Rework Replacer loop to handle escaped braces (#3121)

Fixes #3116

* Rework Replacer loop to ignore escaped braces

* Add benchmark tests for replacer

* Optimise handling of escaped braces

* Handle escaped closing braces

* Remove additional check for closing brace

This commit removes the additional check for input in which the closing
brace appears before the opening brace. This check has been removed for
performance reasons as it is deemed an unlikely edge case.

* Check for escaped closing braces in placeholder name

2 files changed, 127 insertions, 1 deletions

diff --git a/replacer.go b/replacer.go
index aad13e2..8823404 100644
--- a/replacer.go
+++ b/replacer.go
@@ -125,6 +125,14 @@ func (r *Replacer) replace(input, empty string,
 	// iterate the input to find each placeholder
 	var lastWriteCursor int
 	for i := 0; i < len(input); i++ {
+
+		// check for escaped braces
+		if i > 0 && input[i-1] == phEscape && (input[i] == phClose || input[i] == phOpen) {
+			sb.WriteString(input[lastWriteCursor : i-1])
+			lastWriteCursor = i
+			continue
+		}
+
 		if input[i] != phOpen {
 			continue
 		}
@@ -135,6 +143,11 @@ func (r *Replacer) replace(input, empty string,
 			continue
 		}
 
+		// if necessary look for the first closing brace that is not escaped
+		for end > 0 && end < len(input)-1 && input[end-1] == phEscape {
+			end = strings.Index(input[end+1:], string(phClose)) + end + 1
+		}
+
 		// write the substring from the last cursor to this point
 		sb.WriteString(input[lastWriteCursor:i])
 
@@ -237,4 +250,4 @@ var nowFunc = time.Now
 // ReplacerCtxKey is the context key for a replacer.
 const ReplacerCtxKey CtxKey = "replacer"
 
-const phOpen, phClose = '{', '}'
+const phOpen, phClose, phEscape = '{', '}', '\\'
diff --git a/replacer_test.go b/replacer_test.go
index 4b56194..66bb537 100644
--- a/replacer_test.go
+++ b/replacer_test.go
@@ -36,30 +36,86 @@ func TestReplacer(t *testing.T) {
 			expect: "{",
 		},
 		{
+			input:  `\{`,
+			expect: `{`,
+		},
+		{
 			input:  "foo{",
 			expect: "foo{",
 		},
 		{
+			input:  `foo\{`,
+			expect: `foo{`,
+		},
+		{
 			input:  "foo{bar",
 			expect: "foo{bar",
 		},
 		{
+			input:  `foo\{bar`,
+			expect: `foo{bar`,
+		},
+		{
 			input:  "foo{bar}",
 			expect: "foo",
 		},
 		{
+			input:  `foo\{bar\}`,
+			expect: `foo{bar}`,
+		},
+		{
 			input:  "}",
 			expect: "}",
 		},
 		{
+			input:  `\}`,
+			expect: `\}`,
+		},
+		{
 			input:  "{}",
 			expect: "",
 		},
 		{
+			input:  `\{\}`,
+			expect: `{}`,
+		},
+		{
 			input:  `{"json": "object"}`,
 			expect: "",
 		},
 		{
+			input:  `\{"json": "object"}`,
+			expect: `{"json": "object"}`,
+		},
+		{
+			input:  `\{"json": "object"\}`,
+			expect: `{"json": "object"}`,
+		},
+		{
+			input:  `\{"json": "object{bar}"\}`,
+			expect: `{"json": "object"}`,
+		},
+		{
+			input:  `\{"json": \{"nested": "object"\}\}`,
+			expect: `{"json": {"nested": "object"}}`,
+		},
+		{
+			input:  `\{"json": \{"nested": "{bar}"\}\}`,
+			expect: `{"json": {"nested": ""}}`,
+		},
+		{
+			input:  `pre \{"json": \{"nested": "{bar}"\}\}`,
+			expect: `pre {"json": {"nested": ""}}`,
+		},
+		{
+			input:  `\{"json": \{"nested": "{bar}"\}\} post`,
+			expect: `{"json": {"nested": ""}} post`,
+		},
+		{
+			input:  `pre \{"json": \{"nested": "{bar}"\}\} post`,
+			expect: `pre {"json": {"nested": ""}} post`,
+		},
+		{
 			input:  `{{`,
 			expect: "{{",
 		},
@@ -68,10 +124,38 @@ func TestReplacer(t *testing.T) {
 			expect: "",
 		},
 		{
+			input:  `{"json": "object"\}`,
+			expect: "",
+		},
+		{
 			input:  `{unknown}`,
 			empty:  "-",
 			expect: "-",
 		},
+		{
+			input:  `back\slashes`,
+			expect: `back\slashes`,
+		},
+		{
+			input:  `double back\\slashes`,
+			expect: `double back\\slashes`,
+		},
+		{
+			input:  `placeholder {with \{ brace} in name`,
+			expect: `placeholder  in name`,
+		},
+		{
+			input:  `placeholder {with \} brace} in name`,
+			expect: `placeholder  in name`,
+		},
+		{
+			input:  `placeholder {with \} \} braces} in name`,
+			expect: `placeholder  in name`,
+		},
+		{
+			input:  `\{'group':'default','max_age':3600,'endpoints':[\{'url':'https://some.domain.local/a/d/g'\}],'include_subdomains':true\}`,
+			expect: `{'group':'default','max_age':3600,'endpoints':[{'url':'https://some.domain.local/a/d/g'}],'include_subdomains':true}`,
+		},
 	} {
 		actual := rep.ReplaceAll(tc.input, tc.empty)
 		if actual != tc.expect {
@@ -81,6 +165,35 @@ func TestReplacer(t *testing.T) {
 	}
 }
 
+func BenchmarkReplacer(b *testing.B) {
+	type testCase struct {
+		name, input, empty string
+	}
+
+	rep := testReplacer()
+
+	for _, bm := range []testCase{
+		{
+			name:  "no placeholder",
+			input: `simple string`,
+		},
+		{
+			name:  "placeholder",
+			input: `{"json": "object"}`,
+		},
+		{
+			name:  "escaped placeholder",
+			input: `\{"json": \{"nested": "{bar}"\}\}`,
+		},
+	} {
+		b.Run(bm.name, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				rep.ReplaceAll(bm.input, bm.empty)
+			}
+		})
+	}
+}
+
 func TestReplacerSet(t *testing.T) {
 	rep := testReplacer()