_test.go:70 +0x402
testing.tRunner(0xc082074090, 0x67cdb8)
        C:/Go/src/testing/testing.go:473 +0x9f
created by testing.RunTests
        C:/Go/src/testing/testing.go:582 +0x899
exit status 2
```

The root cause is that [aggregate.Error()](https://github.com/kubernetes/kubernetes/blob/master/pkg/util/errors/errors.go#L47) doesn't check if error list contains nil.

We can blame the user didn't check if `err` is nil before passing it to 

```
errors.NewAggregate([]error{err})
``` 

but I think we can check again inside in case of user forget to check outside.

pkg/util/errors/errors.go

@@ -31,11 +31,23 @@ type Aggregate interface {
 // NewAggregate converts a slice of errors into an Aggregate interface, which
 // is itself an implementation of the error interface.  If the slice is empty,
 // this returns nil.
+// It will check if any of the element of input error list is nil, to avoid
+// nil pointer panic when call Error().
 func NewAggregate(errlist []error) Aggregate {
 	if len(errlist) == 0 {
 		return nil
 	}
-	return aggregate(errlist)
+	// In case of input error list contains nil
+	var errs []error
+	for _, e := range errlist {
+		if e != nil {
+			errs = append(errs, e)
+		}
+	}
+	if len(errs) == 0 {
+		return nil
+	}
+	return aggregate(errs)
 }
 
 // This helper implements the error and Errors interfaces.  Keeping it private

pkg/util/errors/errors_test.go

@@ -50,6 +50,46 @@ func TestEmptyAggregate(t *testing.T) {
 	}
 }
 
+func TestAggregateWithNil(t *testing.T) {
+	var slice []error
+	slice = []error{nil}
+	var agg Aggregate
+	var err error
+
+	agg = NewAggregate(slice)
+	if agg != nil {
+		t.Errorf("expected nil, got %#v", agg)
+	}
+	err = NewAggregate(slice)
+	if err != nil {
+		t.Errorf("expected nil, got %#v", err)
+	}
+
+	// Append a non-nil error
+	slice = append(slice, fmt.Errorf("err"))
+	agg = NewAggregate(slice)
+	if agg == nil {
+		t.Errorf("expected non-nil")
+	}
+	if s := agg.Error(); s != "err" {
+		t.Errorf("expected 'err', got %q", s)
+	}
+	if s := agg.Errors(); len(s) != 1 {
+		t.Errorf("expected one-element slice, got %#v", s)
+	}
+	if s := agg.Errors()[0].Error(); s != "err" {
+		t.Errorf("expected 'err', got %q", s)
+	}
+
+	err = agg.(error)
+	if err == nil {
+		t.Errorf("expected non-nil")
+	}
+	if s := err.Error(); s != "err" {
+		t.Errorf("expected 'err', got %q", s)
+	}
+}
+
 func TestSingularAggregate(t *testing.T) {
 	var slice []error = []error{fmt.Errorf("err")}
 	var agg Aggregate