Allow user to handle/ignore errors in resources.Get

In Hugo 0.90.0 we introduced remote support in `resources.Get`.

But with remote resources comes with a higher chance of failing a build (network issues, remote server down etc.).

Before this commit we always failed the build on any unexpected error.

This commit allows the user to check for any error (and potentially fall back to a default local resource):

```htmlbars
{{ $result := resources.Get "https://gohugo.io/img/hugo-logo.png" }}
{{ with $result }}
        {{ if .Err }}
        {{/* log the error, insert a default image etc. *}}
        {{ else }}
        <img src="{{ .RelPermalink }}" width="{{ .Width }}" height="{{ .Height }}" alt="">
        {{ end }}
{{ end }}
```

Note that the default behaviour is still to fail the build, but we will delay that error until you start using the `Resource`.

Fixes #9529

7 files changed, 146 insertions, 3 deletions

diff --git a/resources/errorResource.go b/resources/errorResource.go
new file mode 100644
index 000000000..705547d4c
--- /dev/null
+++ b/resources/errorResource.go
@@ -0,0 +1,129 @@
+// Copyright 2021 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 resources
+
+import (
+	"image"
+
+	"github.com/gohugoio/hugo/common/hugio"
+	"github.com/gohugoio/hugo/common/maps"
+	"github.com/gohugoio/hugo/media"
+
+	"github.com/gohugoio/hugo/resources/images/exif"
+
+	"github.com/gohugoio/hugo/resources/resource"
+)
+
+var (
+	_ error = (*errorResource)(nil)
+	// Imnage covers all current Resource implementations.
+	_ resource.Image = (*errorResource)(nil)
+	// The list of user facing and exported interfaces in resource.go
+	// Note that if we're missing some interface here, the user will still
+	// get an error, but not as pretty.
+	_ resource.ContentResource         = (*errorResource)(nil)
+	_ resource.ReadSeekCloserResource  = (*errorResource)(nil)
+	_ resource.ResourcesLanguageMerger = (*resource.Resources)(nil)
+	// Make sure it also fails when passed to a pipe function.
+	_ ResourceTransformer = (*errorResource)(nil)
+)
+
+// NewErrorResource wraps err in a Resource where all but the Err method will panic.
+func NewErrorResource(err error) resource.Resource {
+	return &errorResource{error: err}
+}
+
+type errorResource struct {
+	error
+}
+
+func (e *errorResource) Err() error {
+	return e.error
+}
+
+func (e *errorResource) ReadSeekCloser() (hugio.ReadSeekCloser, error) {
+	panic(e.error)
+}
+
+func (e *errorResource) Content() (interface{}, error) {
+	panic(e.error)
+}
+
+func (e *errorResource) ResourceType() string {
+	panic(e.error)
+}
+
+func (e *errorResource) MediaType() media.Type {
+	panic(e.error)
+}
+
+func (e *errorResource) Permalink() string {
+	panic(e.error)
+}
+
+func (e *errorResource) RelPermalink() string {
+	panic(e.error)
+}
+
+func (e *errorResource) Name() string {
+	panic(e.error)
+}
+
+func (e *errorResource) Title() string {
+	panic(e.error)
+}
+
+func (e *errorResource) Params() maps.Params {
+	panic(e.error)
+}
+
+func (e *errorResource) Data() interface{} {
+	panic(e.error)
+}
+
+func (e *errorResource) Height() int {
+	panic(e.error)
+}
+
+func (e *errorResource) Width() int {
+	panic(e.error)
+}
+
+func (e *errorResource) Fill(spec string) (resource.Image, error) {
+	panic(e.error)
+}
+
+func (e *errorResource) Fit(spec string) (resource.Image, error) {
+	panic(e.error)
+}
+
+func (e *errorResource) Resize(spec string) (resource.Image, error) {
+	panic(e.error)
+}
+
+func (e *errorResource) Filter(filters ...interface{}) (resource.Image, error) {
+	panic(e.error)
+}
+
+func (e *errorResource) Exif() *exif.Exif {
+	panic(e.error)
+}
+
+func (e *errorResource) DecodeImage() (image.Image, error) {
+	panic(e.error)
+}
+
+func (e *errorResource) Transform(...ResourceTransformation) (ResourceTransformer, error) {
+	panic(e.error)
+}
diff --git a/resources/page/page_nop.go b/resources/page/page_nop.go
index 94515d27d..4f91883a0 100644
--- a/resources/page/page_nop.go
+++ b/resources/page/page_nop.go
@@ -48,6 +48,10 @@ var (
 // PageNop implements Page, but does nothing.
 type nopPage int
 
