luks: Create a very simple JSON library

LUKS version 2 uses JSON as a means of communicating the key
information. Add a simple library which can print JSON in a
human-readable format.

Note that it does not fully parse the JSON fragment. That may be
considered later, if needed.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

lib/json.c

@@ -0,0 +1,122 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * JSON pretty-printer
+ *
+ * Copyright (C) 2025 Canonical Ltd
+ * Written by Simon Glass <simon.glass@canonical.com>
+ */
+
+#include <ctype.h>
+#include <log.h>
+
+/**
+ * print_indent() - Print indentation spaces
+ *
+ * @indent:	Indentation level (each level is 2 spaces)
+ */
+static void print_indent(int indent)
+{
+	for (int i = 0; i < indent * 2; i++)
+		putc(' ');
+}
+
+void json_print_pretty(const char *json, int len)
+{
+	int indent = 0;
+	bool in_string = false;
+	bool escaped = false;
+	bool after_open = false;
+	int i;
+
+	for (i = 0; i < len && json[i]; i++) {
+		char c = json[i];
+
+		/* Handle escape sequences */
+		if (escaped) {
+			putc(c);
+			escaped = false;
+			continue;
+		}
+
+		if (c == '\\') {
+			putc(c);
+			escaped = true;
+			continue;
+		}
+
+		/* Track whether we're inside a string */
+		if (c == '"') {
+			in_string = !in_string;
+			if (after_open) {
+				print_indent(indent);
+				after_open = false;
+			}
+			putc(c);
+			continue;
+		}
+
+		/* Don't format inside strings */
+		if (in_string) {
+			putc(c);
+			continue;
+		}
+
+		/* Format structural characters */
+		switch (c) {
+		case '{':
+		case '[':
+			if (after_open) {
+				print_indent(indent);
+				after_open = false;
+			}
+			putc(c);
+			putc('\n');
+			indent++;
+			after_open = true;
+			break;
+
+		case '}':
+		case ']':
+			if (!after_open) {
+				putc('\n');
+				indent--;
+				print_indent(indent);
+			} else {
+				indent--;
+			}
+			putc(c);
+			after_open = false;
+			break;
+
+		case ',':
+			putc(c);
+			putc('\n');
+			print_indent(indent);
+			after_open = false;
+			break;
+
+		case ':':
+			putc(c);
+			putc(' ');
+			after_open = false;
+			break;
+
+		case ' ':
+		case '\t':
+		case '\n':
+		case '\r':
+			/* Skip whitespace outside strings */
+			break;
+
+		default:
+			if (after_open) {
+				print_indent(indent);
+				after_open = false;
+			}
+			putc(c);
+			break;
+		}
+	}
+
+	putc('\n');
+}