net/http: fix Transport panic with nil Request.Header

For Go 1.13 we introduced Header.Clone and it returns
nil if a nil Header is cloned. Unfortunately, though,
this exported Header.Clone nil behavior differed from
the old Go 1.12 and earlier internal header clone
behavior which always returned non-nil Headers.
This CL fixes the places where that distinction mattered.

Fixes #34878

Change-Id: Id19dea2272948c8dd10883b18ea7f7b8b33ea8eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/200977
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 32 insertions, 0 deletions

diff --git a/src/net/http/header_test.go b/src/net/http/header_test.go
index 51fcab103b..4789362919 100644
--- a/src/net/http/header_test.go
+++ b/src/net/http/header_test.go
@@ -7,6 +7,7 @@ package http
 import (
 	"bytes"
 	"internal/race"
+	"reflect"
 	"runtime"
 	"testing"
 	"time"
@@ -219,3 +220,34 @@ func TestHeaderWriteSubsetAllocs(t *testing.T) {
 		t.Errorf("allocs = %g; want 0", n)
 	}
 }
+
+// Issue 34878: test that every call to
+// cloneOrMakeHeader never returns a nil Header.
+func TestCloneOrMakeHeader(t *testing.T) {
+	tests := []struct {
+		name     string
+		in, want Header
+	}{
+		{"nil", nil, Header{}},
+		{"empty", Header{}, Header{}},
+		{
+			name: "non-empty",
+			in:   Header{"foo": {"bar"}},
+			want: Header{"foo": {"bar"}},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := cloneOrMakeHeader(tt.in)
+			if got == nil {
+				t.Fatal("unexpected nil Header")
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Fatalf("Got:  %#v\nWant: %#v", got, tt.want)
+			}
+			got.Add("A", "B")
+			got.Get("A")
+		})
+	}
+}