Fix a panic in NotifyCreateIssueComment (caused by string truncation) (#17928)

* Fix a panic in NotifyCreateIssueComment (caused by string truncation)

* more unit tests

* refactor

* fix some edge cases

* use SplitStringAtByteN for comment content

modules/util/truncate.go

@@ -6,20 +6,23 @@ package util
 
 import "unicode/utf8"
 
+// in UTF8 "…" is 3 bytes so doesn't really gain us anything...
+const utf8Ellipsis = "…"
+const asciiEllipsis = "..."
+
 // SplitStringAtByteN splits a string at byte n accounting for rune boundaries. (Combining characters are not accounted for.)
 func SplitStringAtByteN(input string, n int) (left, right string) {
 	if len(input) <= n {
-		left = input
-		return
+		return input, ""
 	}
 
 	if !utf8.ValidString(input) {
-		left = input[:n-3] + "..."
-		right = "..." + input[n-3:]
-		return
+		if n-3 < 0 {
+			return input, ""
+		}
+		return input[:n-3] + asciiEllipsis, asciiEllipsis + input[n-3:]
 	}
 
-	// in UTF8 "…" is 3 bytes so doesn't really gain us anything...
 	end := 0
 	for end <= n-3 {
 		_, size := utf8.DecodeRuneInString(input[end:])
@@ -29,7 +32,29 @@ func SplitStringAtByteN(input string, n int) (left, right string) {
 		end += size
 	}
 
-	left = input[:end] + "…"
-	right = "…" + input[end:]
-	return
+	return input[:end] + utf8Ellipsis, utf8Ellipsis + input[end:]
+}
+
+// SplitStringAtRuneN splits a string at rune n accounting for rune boundaries. (Combining characters are not accounted for.)
+func SplitStringAtRuneN(input string, n int) (left, right string) {
+	if !utf8.ValidString(input) {
+		if len(input) <= n || n-3 < 0 {
+			return input, ""
+		}
+		return input[:n-3] + asciiEllipsis, asciiEllipsis + input[n-3:]
+	}
+
+	if utf8.RuneCountInString(input) <= n {
+		return input, ""
+	}
+
+	count := 0
+	end := 0
+	for count < n-1 {
+		_, size := utf8.DecodeRuneInString(input[end:])
+		end += size
+		count++
+	}
+
+	return input[:end] + utf8Ellipsis, utf8Ellipsis + input[end:]
 }