Pass minification errors to the user

Previously, *minifyTransformation.Transform suppressed the
error returned by t.m.Minify. This meant that when minification
returned an error, the error would not reach the user. Instead,
minification would silently fail. For example, if a JavaScript
file included a call to the Date constructor with:

new Date(2020, 04, 02)

The package that the minification library uses to parse JS files,
github.com/tdewolff/parse would return an error, since "04" would
be parsed as a legacy octal. However, the JS file would remain
un-minified with no error.

Fixing this is not as simple as replacing "_" with an "err" in
*minifyTransformation.Transform, however (though this is
necessary). If we only returned this error from Transform,
then hugolib.TestResourceMinifyDisabled would fail. Instead of
being a no-op, as TestResourceMinifyDisabled expects, using the
"minify" template function with a "disableXML=true" config
setting instead returns the error, "minifier does not exist for
mimetype."

The "minifier does not exist" error is returned because of the
way minifiers.New works. If the user's config disables
minification for a particular MIME type, minifiers.New does
not add it to the resulting Client's *minify.M. However, this
also means that when the "minify" template function is executed,
 a *resourceAdapter's transformations still add a minification.
When it comes time to call the minify.Minifier for a specific
MIME type via *M.MinifyMimetype, the github.com/tdewolff/minify
library throws the "does not exist" error for the missing MIME
type.

The solution was to change minifiers.New so, instead of skipping
a minifier for each disabled MIME type, it adds  a NoOpMinifier,
which simply copies the source to the destination without
minification. This means that when the "minify" template
function is used for a particular resource, and that resource's
MIME type has minification disabled, minification is genuinely
skipped, and does not result in an error.

In order to add this, I've fixed a possibly unwanted interaction
between minifiers.TestConfigureMinify and
hugolib.TestResourceMinifyDisabled. The latter disables
minification and expects minification to be a no-op. The former
disables minification and expects it to result in an error. The
only reason hugolib.TestResourceMinifyDisabled passes in the
original code is that the "does not exist" error is suppressed.
However, we shouldn't suppress minification errors, since they
can leave users perplexed. I've changed the test assertion in
minifiers.TestConfigureMinify to expect no errors and a no-op
if minification is disabled for a particular MIME type.

Fixes #8954

2 files changed, 52 insertions, 32 deletions

diff --git a/minifiers/minifiers.go b/minifiers/minifiers.go
index 3ac285ecb..5a5cec121 100644
--- a/minifiers/minifiers.go
+++ b/minifiers/minifiers.go
@@ -57,6 +57,20 @@ func (m Client) Minify(mediatype media.Type, dst io.Writer, src io.Reader) error
 	return m.m.Minify(mediatype.Type(), dst, src)
 }
 
+// noopMinifier implements minify.Minifier [1], but doesn't minify content. This means
+// that we can avoid missing minifiers for any MIME types in our minify.M, which
+// causes minify to return errors, while still allowing minification to be
+// disabled for specific types.
+//
+// [1]: https://pkg.go.dev/github.com/tdewolff/minify#Minifier
+type noopMinifier struct{}
+
+// Minify copies r into w without transformation.
+func (m noopMinifier) Minify(_ *minify.M, w io.Writer, r io.Reader, _ map[string]string) error {
+	_, err := io.Copy(w, r)
+	return err
+}
+
 // New creates a new Client with the provided MIME types as the mapping foundation.
 // The HTML minifier is also registered for additional HTML types (AMP etc.) in the
 // provided list of output formats.
@@ -69,47 +83,53 @@ func New(mediaTypes media.Types, outputFormats output.Formats, cfg config.Provid
 	}
 
