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/context.go

@@ -0,0 +1,73 @@
+package dcontext
+
+import (
+	"context"
+	"sync"
+
+	"github.com/google/uuid"
+)
+
+// instanceContext is a context that provides only an instance id. It is
+// provided as the main background context.
+type instanceContext struct {
+	context.Context
+	id   string    // id of context, logged as "instance.id"
+	once sync.Once // once protect generation of the id
+}
+
+func (ic *instanceContext) Value(key interface{}) interface{} {
+	if key == "instance.id" {
+		ic.once.Do(func() {
+			// We want to lazy initialize the UUID such that we don't
+			// call a random generator from the package initialization
+			// code. For various reasons random could not be available
+			// https://github.com/distribution/distribution/issues/782
+			ic.id = uuid.NewString()
+		})
+		return ic.id
+	}
+
+	return ic.Context.Value(key)
+}
+
+var background = &instanceContext{
+	Context: context.Background(),
+}
+
+// Background returns a non-nil, empty Context. The background context
+// provides a single key, "instance.id" that is globally unique to the
+// process.
+func Background() context.Context {
+	return background
+}
+
+// stringMapContext is a simple context implementation that checks a map for a
+// key, falling back to a parent if not present.
+type stringMapContext struct {
+	context.Context
+	m map[string]interface{}
+}
+
+// WithValues returns a context that proxies lookups through a map. Only
+// supports string keys.
+func WithValues(ctx context.Context, m map[string]interface{}) context.Context {
+	mo := make(map[string]interface{}, len(m)) // make our own copy.
+	for k, v := range m {
+		mo[k] = v
+	}
+
+	return stringMapContext{
+		Context: ctx,
+		m:       mo,
+	}
+}
+
+func (smc stringMapContext) Value(key interface{}) interface{} {
+	if ks, ok := key.(string); ok {
+		if v, ok := smc.m[ks]; ok {
+			return v
+		}
+	}
+
+	return smc.Context.Value(key)
+}