Rewrite clean-nginx-conf.sh in Go to speed up admission webhook (#7076) (#7322)

* Rewrite clean-nginx-conf.sh to speed up admission webhook

* Less diff with original clean-nginx-conf.sh

* Add error handling, add documentation, add unit test

* indent code

* Don't ignore Getwd() error

internal/ingress/controller/template/template.go

@@ -23,12 +23,12 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"math/rand" // #nosec
 	"net"
 	"net/url"
 	"os"
-	"os/exec"
 	"reflect"
 	"regexp"
 	"sort"
@@ -50,9 +50,15 @@ import (
 )
 
 const (
-	slash         = "/"
-	nonIdempotent = "non_idempotent"
-	defBufferSize = 65535
+	slash                   = "/"
+	nonIdempotent           = "non_idempotent"
+	defBufferSize           = 65535
+	writeIndentOnEmptyLines = true // backward-compatibility
+)
+
+const (
+	stateCode = iota
+	stateComment
 )
 
 // TemplateWriter is the interface to render a template
@@ -86,6 +92,87 @@ func NewTemplate(file string) (*Template, error) {
 	}, nil
 }
 
+// 1. Removes carriage return symbol (\r)
+// 2. Collapses multiple empty lines to single one
+// 3. Re-indent
+// (ATW: always returns nil)
+func cleanConf(in *bytes.Buffer, out *bytes.Buffer) error {
+	depth := 0
+	lineStarted := false
+	emptyLineWritten := false
+	state := stateCode
+	for {
+		c, err := in.ReadByte()
+		if err != nil {
+			if err == io.EOF {
+				return nil
+			}
+			return err // unreachable
+		}
+
+		needOutput := false
+		nextDepth := depth
+		nextLineStarted := lineStarted
+
+		switch state {
+		case stateCode:
+			switch c {
+			case '{':
+				needOutput = true
+				nextDepth = depth + 1
+				nextLineStarted = true
+			case '}':
+				needOutput = true
+				depth--
+				nextDepth = depth
+				nextLineStarted = true
+			case ' ', '\t':
+				needOutput = lineStarted
+			case '\r':
+			case '\n':
+				needOutput = !(!lineStarted && emptyLineWritten)
+				nextLineStarted = false
+			case '#':
+				needOutput = true
+				nextLineStarted = true
+				state = stateComment
+			default:
+				needOutput = true
+				nextLineStarted = true
+			}
+		case stateComment:
+			switch c {
+			case '\r':
+			case '\n':
+				needOutput = true
+				nextLineStarted = false
+				state = stateCode
+			default:
+				needOutput = true
+			}
+		}
+
+		if needOutput {
+			if !lineStarted && (writeIndentOnEmptyLines || c != '\n') {
+				for i := 0; i < depth; i++ {
+					err = out.WriteByte('\t') // always nil
+					if err != nil {
+						return err
+					}
+				}
+			}
+			emptyLineWritten = !lineStarted
+			err = out.WriteByte(c) // always nil
+			if err != nil {
+				return err
+			}
+		}
+
+		depth = nextDepth
+		lineStarted = nextLineStarted
+	}
+}
+
 // Write populates a buffer using a template with NGINX configuration
 // and the servers and upstreams created by Ingress rules
 func (t *Template) Write(conf config.TemplateConfig) ([]byte, error) {
@@ -110,12 +197,9 @@ func (t *Template) Write(conf config.TemplateConfig) ([]byte, error) {
 
 	// squeezes multiple adjacent empty lines to be single
 	// spaced this is to avoid the use of regular expressions
-	cmd := exec.Command("/ingress-controller/clean-nginx-conf.sh")
-	cmd.Stdin = tmplBuf
-	cmd.Stdout = outCmdBuf
-	if err := cmd.Run(); err != nil {
-		klog.Warningf("unexpected error cleaning template: %v", err)
-		return tmplBuf.Bytes(), nil
+	err = cleanConf(tmplBuf, outCmdBuf)
+	if err != nil {
+		return nil, err
 	}
 
 	return outCmdBuf.Bytes(), nil