Re: JSON/YAML/TOML/etc. parsing performance

[Philipp Stephani <p.stephani2@gmail.com>]

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Philipp Stephani <p.stephani2@gmail.com> schrieb am So., 17. Sep. 2017 um
20:46 Uhr:

> Ted Zlatanov <tzz@lifelogs.com> schrieb am Sa., 16. Sep. 2017 um
> 17:55 Uhr:
>
>> I wanted to ask if there's any chance of improving the parsing
>> performance of JSON, YAML, TOML, and similar data formats. It's pretty
>> poor today.
>>
>> That could be done in the core with C code, improved Lisp code,
>> integration with an external library, or a mix of those.
>>
>
> I don't know much about the others, but given the importance of JSON as
> data exchange and serialization format, I think it's worthwhile to invest
> some time here. I've implemented a wrapper around the json-c library
> (license: Expat/X11/MIT), resulting in significant speedups using the test
> data from https://github.com/miloyip/nativejson-benchmark: a factor of
> 3.9 to 6.4 for parsing, and a factor of 27 to 67 for serializing. If people
> agree that this is useful I can send a patch.
>

I've discovered that the interface and documentation of Jansson are much
better than the ones of json-c, so I switched to Jansson. I've attached a
patch.

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From 67ad4e22c5a0b5dcc0dea2abdef32ee3c636fade Mon Sep 17 00:00:00 2001
From: Philipp Stephani <phst@google.com>
Date: Mon, 18 Sep 2017 10:51:39 +0200
Subject: [PATCH] Implement native JSON support using Jansson

* configure.ac: New option --with-json.

* src/json.c (Fjson_serialize, Fjson_insert, Fjson_parse_string)
(Fjson_parse_buffer): New defuns.
(json_out_of_memory, json_parse_error, json_release_object)
(check_string_without_embedded_nulls, json_check, lisp_to_json)
(json_insert, json_insert_callback, json_to_lisp)
(json_read_buffer_callback, Fjson_parse_buffer, define_error): New
helper function.
(syms_of_json): New file.

* src/lisp.h: Declaration for syms_of_json.

* src/emacs.c (main): Enable JSON functions.

* src/Makefile.in (JSON_LIBS, JSON_CFLAGS, JSON_OBJ, EMACS_CFLAGS)
(base_obj, LIBES): Compile json.c if --with-json is enabled.

* test/src/json-tests.el (json-serialize/roundtrip)
(json-serialize/object, json-parse-string/object): New unit tests.
---
 configure.ac           |  20 ++-
 src/Makefile.in        |  11 +-
 src/emacs.c            |   4 +
 src/json.c             | 442 +++++++++++++++++++++++++++++++++++++++++++++++++
 src/lisp.h             |   5 +
 test/src/json-tests.el |  61 +++++++
 6 files changed, 539 insertions(+), 4 deletions(-)
 create mode 100644 src/json.c
 create mode 100644 test/src/json-tests.el

diff --git a/configure.ac b/configure.ac
index 35b7e69daf..c9ce5ee120 100644
--- a/configure.ac
+++ b/configure.ac
@@ -348,6 +348,7 @@ AC_DEFUN
 OPTION_DEFAULT_OFF([cairo],[compile with Cairo drawing (experimental)])
 OPTION_DEFAULT_ON([xml2],[don't compile with XML parsing support])
 OPTION_DEFAULT_ON([imagemagick],[don't compile with ImageMagick image support])
+OPTION_DEFAULT_ON([json], [don't compile with native JSON support])
 
 OPTION_DEFAULT_ON([xft],[don't use XFT for anti aliased fonts])
 OPTION_DEFAULT_ON([libotf],[don't use libotf for OpenType font support])
@@ -2856,6 +2857,22 @@ AC_DEFUN
 AC_SUBST(LIBSYSTEMD_LIBS)
 AC_SUBST(LIBSYSTEMD_CFLAGS)
 
+HAVE_JSON=no
+JSON_OBJ=
+
+if test "${with_json}" = yes; then
+  EMACS_CHECK_MODULES([JSON], [jansson >= 2.5],
+    [HAVE_JSON=yes], [HAVE_JSON=no])
+  if test "${HAVE_JSON}" = yes; then
+    AC_DEFINE(HAVE_JSON, 1, [Define if using Jansson.])
+    JSON_OBJ=json.o
+  fi
+fi
+
+AC_SUBST(JSON_LIBS)
+AC_SUBST(JSON_CFLAGS)
+AC_SUBST(JSON_OBJ)
+
 NOTIFY_OBJ=
 NOTIFY_SUMMARY=no
 
