libnet/internal/resolvconf: optimize Generate() without text/template

Rewrite this function to not use text/template, which is ~74× faster,
~25× less memory, and ~56× fewer allocations.

Before/After:

    BenchmarkGenerate-10       42550     27439 ns/op    18083 B/op    394 allocs/op
    BenchmarkGenerate-10     3148328       373 ns/op      712 B/op      7 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/libnetwork/internal/resolvconf/resolvconf.go

@@ -21,7 +21,6 @@ import (
 	"bufio"
 	"bytes"
 	"context"
-	"fmt"
 	"io"
 	"io/fs"
 	"net/netip"
@@ -29,7 +28,6 @@ import (
 	"slices"
 	"strconv"
 	"strings"
-	"text/template"
 
 	"github.com/containerd/log"
 	"github.com/moby/sys/atomicwriter"
@@ -71,7 +69,7 @@ type ExtDNSEntry struct {
 
 func (ed ExtDNSEntry) String() string {
 	if ed.HostLoopback {
-		return fmt.Sprintf("host(%s)", ed.Addr)
+		return "host(" + ed.Addr.String() + ")"
 	}
 	return ed.Addr.String()
 }
@@ -291,70 +289,106 @@ func (rc *ResolvConf) TransformForIntNS(
 // is true, the file will include header information if supplied, and a trailing
 // comment that describes how the file was constructed and lists external resolvers.
 func (rc *ResolvConf) Generate(comments bool) ([]byte, error) {
-	s := struct {
-		Md          *metadata
-		NameServers []netip.Addr
-		Search      []string
-		Options     []string
-		Other       []string
-		Overrides   []string
-		Comments    bool
-	}{
-		Md:          &rc.md,
-		NameServers: rc.nameServers,
-		Search:      rc.search,
-		Options:     rc.options,
-		Other:       rc.other,
-		Comments:    comments,
+	var b bytes.Buffer
+	b.Grow(512) // estimated size for a regular resolv.conf we produce.
+
+	if comments && rc.md.Header != "" {
+		b.WriteString(rc.md.Header + "\n")
+		b.WriteByte('\n')
 	}
-	if rc.md.NSOverride {
-		s.Overrides = append(s.Overrides, "nameservers")
+	for _, ns := range rc.nameServers {
+		b.WriteString("nameserver ")
+		b.WriteString(ns.String())
+		b.WriteByte('\n')
 	}
-	if rc.md.SearchOverride {
-		s.Overrides = append(s.Overrides, "search")
+	if len(rc.search) > 0 {
+		b.WriteString("search ")
+		for i, s := range rc.search {
+			if i > 0 {
+				b.WriteByte(' ')
+			}
+			b.WriteString(s)
+		}
+		b.WriteByte('\n')
 	}
-	if rc.md.OptionsOverride {
-		s.Overrides = append(s.Overrides, "options")
+	if len(rc.options) > 0 {
+		b.WriteString("options ")
+		for i, s := range rc.options {
+			if i > 0 {
+				b.WriteByte(' ')
+			}
+			b.WriteString(s)
+		}
+		b.WriteByte('\n')
+	}
+	for _, o := range rc.other {
+		b.WriteString(o)
+		b.WriteByte('\n')
 	}
 
-	const templateText = `{{if .Comments}}{{with .Md.Header}}{{.}}
+	if comments {
+		b.WriteByte('\n')
+		b.WriteString("# Based on host file: '" + rc.md.SourcePath + "'")
+		if rc.md.Transform != "" {
+			b.WriteString(" (" + rc.md.Transform + ")")
+		}
+		b.WriteByte('\n')
+		for _, w := range rc.md.Warnings {
+			b.WriteString("# ")
+			b.WriteString(w)
+			b.WriteByte('\n')
+		}
+		if len(rc.md.ExtNameServers) > 0 {
+			b.WriteString("# ExtServers: [")
+			for i, ext := range rc.md.ExtNameServers {
+				if i > 0 {
+					b.WriteByte(' ')
+				}
+				b.WriteString(ext.String())
+			}
+			b.WriteByte(']')
+			b.WriteByte('\n')
+		}
+		if len(rc.md.InvalidNSs) > 0 {
+			b.WriteString("# Invalid nameservers: [")
+			for i, ext := range rc.md.InvalidNSs {
+				if i > 0 {
+					b.WriteByte(' ')
+				}
+				b.WriteString(ext)
+			}
+			b.WriteByte(']')
+			b.WriteByte('\n')
+		}
 
-{{end}}{{end}}{{range .NameServers -}}
-nameserver {{.}}
-{{end}}{{with .Search -}}
-search {{join . " "}}
-{{end}}{{with .Options -}}
-options {{join . " "}}
-{{end}}{{with .Other -}}
-{{join . "\n"}}
-{{end}}{{if .Comments}}
-# Based on host file: '{{.Md.SourcePath}}'{{with .Md.Transform}} ({{.}}){{end}}
-{{range .Md.Warnings -}}
-# {{.}}
-{{end -}}
-{{with .Md.ExtNameServers -}}
-# ExtServers: {{.}}
-{{end -}}
-{{with .Md.InvalidNSs -}}
-# Invalid nameservers: {{.}}
-{{end -}}
-# Overrides: {{.Overrides}}
-{{with .Md.NDotsFrom -}}
-# Option ndots from: {{.}}
-{{end -}}
-{{end -}}
-`
+		b.WriteString("# Overrides: [")
+		var overrides int
+		if rc.md.NSOverride {
+			b.WriteString("nameservers")
+			overrides++
+		}
+		if rc.md.SearchOverride {
+			if overrides > 0 {
+				b.WriteByte(' ')
+			}
+			b.WriteString("search")
+			overrides++
+		}
+		if rc.md.OptionsOverride {
+			if overrides > 0 {
+				b.WriteByte(' ')
+			}
+			b.WriteString("options")
+		}
+		b.WriteByte(']')
+		b.WriteByte('\n')
 
-	funcs := template.FuncMap{"join": strings.Join}
-	var buf bytes.Buffer
-	templ, err := template.New("summary").Funcs(funcs).Parse(templateText)
-	if err != nil {
-		return nil, errSystem{err}
+		if rc.md.NDotsFrom != "" {
+			b.WriteString("# Option ndots from: " + rc.md.NDotsFrom + "\n")
+		}
 	}
-	if err := templ.Execute(&buf, s); err != nil {
-		return nil, errSystem{err}
-	}
-	return buf.Bytes(), nil
+
+	return b.Bytes(), nil
 }
 
 // WriteFile generates content and writes it to path. If hashPath is non-zero, it