flattener: Recompile with latest gopherjs (#1635)

docs/flattener/js.go

@@ -100,7 +100,7 @@ func renderResults() {
 		content += fmt.Sprintf(`
 <h3> %s flattened (length %d, %d lookups)</h3><code>%s</code>	
 `, mode, len(flat.TXT()), lookups, flat.TXT())
-		split := flat.TXTSplit("_spf%d." + domain)
+		split := flat.TXTSplit("_spf%d."+domain, 0, 255)
 		if len(split) > 1 {
 			lookups += len(split) - 1
 			content += fmt.Sprintf("<h3>%s flattened split (%d lookups)</h3>", mode, lookups)

docs/release-engineering.md

@@ -197,6 +197,34 @@ If you are at Stack Overflow:
   * Run this build: `dnscontrol_embed - Promote most recent artifact into ExternalDNS repo`
 
 
+# Tip: How to rebuild flattener
+
+Rebuilding flatter requites go1.17.1 and the gopherjs compiler.
+
+Install go1.17.1:
+
+```
+go install golang.org/dl/go1.17.1@latest
+go1.17.1 download
+```
+
+Install gopherjs:
+
+```
+go install github.com/gopherjs/gopherjs@latest
+```
+
+Build the software:
+
+NOTE: GOOS can't be Darwin because GOPHERJS doesn't support it.
+
+```
+export GOPHERJS_GOROOT="$(go1.17.1 env GOROOT)"
+export GOOS=linux
+gopherjs build
+```
+
+
 # Tip: How to update modules
 
 List out-of-date modules and update any that seem worth updating: