Move context package internal

Our context package predates the establishment of current best practices
regarding context usage and it shows. It encourages bad practices such
as using contexts to propagate non-request-scoped values like the
application version and using string-typed keys for context values. Move
the package internal to remove it from the API surface of
distribution/v3@v3.0.0 so we are free to iterate on it without being
constrained by compatibility.

Signed-off-by: Cory Snider <csnider@mirantis.com>

internal/dcontext/trace_test.go

@@ -0,0 +1,103 @@
+package dcontext
+
+import (
+	"runtime"
+	"testing"
+	"time"
+)
+
+// TestWithTrace ensures that tracing has the expected values in the context.
+func TestWithTrace(t *testing.T) {
+	t.Parallel()
+	pc, file, _, _ := runtime.Caller(0) // get current caller.
+	f := runtime.FuncForPC(pc)
+
+	base := []valueTestCase{
+		{
+			key:           "trace.id",
+			notnilorempty: true,
+		},
+
+		{
+			key:           "trace.file",
+			expected:      file,
+			notnilorempty: true,
+		},
+		{
+			key:           "trace.line",
+			notnilorempty: true,
+		},
+		{
+			key:           "trace.start",
+			notnilorempty: true,
+		},
+	}
+
+	ctx, done := WithTrace(Background())
+	defer done("this will be emitted at end of test")
+
+	tests := append(base, valueTestCase{
+		key:      "trace.func",
+		expected: f.Name(),
+	})
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.key, func(t *testing.T) {
+			t.Parallel()
+			v := ctx.Value(tc.key)
+			if tc.notnilorempty {
+				if v == nil || v == "" {
+					t.Fatalf("value was nil or empty: %#v", v)
+				}
+				return
+			}
+
+			if v != tc.expected {
+				t.Fatalf("unexpected value: %v != %v", v, tc.expected)
+			}
+		})
+	}
+
+	tracedFn := func() {
+		parentID := ctx.Value("trace.id") // ensure the parent trace id is correct.
+
+		pc, _, _, _ := runtime.Caller(0) // get current caller.
+		f := runtime.FuncForPC(pc)
+		ctx, done := WithTrace(ctx)
+		defer done("this should be subordinate to the other trace")
+		time.Sleep(time.Second)
+		tests := append(base, valueTestCase{
+			key:      "trace.func",
+			expected: f.Name(),
+		}, valueTestCase{
+			key:      "trace.parent.id",
+			expected: parentID,
+		})
+		for _, tc := range tests {
+			tc := tc
+			t.Run(tc.key, func(t *testing.T) {
+				t.Parallel()
+				v := ctx.Value(tc.key)
+				if tc.notnilorempty {
+					if v == nil || v == "" {
+						t.Fatalf("value was nil or empty: %#v", v)
+					}
+					return
+				}
+
+				if v != tc.expected {
+					t.Fatalf("unexpected value: %v != %v", v, tc.expected)
+				}
+			})
+		}
+	}
+	tracedFn()
+
+	time.Sleep(time.Second)
+}
+
+type valueTestCase struct {
+	key           string
+	expected      interface{}
+	notnilorempty bool // just check not empty/not nil
+}