+func (p *nopPage) Err() error {
+	return nil
+}
+
 func (p *nopPage) Aliases() []string {
 	return nil
 }
diff --git a/resources/page/testhelpers_test.go b/resources/page/testhelpers_test.go
index 1a0a6586a..a21ab2ff3 100644
--- a/resources/page/testhelpers_test.go
+++ b/resources/page/testhelpers_test.go
@@ -29,7 +29,6 @@ import (
 	"github.com/bep/gitmap"
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/resources/resource"
-	
 
 	"github.com/gohugoio/hugo/navigation"
 
@@ -121,6 +120,10 @@ type testPage struct {
 	sectionEntries []string
 }
 
+func (p *testPage) Err() error {
+	return nil
+}
+
 func (p *testPage) Aliases() []string {
 	panic("not implemented")
 }
diff --git a/resources/resource.go b/resources/resource.go
index 28b9a8879..1f6246859 100644
--- a/resources/resource.go
+++ b/resources/resource.go
@@ -230,6 +230,10 @@ func (l *genericResource) Content() (interface{}, error) {
 	return l.content, nil
 }
 
+func (r *genericResource) Err() error {
+	return nil
+}
+
 func (l *genericResource) Data() interface{} {
 	return l.data
 }
diff --git a/resources/resource/resourcetypes.go b/resources/resource/resourcetypes.go
index 8ab77e436..788cdb86a 100644
--- a/resources/resource/resourcetypes.go
+++ b/resources/resource/resourcetypes.go
@@ -45,6 +45,7 @@ type Resource interface {
 	ResourceMetaProvider
 	ResourceParamsProvider
 	ResourceDataProvider
+	Err() error
 }
 
 // Image represents an image resource.
diff --git a/resources/resource_factories/create/create.go b/resources/resource_factories/create/create.go
index dc03568ac..f7bde9ee6 100644
--- a/resources/resource_factories/create/create.go
+++ b/resources/resource_factories/create/create.go
@@ -35,7 +35,6 @@ import (
 	"github.com/gohugoio/hugo/hugofs"
 
 	"github.com/gohugoio/hugo/cache/filecache"
-	"github.com/gohugoio/hugo/common/herrors"
 	"github.com/gohugoio/hugo/common/hugio"
 	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/common/types"
@@ -155,7 +154,6 @@ func (c *Client) FromString(targetPath, content string) (resource.Resource, erro
 // FromRemote expects one or n-parts of a URL to a resource
 // If you provide multiple parts they will be joined together to the final URL.
 func (c *Client) FromRemote(uri string, options map[string]interface{}) (resource.Resource, error) {
-	defer herrors.Recover()
 	rURL, err := url.Parse(uri)
 	if err != nil {
 		return nil, errors.Wrapf(err, "failed to parse URL for resource %s", uri)
diff --git a/resources/transform.go b/resources/transform.go
index 0d555b2b5..0569fb35e 100644
--- a/resources/transform.go
+++ b/resources/transform.go
@@ -167,6 +167,10 @@ func (r *resourceAdapter) Content() (interface{}, error) {
 	return r.target.Content()
 }
 
+func (r *resourceAdapter) Err() error {
+	return nil
+}
+
 func (r *resourceAdapter) Data() interface{} {
 	r.init(false, false)
 	return r.target.Data()