@@ -5368,7 +5385,7 @@ AC_DEFUN
 for opt in XAW3D XPM JPEG TIFF GIF PNG RSVG CAIRO IMAGEMAGICK SOUND GPM DBUS \
   GCONF GSETTINGS NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT \
   LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS X_TOOLKIT OLDXMENU X11 NS MODULES \
-  XWIDGETS LIBSYSTEMD CANNOT_DUMP LCMS2; do
+  XWIDGETS LIBSYSTEMD JSON CANNOT_DUMP LCMS2; do
 
     case $opt in
       CANNOT_DUMP) eval val=\${$opt} ;;
@@ -5418,6 +5435,7 @@ AC_DEFUN
   Does Emacs use -lotf?                                   ${HAVE_LIBOTF}
   Does Emacs use -lxft?                                   ${HAVE_XFT}
   Does Emacs use -lsystemd?                               ${HAVE_LIBSYSTEMD}
+  Does Emacs use -ljanssoon?                              ${HAVE_JSON}
   Does Emacs directly use zlib?                           ${HAVE_ZLIB}
   Does Emacs have dynamic modules support?                ${HAVE_MODULES}
   Does Emacs use toolkit scroll bars?                     ${USE_TOOLKIT_SCROLL_BARS}
diff --git a/src/Makefile.in b/src/Makefile.in
index 0e55ad4bb2..4d33682629 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -312,6 +312,10 @@ LIBGNUTLS_CFLAGS =
 LIBSYSTEMD_LIBS = @LIBSYSTEMD_LIBS@
 LIBSYSTEMD_CFLAGS = @LIBSYSTEMD_CFLAGS@
 
+JSON_LIBS = @JSON_LIBS@
+JSON_CFLAGS = @JSON_CFLAGS@
+JSON_OBJ = @JSON_OBJ@
+
 INTERVALS_H = dispextern.h intervals.h composite.h
 
 GETLOADAVG_LIBS = @GETLOADAVG_LIBS@
@@ -363,7 +367,7 @@ EMACS_CFLAGS=
   $(WEBKIT_CFLAGS) \
   $(SETTINGS_CFLAGS) $(FREETYPE_CFLAGS) $(FONTCONFIG_CFLAGS) \
   $(LIBOTF_CFLAGS) $(M17N_FLT_CFLAGS) $(DEPFLAGS) \
-  $(LIBSYSTEMD_CFLAGS) \
+  $(LIBSYSTEMD_CFLAGS) $(JSON_CFLAGS) \
   $(LIBGNUTLS_CFLAGS) $(NOTIFY_CFLAGS) $(CAIRO_CFLAGS) \
   $(WERROR_CFLAGS)
 ALL_CFLAGS = $(EMACS_CFLAGS) $(WARN_CFLAGS) $(CFLAGS)
@@ -397,7 +401,7 @@ base_obj =
 	thread.o systhread.o \
 	$(if $(HYBRID_MALLOC),sheap.o) \
 	$(MSDOS_OBJ) $(MSDOS_X_OBJ) $(NS_OBJ) $(CYGWIN_OBJ) $(FONT_OBJ) \
-	$(W32_OBJ) $(WINDOW_SYSTEM_OBJ) $(XGSELOBJ)
+	$(W32_OBJ) $(WINDOW_SYSTEM_OBJ) $(XGSELOBJ) $(JSON_OBJ)
 obj = $(base_obj) $(NS_OBJC_OBJ)
 
 ## Object files used on some machine or other.
@@ -493,7 +497,8 @@ LIBES =
    $(LIBS_TERMCAP) $(GETLOADAVG_LIBS) $(SETTINGS_LIBS) $(LIBSELINUX_LIBS) \
    $(FREETYPE_LIBS) $(FONTCONFIG_LIBS) $(LIBOTF_LIBS) $(M17N_FLT_LIBS) \
    $(LIBGNUTLS_LIBS) $(LIB_PTHREAD) $(GETADDRINFO_A_LIBS) $(LIBLCMS2) \
-   $(NOTIFY_LIBS) $(LIB_MATH) $(LIBZ) $(LIBMODULES) $(LIBSYSTEMD_LIBS)
+   $(NOTIFY_LIBS) $(LIB_MATH) $(LIBZ) $(LIBMODULES) $(LIBSYSTEMD_LIBS) \
+   $(JSON_LIBS)
 
