Support Go time format strings in permalinks

In the vein of an ancient TODO about supporting custom date formatting with
strftime, this allows `:`-prefixed Go time format strings in permalink segments.
This allows users to customize date-based permalinks any way they need to.

For example, with a date of 2019-11-09, the permalink `/:06/:1/:2` will render
as `/19/11/9`.

See:

07978e4a49 (diff-0688a3b65c7f5d01aa216f8d9b57fd00R111-R112)
https://discourse.gohugo.io/t/implementing-additional-date-formats-for-permalinks/17860
https://github.com/gohugoio/hugo/pull/6488

resources/page/permalinks.go

@@ -20,6 +20,7 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/pkg/errors"
 
@@ -38,6 +39,24 @@ type PermalinkExpander struct {
 	ps *helpers.PathSpec
 }
 
+// Time for checking date formats. Every field is different than the
+// Go reference time for date formatting. This ensures that formatting this date
+// with a Go time format always has a different output than the format itself.
+var referenceTime = time.Date(2019, time.November, 9, 23, 1, 42, 1, time.UTC)
+
+// Return the callback for the given permalink attribute and a boolean indicating if the attribute is valid or not.
+func (p PermalinkExpander) callback(attr string) (pageToPermaAttribute, bool) {
+	if callback, ok := p.knownPermalinkAttributes[attr]; ok {
+		return callback, true
+	}
+
+	if referenceTime.Format(attr) != attr {
+		return p.pageToPermalinkDate, true
+	}
+
+	return nil, false
+}
+
 // NewPermalinkExpander creates a new PermalinkExpander configured by the given
 // PathSpec.
 func NewPermalinkExpander(ps *helpers.PathSpec) (PermalinkExpander, error) {
@@ -109,7 +128,7 @@ func (l PermalinkExpander) parse(patterns map[string]string) (map[string]func(Pa
 			replacement := m[0]
 			attr := replacement[1:]
 			replacements[i] = replacement
-			callback, ok := l.knownPermalinkAttributes[attr]
+			callback, ok := l.callback(attr)
 
 			if !ok {
 				return nil, &permalinkExpandError{pattern: pattern, err: errPermalinkAttributeUnknown}
@@ -173,8 +192,8 @@ func (l PermalinkExpander) validate(pp string) bool {
 		}
 
 		for _, match := range matches {
-			k := strings.ToLower(match[0][1:])
-			if _, ok := l.knownPermalinkAttributes[k]; !ok {
+			k := match[0][1:]
+			if _, ok := l.callback(k); !ok {
 				return false
 			}
 		}
@@ -214,9 +233,8 @@ func (l PermalinkExpander) pageToPermalinkDate(p Page, dateField string) (string
 	case "yearday":
 		return strconv.Itoa(p.Date().YearDay()), nil
 	}
-	//TODO: support classic strftime escapes too
-	// (and pass those through despite not being in the map)
-	panic("coding error: should not be here")
+
+	return p.Date().Format(dateField), nil
 }
 
 // pageToPermalinkTitle returns the URL-safe form of the title

resources/page/permalinks_test.go

@@ -32,16 +32,21 @@ var testdataPermalinks = []struct {
 	{":title", true, "spf13-vim-3.0-release-and-new-website"},
 	{"/:year-:month-:title", true, "/2012-04-spf13-vim-3.0-release-and-new-website"},
 	{"/:year/:yearday/:month/:monthname/:day/:weekday/:weekdayname/", true, "/2012/97/04/April/06/5/Friday/"}, // Dates
-	{"/:section/", true, "/blue/"},                                // Section
-	{"/:title/", true, "/spf13-vim-3.0-release-and-new-website/"}, // Title
-	{"/:slug/", true, "/the-slug/"},                               // Slug
-	{"/:filename/", true, "/test-page/"},                          // Filename
+	{"/:section/", true, "/blue/"},                                  // Section
+	{"/:title/", true, "/spf13-vim-3.0-release-and-new-website/"},   // Title
+	{"/:slug/", true, "/the-slug/"},                                 // Slug
+	{"/:filename/", true, "/test-page/"},                            // Filename
+	{"/:06-:1-:2-:Monday", true, "/12-4-6-Friday"},                  // Dates with Go formatting
+	{"/:2006_01_02_15_04_05.000", true, "/2012_04_06_03_01_59.000"}, // Complicated custom date format
 	// TODO(moorereason): need test scaffolding for this.
 	//{"/:sections/", false, "/blue/"},                              // Sections
 
 	// Failures
 	{"/blog/:fred", false, ""},
 	{"/:year//:title", false, ""},
+	{"/:TITLE", false, ""},      // case is not normalized
+	{"/:2017", false, ""},       // invalid date format
+	{"/:2006-01-02", false, ""}, // valid date format but invalid attribute name
 }
 
 func TestPermalinkExpansion(t *testing.T) {
@@ -51,7 +56,7 @@ func TestPermalinkExpansion(t *testing.T) {
 
 	page := newTestPageWithFile("/test-page/index.md")
 	page.title = "Spf13 Vim 3.0 Release and new website"
-	d, _ := time.Parse("2006-01-02", "2012-04-06")
+	d, _ := time.Parse("2006-01-02 15:04:05", "2012-04-06 03:01:59")
 	page.date = d
 	page.section = "blue"
 	page.slug = "The Slug"