--- /dev/null
+JSON Encoder
+============
+
+Approach
+--------
+
+The JSON encoder exists to support QLOG implementation. There is no intention to
+implement a decoder at this time. The encoder is intended to support
+zero-allocation automation using immediate calls without the use of an
+intermediate syntax tree representation. This enables highly efficient
+serialization when called from QUIC code without dynamic memory allocation.
+
+An example usage is as follows:
+
+```c
+int generate_json(BIO *b)
+{
+ int ret = 1;
+ JSON_ENC z;
+
+ if (!ossl_json_init(&z, b, 0))
+ return 0;
+
+ ossl_json_object_begin(&z);
+ {
+ ossl_json_key(&z, "key");
+ ossl_json_str(&z, "value");
+
+ ossl_json_key(&z, "key2");
+ ossl_json_u64(&z, 42);
+
+ ossl_json_key(&z, "key3");
+ ossl_json_array_begin(&z);
+ {
+ ossl_json_null(&z);
+ ossl_json_f64(&z, 42.0);
+ ossl_json_str(&z, "string");
+ }
+ ossl_json_array_end(&z);
+ }
+ ossl_json_object_end(&z);
+
+ if (ossl_json_get_error_flag(&z))
+ ret = 0;
+
+ ossl_json_cleanup(&z);
+ return ret;
+}
+```
+
+The zero-allocation, immediate-output design means that most API calls
+correspond directly to immediately generated output; however there is some
+minimal state tracking. The API guarantees that it will never generate invalid
+JSON, with two exceptions:
+
+- it is the caller's responsibility to avoid generating duplicate keys;
+- it is the caller's responsibility to provide valid UTF-8 strings.
+
+Since the JSON encoder is for internal use only, its structure is defined in
+headers and can be incorporated into other objects without a heap allocation.
+The JSON encoder maintains an internal write buffer and a small state tracking
+stack (1 bit per level of depth in a JSON hierarchy).
+
+JSON-SEQ
+--------
+
+The encoder supports JSON-SEQ (RFC 7464), as this is an optimal format for
+outputting QLOG for our purposes.
+
+Number Handling
+---------------
+
+It is an unfortunate reality that many JSON implementations are not able to
+handle integers outside `[-2**53 + 1, 2**53 - 1]`. This leads to the I-JSON
+specification, RFC 7493, which recommends that values outside these ranges are
+encoded as strings.
+
+An optional I-JSON mode is offered, in which case integers outside these ranges
+are automatically serialized as strings instead.
+
+Error Handling
+--------------
+
+Error handling is deferred to improve ergonomics. If any call to a JSON encoder
+fails, all future calls also fail and the caller is expected to ascertain that
+the encoding process failed by calling `ossl_json_get_error_flag`.
+
+API
+---
+
+The API is documented in `include/internal/json_enc.h`.
--- /dev/null
+/*
+ * Copyright 2023 The OpenSSL Project Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License 2.0 (the "License"). You may not use
+ * this file except in compliance with the License. You can obtain a copy
+ * in the file LICENSE in the source distribution or at
+ * https://www.openssl.org/source/license.html
+ */
+
+#ifndef JSON_ENC_H
+# define JSON_ENC_H
+
+# include <openssl/bio.h>
+
+/*
+ * JSON Encoder
+ * ============
+ *
+ * This JSON encoder is used for QLOG. It supports ordinary JSON (RFC 7159),
+ * JSON-SEQ (RFC 7464) and I-JSON (RFC 7493). It supports only basic ASCII.
+ */
+
+struct json_write_buf {
+ BIO *bio;
+ char *buf;
+ size_t alloc, cur;
+};
+
+typedef struct json_enc_st {
+ uint32_t flags;
+ /* error: 1 if an error has occurred. */
+ /* state: current state. */
+ /* stack stores a bitmap. 0=object, 1=array. */
+ /* stack cur size: stack_end_byte bytes, stack_end_bit bits. */
+ /* stack alloc size: stack_bytes bytes. */
+ unsigned char error, stack_end_bit, state, *stack, defer_indent;
+ unsigned char stack_small[16];
+ struct json_write_buf wbuf;
+ size_t stack_end_byte, stack_bytes;
+} JSON_ENC;
+
+/*
+ * ossl_json_init
+ * --------------
+ *
+ * Initialises a JSON encoder.
+ *
+ * If the flag JSON_FLAG_SEQ is passed, the output is in JSON-SEQ. The caller
+ * should use the encoder as though it is encoding members of a JSON array (but
+ * without calling ossl_json_array_begin() or ossl_json_array_end()). Each
+ * top-level JSON item (e.g. JSON object) encoded will be separated correctly as
+ * per the JSON-SEQ format.
+ *
+ * If the flag JSON_FLAG_SEQ is not passed, the output is in JSON format.
+ * Generally the caller should encode only a single output item (e.g.
+ * a JSON object).
+ *
+ * By default, JSON output is maximally compact. If JSON_FLAG_PRETTY is set,
+ * JSON/JSON-SEQ output is spaced for optimal human readability.
+ *
+ * If JSON_FLAG_IJSON is set, integers outside the range `[-2**53 + 1, 2**53 -
+ * 1]` are automatically converted to decimal strings before serialization.
+ */
+#define JSON_FLAG_NONE 0
+#define JSON_FLAG_SEQ (1U << 0)
+#define JSON_FLAG_PRETTY (1U << 1)
+#define JSON_FLAG_IJSON (1U << 2)
+
+int ossl_json_init(JSON_ENC *json, BIO *bio, uint32_t flags);
+
+/*
+ * ossl_json_cleanup
+ * -----------------
+ *
+ * Destroys a JSON encoder.