 ## FORCE it so that admin/unidata can decide whether these files
 ## are up-to-date.  Although since charprop depends on bootstrap-emacs,
diff --git a/src/emacs.c b/src/emacs.c
index 1ad8af70a7..eb5f1128f6 100644
--- a/src/emacs.c
+++ b/src/emacs.c
@@ -1610,6 +1610,10 @@ Using an Emacs configured with --with-x-toolkit=lucid does not have this problem
       syms_of_threads ();
       syms_of_profiler ();
 
+#ifdef HAVE_JSON
+      syms_of_json ();
+#endif
+
       keys_of_casefiddle ();
       keys_of_cmds ();
       keys_of_buffer ();
diff --git a/src/json.c b/src/json.c
new file mode 100644
index 0000000000..628de82921
--- /dev/null
+++ b/src/json.c
@@ -0,0 +1,442 @@
+/* JSON parsing and serialization.
+
+Copyright (C) 2017 Free Software Foundation, Inc.
+
+This file is part of GNU Emacs.
+
+GNU Emacs is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or (at
+your option) any later version.
+
+GNU Emacs is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <jansson.h>
+
+#include "lisp.h"
+#include "buffer.h"
+
+static _Noreturn void
+json_out_of_memory (void)
+{
+  xsignal0 (Qjson_out_of_memory);
+}
+
+static _Noreturn void
+json_parse_error (const json_error_t *error)
+{
+  xsignal (Qjson_parse_error,
+           list5 (build_string (error->text), build_string (error->source),
+                  make_natnum (error->line), make_natnum (error->column),
+                  make_natnum (error->position)));
+}
+
+static void
+json_release_object (void *object)
+{
+  json_decref (object);
+}
+
+static void
+check_string_without_embedded_nulls (Lisp_Object object)
+{
+  CHECK_STRING (object);
+  CHECK_TYPE (memchr (SDATA (object), '\0', SBYTES (object)) == NULL,
+              Qstring_without_embedded_nulls_p, object);
+}
+
+static json_t *
+json_check (json_t *object)
+{
+  if (object == NULL)
+    json_out_of_memory ();
+  return object;
+}
+
+static json_t *
+lisp_to_json (Lisp_Object lisp)
+{
+  if (NILP (lisp))
+    return json_check (json_null ());
+  else if (EQ (lisp, QCjson_false))
+    return json_check (json_false ());
+  else if (EQ (lisp, Qt))
+    return json_check (json_true ());
+  else if (INTEGERP (lisp))
+    {
+      CHECK_TYPE_RANGED_INTEGER (json_int_t, lisp);
+      return json_check (json_integer (XINT (lisp)));
+    }
+  else if (FLOATP (lisp))
+    return json_check (json_real (XFLOAT_DATA (lisp)));
+  else if (STRINGP (lisp))
+    {
+      ptrdiff_t size = SBYTES (lisp);
+      eassert (size >= 0);
+      if (size > SIZE_MAX)
+        xsignal1 (Qoverflow_error, build_pure_c_string ("string is too long"));
+      return json_check (json_stringn (SSDATA (lisp), size));
+    }
+  else if (VECTORP (lisp))
+    {
+      if (++lisp_eval_depth > max_lisp_eval_depth)
+        xsignal0 (Qjson_object_too_deep);
+      ptrdiff_t size = ASIZE (lisp);
+      eassert (size >= 0);
+      if (size > SIZE_MAX)
+        xsignal1 (Qoverflow_error, build_pure_c_string ("vector is too long"));
+      json_t *json = json_check (json_array ());
+      for (ptrdiff_t i = 0; i < size; ++i)
+        {
+          int status
+            = json_array_append_new (json, lisp_to_json (AREF (lisp, i)));
+          if (status == -1)
+            json_out_of_memory ();
+          eassert (status == 0);
+        }
+      eassert (json_array_size (json) == size);
+      --lisp_eval_depth;
+      return json;
+    }
+  else if (HASH_TABLE_P (lisp))
+    {
+      if (++lisp_eval_depth > max_lisp_eval_depth)
+        xsignal0 (Qjson_object_too_deep);
+      json_t* json = json_check (json_object ());
+      struct Lisp_Hash_Table *h = XHASH_TABLE (lisp);
+      for (ptrdiff_t i = 0; i < HASH_TABLE_SIZE (h); ++i)
+        if (!NILP (HASH_HASH (h, i)))
