Add render template hooks for links and images

This commit also

* revises the change detection for templates used by content files in server mode.
* Adds a Page.RenderString method

Fixes #6545
Fixes #4663
Closes #6043

2 files changed, 173 insertions, 0 deletions

diff --git a/identity/identity.go b/identity/identity.go
new file mode 100644
index 000000000..d06710efe
--- /dev/null
+++ b/identity/identity.go
@@ -0,0 +1,131 @@
+package identity
+
+import (
+	"path/filepath"
+	"strings"
+	"sync"
+)
+
+// NewIdentityManager creates a new Manager starting at id.
+func NewManager(id Provider) Manager {
+	return &identityManager{
+		Provider: id,
+		ids:      Identities{id.GetIdentity(): id},
+	}
+}
+
+// NewPathIdentity creates a new Identity with the two identifiers
+// type and path.
+func NewPathIdentity(typ, pat string) PathIdentity {
+	pat = strings.ToLower(strings.TrimPrefix(filepath.ToSlash(pat), "/"))
+	return PathIdentity{Type: typ, Path: pat}
+}
+
+// Identities stores identity providers.
+type Identities map[Identity]Provider
+
+func (ids Identities) search(id Identity) Provider {
+	if v, found := ids[id]; found {
+		return v
+	}
+	for _, v := range ids {
+		switch t := v.(type) {
+		case IdentitiesProvider:
+			if nested := t.GetIdentities().search(id); nested != nil {
+				return nested
+			}
+		}
+	}
+	return nil
+}
+
+// IdentitiesProvider provides all Identities.
+type IdentitiesProvider interface {
+	GetIdentities() Identities
+}
+
+// Identity represents an thing that can provide an identify. This can be
+// any Go type, but the Identity returned by GetIdentify must be hashable.
+type Identity interface {
+	Provider
+	Name() string
+}
+
+// Manager manages identities, and is itself a Provider of Identity.
+type Manager interface {
+	IdentitiesProvider
+	Provider
+	Add(ids ...Provider)
+	Search(id Identity) Provider
+	Reset()
+}
+
+// A PathIdentity is a common identity identified by a type and a path, e.g. "layouts" and "_default/single.html".
+type PathIdentity struct {
+	Type string
+	Path string
+}
+
+// GetIdentity returns itself.
+func (id PathIdentity) GetIdentity() Identity {
+	return id
+}
+
+// Name returns the Path.
+func (id PathIdentity) Name() string {
+	return id.Path
+}
+
+// A KeyValueIdentity a general purpose identity.
+type KeyValueIdentity struct {
+	Key   string
+	Value string
+}
+
+// GetIdentity returns itself.
+func (id KeyValueIdentity) GetIdentity() Identity {
+	return id
+}
+
+// Name returns the Key.
+func (id KeyValueIdentity) Name() string {
+	return id.Key
+}
+
+// Provider provides the hashable Identity.
+type Provider interface {
+	GetIdentity() Identity
+}
+
+type identityManager struct {
+	sync.Mutex
+	Provider
+	ids Identities
+}
+
+func (im *identityManager) Add(ids ...Provider) {
+	im.Lock()
+	for _, id := range ids {
+		im.ids[id.GetIdentity()] = id
+	}
+	im.Unlock()
+}
+
+func (im *identityManager) Reset() {
+	im.Lock()
+	id := im.GetIdentity()
+	im.ids = Identities{id.GetIdentity(): id}
+	im.Unlock()
+}
+
+func (im *identityManager) GetIdentities() Identities {
+	im.Lock()
+	defer im.Unlock()
+	return im.ids
+}
+
+func (im *identityManager) Search(id Identity) Provider {
+	im.Lock()
+	defer im.Unlock()
+	return im.ids.search(id.GetIdentity())
+}
diff --git a/identity/identity_test.go b/identity/identity_test.go
new file mode 100644
index 000000000..adebcad91
--- /dev/null
+++ b/identity/identity_test.go
@@ -0,0 +1,42 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package identity
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestIdentityManager(t *testing.T) {
+	c := qt.New(t)
+
+	id1 := testIdentity{name: "id1"}
+	im := NewManager(id1)
+
+	c.Assert(im.Search(id1).GetIdentity(), qt.Equals, id1)
+	c.Assert(im.Search(testIdentity{name: "notfound"}), qt.Equals, nil)
+}
+
+type testIdentity struct {
+	name string
+}
+
+func (id testIdentity) GetIdentity() Identity {
+	return id
+}
+
+func (id testIdentity) Name() string {
+	return id.name
+}