flamegraph: give the page a favicon

internal/flamegraph/assets.go

@@ -25,6 +25,19 @@ import (
 // one line of warning. The legend, the keyboard map, the notes and the label
 // table are all one keypress (or one icon) away.
 
+// faviconHRef is the perf-agent mark, inlined as a data: URI so the page
+// keeps its one-file contract -- a <link rel="icon" href="brand/..."> would
+// render a broken tab icon from file:// and leak a request everywhere else.
+//
+// It is the 16px mark from docs/brand/perf-agent-favicon.svg, which is drawn
+// for this size rather than scaled down from the full one: the full mark
+// loses its top rows below about 24px and reads as a smudge in a tab strip.
+//
+// Base64 rather than percent-encoding because the markup contains #, < and
+// " -- the three characters that make a raw SVG data: URI fragile inside an
+// HTML attribute.
+const faviconHRef = "data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZpZXdCb3g9IjAgMCAxNiAxNiI+PHJlY3QgeD0iMSIgeT0iMTEiIHdpZHRoPSIxNCIgaGVpZ2h0PSI0IiByeD0iMS4zIiBmaWxsPSIjZDE0OTdmIi8+PHJlY3QgeD0iMSIgeT0iNiIgd2lkdGg9IjYiIGhlaWdodD0iNCIgcng9IjEuMyIgZmlsbD0iI2U4ODczYSIvPjxyZWN0IHg9IjgiIHk9IjYiIHdpZHRoPSI3IiBoZWlnaHQ9IjQiIHJ4PSIxLjMiIGZpbGw9IiMzZmEwYWIiLz48cmVjdCB4PSI5IiB5PSIxIiB3aWR0aD0iNiIgaGVpZ2h0PSI0IiByeD0iMS4zIiBmaWxsPSIjNDRhODVlIi8+PC9zdmc+"
+
 // darkChrome is the page chrome's dark theme, written once and emitted
 // twice: once behind prefers-color-scheme for readers who never touch the
 // D key, once behind [data-theme="dark"] for readers who do. Two copies of

internal/flamegraph/render.go

@@ -141,7 +141,8 @@ func RenderHTML(w io.Writer, res *foldedstacks.Result, opts Options) error {
 	degenerate := res.Degenerate()
 
 	ew.s("<!DOCTYPE html>\n<html lang=\"en\">\n<head>\n<meta charset=\"utf-8\">\n")
-	ew.s("<meta name=\"viewport\" content=\"width=device-width, initial-scale=1\">\n<title>")
+	ew.s("<meta name=\"viewport\" content=\"width=device-width, initial-scale=1\">\n")
+	ew.s("<link rel=\"icon\" href=\"" + faviconHRef + "\">\n<title>")
 	ew.esc(opts.Title)
 	ew.s("</title>\n<style>\n")
 	ew.s(styleSheet)

internal/flamegraph/render_test.go

@@ -70,12 +70,26 @@ func TestRenderHTMLFetchesNothing(t *testing.T) {
 	), Options{})
 
 	for _, forbidden := range []string{
-		"<script src", "<link ", "@import", "url(http", "url('http", "url(\"http",
+		"<script src", "@import", "url(http", "url('http", "url(\"http",
 		"fonts.googleapis.com", "fonts.gstatic.com", "cdn.", "fetch(", "XMLHttpRequest",
 		"<img", "<iframe",
 	} {
 		assert.NotContains(t, got, forbidden, "the page must load nothing from the network")
 	}
+
+	// <link> is allowed only when it fetches nothing. The favicon is one, and
+	// it is inlined as a data: URI; a stylesheet or an icon by path would
+	// both be a network request from anywhere but the directory it was
+	// written in, which is the whole point of the one-file contract. So the
+	// rule is not "no <link>" but "no <link> that resolves off-page".
+	links := regexp.MustCompile(`<link\b[^>]*>`).FindAllString(got, -1)
+	for _, link := range links {
+		href := regexp.MustCompile(`href="([^"]*)"`).FindStringSubmatch(link)
+		if assert.Len(t, href, 2, "every <link> must carry an href: %s", link) {
+			assert.True(t, strings.HasPrefix(href[1], "data:"),
+				"a <link> must resolve to a data: URI, got %q", href[1])
+		}
+	}
 	// Not one URL of any kind is left in the document. There used to be
 	// exactly one — the SVG XML namespace, an identifier rather than a fetch
 	// — and the geometry that needed it is gone.