Add string escaping to JSON formatter (SmingHub#2875)

JSON does not support multiline text so control characters must be escaped.
This PR adds a generic `escapeControls` function to the `Formatter` classes, which is used by `Format::json::escape()`.

Sming/Core/Data/Format/Formatter.cpp

@@ -0,0 +1,98 @@
+/****
+ * Sming Framework Project - Open Source framework for high efficiency native ESP8266 development.
+ * Created 2015 by Skurydin Alexey
+ * http://github.com/SmingHub/Sming
+ * All files of the Sming Core are provided under the LGPL v3 license.
+ *
+ * Formatter.cpp
+ *
+ * @author mikee47 <mike@sillyhouse.net> Aug 2024
+ *
+ ****/
+
+#include "Formatter.h"
+
+namespace
+{
+/**
+ * @brief Get character used for standard escapes
+ * @param c Code to be escaped
+ * @retval char Corresponding character, NUL if there isn't a standard escape
+ */
+char escapeChar(char c)
+{
+	switch(c) {
+	case '\0':
+		return '0';
+	case '\'':
+		return '\'';
+	case '\"':
+		return '"';
+	case '\?':
+		return '?';
+	case '\\':
+		return '\\';
+	case '\a':
+		return 'a';
+	case '\b':
+		return 'b';
+	case '\f':
+		return 'f';
+	case '\n':
+		return 'n';
+	case '\r':
+		return 'r';
+	case '\t':
+		return 't';
+	case '\v':
+		return 'v';
+	default:
+		return '\0';
+	}
+}
+
+} // namespace
+
+namespace Format
+{
+unsigned escapeControls(String& value)
+{
+	// Count number of extra characters we'll need to insert
+	unsigned extra{0};
+	for(auto& c : value) {
+		if(escapeChar(c)) {
+			extra += 1; // "\"
+		} else if(uint8_t(c) < 0x20) {
+			extra += 3; // "\xnn"
+		}
+	}
+	if(extra == 0) {
+		return 0;
+	}
+	auto len = value.length();
+	if(!value.setLength(len + extra)) {
+		return 0;
+	}
+	char* out = value.begin();
+	const char* in = out;
+	memmove(out + extra, in, len);
+	in += extra;
+	while(len--) {
+		uint8_t c = *in++;
+		auto esc = escapeChar(c);
+		if(esc) {
+			*out++ = '\\';
+			*out++ = esc;
+		} else if(c < 0x20) {
+			*out++ = '\\';
+			*out++ = 'x';
+			*out++ = hexchar(uint8_t(c) >> 4);
+			*out++ = hexchar(uint8_t(c) & 0x0f);
+		} else {
+			*out++ = c;
+		}
+	}
+	return extra;
+}
+
+} // namespace Format

Sming/Core/Data/Format/Formatter.h

@@ -17,6 +17,13 @@
 
 namespace Format
 {
+/**
+ * @brief Escape standard control codes such as `\n` (below ASCII 0x20)
+ * @param value String to be modified
+ * @retval unsigned Number of control characters found and replaced
+ */
+unsigned escapeControls(String& value);
+
 /**
  * @brief Virtual class to perform format-specific String adjustments
  */

Sming/Core/Data/Format/Json.cpp

@@ -63,11 +63,10 @@ bool IsValidUtf8(const char* str, unsigned length)
  *
  * This can occur if filenames become corrupted, so here we just
  * substitute an underscore _ for anything which fails to match UTF8.
- *
- * TODO: Perform ANSI -> UTF8 conversion?
  */
 void Json::escape(String& value) const
 {
+	escapeControls(value);
 	if(!IsValidUtf8(value.c_str(), value.length())) {
 		debug_w("Invalid UTF8: %s", value.c_str());
 		for(unsigned i = 0; i < value.length(); ++i) {

tests/HostTests/include/modules.h

@@ -29,6 +29,7 @@
 	XX(CStringArray)                                                                                                   \
 	XX(Stream)                                                                                                         \
 	XX(TemplateStream)                                                                                                 \
+	XX(Formatter)                                                                                                      \
 	XX(Serial)                                                                                                         \
 	XX(ObjectMap)                                                                                                      \
 	XX_NET(Base64)                                                                                                     \

tests/HostTests/modules/Formatter.cpp

@@ -0,0 +1,30 @@
+#include <HostTests.h>
+#include <Data/Format/Json.h>
+
+class FormatterTest : public TestGroup
+{
+public:
+	FormatterTest() : TestGroup(_F("Formatter"))
+	{
+	}
+
+	void execute() override
+	{
+		DEFINE_FSTR_LOCAL(text1, "A JSON\ntest string\twith escapes\x12\0\n"
+								 "Worth maybe \xc2\xa3"
+								 "0.53.")
+		DEFINE_FSTR_LOCAL(text1b, "A JSON\\ntest string\\twith escapes\\x12\\0\\n"
+								  "Worth maybe \xc2\xa3"
+								  "0.53.")
+
+		Serial << text1 << endl;
+		String s(text1);
+		Format::json.escape(s);
+		REQUIRE_EQ(s, text1b);
+	}
+};
+
+void REGISTER_TEST(Formatter)
+{
+	registerGroup<FormatterTest>();
+}