+          {
+            Lisp_Object key = HASH_KEY (h, i);
+            /* We can’t specify the length, so the string must be
+               null-terminated.  */
+            check_string_without_embedded_nulls (key);
+            int status = json_object_set_new (json, SSDATA (key),
+                                              lisp_to_json (HASH_VALUE (h, i)));
+            if (status == -1)
+              json_out_of_memory ();
+            eassert (status == 0);
+          }
+      --lisp_eval_depth;
+      return json;
+    }
+  wrong_type_argument (Qjson_value_p, lisp);
+}
+
+DEFUN ("json-serialize", Fjson_serialize, Sjson_serialize, 1, 1, NULL,
+       doc: /* Return the JSON representation of OBJECT as a string.
+OBJECT must be a vector or hashtable, and its elements can recursively
+contain nil, t, `:json-false', numbers, strings, or other vectors and
+hashtables.  nil, t, and `:json-false' will be converted to JSON null,
+true, and false values, respectively.  Vectors will be converted to
+JSON arrays, and hashtables to JSON objects.  Hashtable keys must be
+strings without embedded null characters and must be unique within
+each object.  */)
+  (Lisp_Object object)
+{
+  ptrdiff_t count = SPECPDL_INDEX ();
+
+  json_t *json = lisp_to_json (object);
+  record_unwind_protect_ptr (json_release_object, json);
+
+  char *string = json_dumps (json, JSON_COMPACT);
+  if (string == NULL)
+    json_out_of_memory ();
+  record_unwind_protect_ptr (free, string);
+
+  return unbind_to (count, build_string (string));
+}
+
+struct json_buffer_and_size
+{
+  const char *buffer;
+  size_t size;
+};
+
+static Lisp_Object
+json_insert (Lisp_Object data)
+{
+  const struct json_buffer_and_size *buffer_and_size = XSAVE_POINTER (data, 0);
+  if (FIXNUM_OVERFLOW_P (buffer_and_size->size))
+    xsignal1 (Qoverflow_error, build_pure_c_string ("buffer too large"));
+  Lisp_Object string
+    = make_string (buffer_and_size->buffer, buffer_and_size->size);
+  insert_from_string (string, 0, 0, SCHARS (string), SBYTES (string), false);
+  return Qnil;
+}
+
+struct json_insert_data
+{
+  /* nil if json_insert succeeded, otherwise a cons
+     (ERROR-SYMBOL . ERROR-DATA).  */
+  Lisp_Object error;
+};
+
+static int
+json_insert_callback (const char *buffer, size_t size, void *data)
+{
+  /* This function may not exit nonlocally.  */
+  struct json_insert_data *d = data;
+  struct json_buffer_and_size buffer_and_size
+    = {.buffer = buffer, .size = size};
+  d->error
+    = internal_condition_case_1 (json_insert, make_save_ptr (&buffer_and_size),
+                                 Qt, Fidentity);
+  return 0;
+}
+
+DEFUN ("json-insert", Fjson_insert, Sjson_insert, 1, 1, NULL,
+       doc: /* Insert the JSON representation of OBJECT before point.
+This is the same as (insert (json-serialize OBJECT)), but potentially
+faster.  See the function `json-serialize' for allowed values of
+OBJECT.  */)
+  (Lisp_Object object)
+{
+  ptrdiff_t count = SPECPDL_INDEX ();
+
+  json_t *json = lisp_to_json (object);
+  record_unwind_protect_ptr (json_release_object, json);
+
+  struct json_insert_data data;
+  int status
+    = json_dump_callback (json, json_insert_callback, &data, JSON_COMPACT);
+  if (status == -1)
+    json_out_of_memory ();
+  eassert (status == 0);
+
+  if (!NILP (data.error))
+    xsignal (XCAR (data.error), XCDR (data.error));
+
+  return unbind_to (count, Qnil);
+}
+
+static Lisp_Object
+json_to_lisp (json_t *json)
+{
+  switch (json_typeof (json))
+    {
+    case JSON_NULL:
+      return Qnil;
+    case JSON_FALSE:
+      return QCjson_false;
+    case JSON_TRUE:
+      return Qt;
+    case JSON_INTEGER:
+      {
+        json_int_t value = json_integer_value (json);