+ */
+void ossl_json_cleanup(JSON_ENC *json);
+
+/*
+ * ossl_json_reset
+ * ---------------
+ *
+ * Resets a JSON encoder, as though it has just been initialised, allowing it
+ * to be used again for new output syntactically unrelated to any previous
+ * output. This is similar to calling ossl_json_cleanup followed by
+ * ossl_json_init but may allow internal buffers, etc. to be reused.
+ *
+ * If the JSON encoder has entered an error state, this function MAY allow
+ * recovery from this error state, in which case it will return 1. If this
+ * function returns 0, the JSON encoder is unrecoverable and
+ * ossl_json_cleanup() must be called.
+ *
+ * Automatically calls ossl_json_flush().
+ */
+int ossl_json_reset(JSON_ENC *json);
+
+/*
+ * ossl_json_flush
+ * ---------------
+ *
+ * Flushes the JSON encoder, ensuring that any residual bytes in internal
+ * buffers are written to the provided sink BIO. Flushing may also happen
+ * autonomously as buffers are filled, but the caller must use this function
+ * to guarantee all data has been flushed.
+ */
+int ossl_json_flush(JSON_ENC *json);
+
+/*
+ * ossl_json_flush_cleanup
+ * -----------------------
+ *
+ * Tries to flush as in a call to ossl_json_flush, and then calls
+ * ossl_json_cleanup regardless of the result. The result of the flush call is
+ * returned.
+ */
+int ossl_json_flush_cleanup(JSON_ENC *json);
+
+/*
+ * ossl_json_set_sink
+ * ------------------
+ *
+ * Changes the sink used by the JSON encoder.
+ */
+int ossl_json_set_sink(JSON_ENC *json, BIO *bio);
+
+/*
+ * ossl_json_in_error
+ * ------------------
+ *
+ * To enhance the ergonomics of the JSON API, the JSON object uses an implicit
+ * error tracking model. When a JSON API call fails (for example due to caller
+ * error, such as trying to close an array which was not opened), the JSON
+ * object enters an error state and all further calls are silently ignored.
+ *
+ * The caller can detect this condition after it is finished making builder
+ * calls to the JSON object by calling this function. This function returns 1
+ * if an error occurred. At this point the caller's only recourse is to call
+ * ossl_json_reset() or ossl_json_cleanup().
+ *
+ * Note that partial (i.e., invalid) output may still have been sent to the BIO
+ * in this case. Since the amount of output which can potentially be produced
+ * by a JSON object is unbounded, it is impractical to buffer it all before
+ * flushing. It is expected that errors will ordinarily be either caller errors
+ * (programming errors) or BIO errors.
+ */
+int ossl_json_in_error(JSON_ENC *json);
+
+/*
+ * JSON Builder Calls
+ * ==================
+ *
+ * These functions are used to build JSON output. The functions which have
+ * begin and end function pairs must be called in correctly nested sequence.
+ * When writing an object, ossl_json_key() must be called exactly once before
+ * each call to write a JSON item.
+ *
+ * The JSON library takes responsibility for enforcing correct usage patterns.
+ * If a call is made that does not correspond to the JSON syntax, the JSON
+ * object enters the error state and all subsequent calls are ignored.
+ *
+ * In JSON-SEQ mode, the caller should act as though the library implicitly
+ * places all calls between an ossl_json_array_begin() and
+ * ossl_json_array_end() pair; for example, the normal usage pattern would be
+ * to call ossl_json_object_begin() followed by ossl_json_object_end(), in
+ * repeated sequence.
+ *
+ * The library does not enforce non-generation of duplicate keys. Avoiding this
+ * is the caller's responsibility. It is also the caller's responsibility to
+ * pass valid UTF-8 strings. All other forms of invalid output will cause an
+ * error. Note that due to the immediate nature of the API, partial output may
+ * have already been generated in such a case.
+ */
+
+/* Begin a new JSON object. */
+void ossl_json_object_begin(JSON_ENC *json);
+
+/* End a JSON obejct. Must be matched with a call to ossl_json_object_begin(). */
+void ossl_json_object_end(JSON_ENC *json);
+
+/* Begin a new JSON array. */
+void ossl_json_array_begin(JSON_ENC *json);
+
+/* End a JSON array. Must be matched with a call to ossl_json_array_end(). */
+void ossl_json_array_end(JSON_ENC *json);
+
+/*
+ * Encode a JSON key within an object. Pass a zero-terminated string, which can
+ * be freed immediately following the call to this function.
+ */
+void ossl_json_key(JSON_ENC *json, const char *key);
+
+/* Encode a JSON 'null' value. */
+void ossl_json_null(JSON_ENC *json);
+
+/* Encode a JSON boolean value. */
+void ossl_json_bool(JSON_ENC *json, int value);
+
+/* Encode a JSON integer from a uint64_t. */
+void ossl_json_u64(JSON_ENC *json, uint64_t value);
+
+/* Encode a JSON integer from an int64_t. */
+void ossl_json_i64(JSON_ENC *json, int64_t value);
+
+/* Encode a JSON number from a 64-bit floating point value. */
+void ossl_json_f64(JSON_ENC *json, double value);
+
+/*
+ * Encode a JSON UTF-8 string from a zero-terminated string. The string passed
+ * can be freed immediately following the call to this function.
+ */
+void ossl_json_str(JSON_ENC *json, const char *str);
+
+/*
+ * Encode a JSON UTF-8 string from a string with the given length. The string
+ * passed can be freed immediately following the call to this function.
+ */
+void ossl_json_str_len(JSON_ENC *json, const char *str, size_t str_len);
+
+/*
+ * Encode binary data as a lowercase hex string. data_len is the data length in
+ * bytes.
+ */
+void ossl_json_str_hex(JSON_ENC *json, const void *data, size_t data_len);
+
+#endif
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