 	// We use the Type definition of the media types defined in the site if found.
-	if !conf.DisableCSS {
-		addMinifier(m, mediaTypes, "css", &conf.Tdewolff.CSS)
-	}
-	if !conf.DisableJS {
-		addMinifier(m, mediaTypes, "js", &conf.Tdewolff.JS)
-		m.AddRegexp(regexp.MustCompile("^(application|text)/(x-)?(java|ecma)script$"), &conf.Tdewolff.JS)
-	}
-	if !conf.DisableJSON {
-		addMinifier(m, mediaTypes, "json", &conf.Tdewolff.JSON)
-		m.AddRegexp(regexp.MustCompile(`^(application|text)/(x-|(ld|manifest)\+)?json$`), &conf.Tdewolff.JSON)
-	}
-	if !conf.DisableSVG {
-		addMinifier(m, mediaTypes, "svg", &conf.Tdewolff.SVG)
-	}
-	if !conf.DisableXML {
-		addMinifier(m, mediaTypes, "xml", &conf.Tdewolff.XML)
-	}
+	addMinifier(m, mediaTypes, "css", getMinifier(conf, "css"))
+
+	addMinifier(m, mediaTypes, "js", getMinifier(conf, "js"))
+	m.AddRegexp(regexp.MustCompile("^(application|text)/(x-)?(java|ecma)script$"), getMinifier(conf, "js"))
+
+	addMinifier(m, mediaTypes, "json", getMinifier(conf, "json"))
+	m.AddRegexp(regexp.MustCompile(`^(application|text)/(x-|(ld|manifest)\+)?json$`), getMinifier(conf, "json"))
+
+	addMinifier(m, mediaTypes, "svg", getMinifier(conf, "svg"))
+
+	addMinifier(m, mediaTypes, "xml", getMinifier(conf, "xml"))
 
 	// HTML
-	if !conf.DisableHTML {
-		addMinifier(m, mediaTypes, "html", &conf.Tdewolff.HTML)
-		for _, of := range outputFormats {
-			if of.IsHTML {
-				m.Add(of.MediaType.Type(), &conf.Tdewolff.HTML)
-			}
+	addMinifier(m, mediaTypes, "html", getMinifier(conf, "html"))
+	for _, of := range outputFormats {
+		if of.IsHTML {
+			m.Add(of.MediaType.Type(), getMinifier(conf, "html"))
 		}
 	}
 
 	return Client{m: m, MinifyOutput: conf.MinifyOutput}, nil
 }
 
-func addMinifier(m *minify.M, mt media.Types, suffix string, min minify.Minifier) {
-	types := mt.BySuffix(suffix)
-	for _, t := range types {
-		m.Add(t.Type(), min)
+// getMinifier returns the appropriate minify.MinifierFunc for the MIME
+// type suffix s, given the config c.
+func getMinifier(c minifyConfig, s string) minify.Minifier {
+	switch {
+	case s == "css" && !c.DisableCSS:
+		return &c.Tdewolff.CSS
+	case s == "js" && !c.DisableJS:
+		return &c.Tdewolff.JS
+	case s == "json" && !c.DisableJSON:
+		return &c.Tdewolff.JSON
+	case s == "svg" && !c.DisableSVG:
+		return &c.Tdewolff.SVG
+	case s == "xml" && !c.DisableXML:
+		return &c.Tdewolff.XML
+	case s == "html" && !c.DisableHTML:
+		return &c.Tdewolff.HTML
+	default:
+		return noopMinifier{}
 	}
 }
 
-func addMinifierFunc(m *minify.M, mt media.Types, suffix string, min minify.MinifierFunc) {
+func addMinifier(m *minify.M, mt media.Types, suffix string, min minify.Minifier) {
 	types := mt.BySuffix(suffix)
 	for _, t := range types {
-		m.AddFunc(t.Type(), min)
+		m.Add(t.Type(), min)
 	}
 }
diff --git a/minifiers/minifiers_test.go b/minifiers/minifiers_test.go
index 81edba510..ece8cbd08 100644
--- a/minifiers/minifiers_test.go
+++ b/minifiers/minifiers_test.go
@@ -93,9 +93,9 @@ func TestConfigureMinify(t *testing.T) {
 		expectedMinString string
 		errorExpected     bool
 	}{
-		{media.HTMLType, "<hello> Hugo! </hello>", "<hello> Hugo! </hello>", false}, // configured minifier
-		{media.CSSType, " body { color: blue; }  ", "body{color:blue}", false},      // default minifier
-		{media.XMLType, " <hello>  Hugo!   </hello>  ", "", true},                   // disable Xml minification
+		{media.HTMLType, "<hello> Hugo! </hello>", "<hello> Hugo! </hello>", false},            // configured minifier
+		{media.CSSType, " body { color: blue; }  ", "body{color:blue}", false},                 // default minifier
+		{media.XMLType, " <hello>  Hugo!   </hello>  ", " <hello>  Hugo!   </hello>  ", false}, // disable Xml minification
 	} {
 		var b bytes.Buffer
 		if !test.errorExpected {