+        if (FIXNUM_OVERFLOW_P (value))
+          xsignal1 (Qoverflow_error,
+                    build_pure_c_string ("JSON integer is too large"));
+        return make_number (value);
+      }
+    case JSON_REAL:
+      return make_float (json_real_value (json));
+    case JSON_STRING:
+      {
+        size_t size = json_string_length (json);
+        if (FIXNUM_OVERFLOW_P (size))
+          xsignal1 (Qoverflow_error,
+                    build_pure_c_string ("JSON string is too long"));
+        return make_string (json_string_value (json), size);
+      }
+    case JSON_ARRAY:
+      {
+        if (++lisp_eval_depth > max_lisp_eval_depth)
+          xsignal0 (Qjson_object_too_deep);
+        size_t size = json_array_size (json);
+        if (FIXNUM_OVERFLOW_P (size))
+          xsignal1 (Qoverflow_error,
+                    build_pure_c_string ("JSON array is too long"));
+        Lisp_Object result = Fmake_vector (make_natnum (size), Qunbound);
+        for (ptrdiff_t i = 0; i < size; ++i)
+          ASET (result, i,
+                json_to_lisp (json_array_get (json, i)));
+        --lisp_eval_depth;
+        return result;
+      }
+    case JSON_OBJECT:
+      {
+        if (++lisp_eval_depth > max_lisp_eval_depth)
+          xsignal0 (Qjson_object_too_deep);
+        size_t size = json_object_size (json);
+        if (FIXNUM_OVERFLOW_P (size))
+          xsignal1 (Qoverflow_error,
+                    build_pure_c_string ("JSON object has too many elements"));
+        Lisp_Object result = CALLN (Fmake_hash_table, QCtest, Qequal,
+                                    QCsize, make_natnum (size));
+        struct Lisp_Hash_Table *h = XHASH_TABLE (result);
+        const char *key_str;
+        json_t *value;
+        json_object_foreach (json, key_str, value)
+          {
+            Lisp_Object key = build_string (key_str);
+            EMACS_UINT hash;
+            ptrdiff_t i = hash_lookup (h, key, &hash);
+            eassert (i < 0);
+            hash_put (h, key, json_to_lisp (value), hash);
+          }
+        --lisp_eval_depth;
+        return result;
+      }
+    }
+  /* Can’t get here.  */
+  emacs_abort ();
+}
+
+DEFUN ("json-parse-string", Fjson_parse_string, Sjson_parse_string, 1, 1, NULL,
+       doc: /* Parse the JSON STRING into a Lisp object.
+This is essentially the reverse operation of `json-serialize', which
+see.  The returned object will be a vector or hashtable.  Its elements
+will be nil, t, `:json-false', numbers, strings, or further vectors
+and hashtables.  If there are duplicate keys in an object, all but the
+last one are ignored.  If STRING doesn't contain a valid JSON object,
+an error of type `json-parse-error' is signaled.  */)
+  (Lisp_Object string)
+{
+  ptrdiff_t count = SPECPDL_INDEX ();
+  check_string_without_embedded_nulls (string);
+
+  json_error_t error;
+  json_t *object = json_loads (SSDATA (string), 0, &error);
+  if (object == NULL)
+    json_parse_error (&error);
+
+  /* Avoid leaking the object in case of further errors.  */
+  if (object != NULL)
+    record_unwind_protect_ptr (json_release_object, object);
+
+  return unbind_to (count, json_to_lisp (object));
+}
+
+struct json_read_buffer_data
+{
+  ptrdiff_t point;
+};
+
+static size_t
+json_read_buffer_callback (void *buffer, size_t buflen, void *data)
+{
+  struct json_read_buffer_data *d = data;
+
+  /* First, parse from point to the gap or the end of the accessible
+     portion, whatever is closer.  */
+  ptrdiff_t point = d->point;
+  ptrdiff_t end;
+  {
+    bool overflow = INT_ADD_WRAPV (BUFFER_CEILING_OF (point), 1, &end);
+    eassert (!overflow);
+  }
+  size_t count;
+  {
+    bool overflow = INT_SUBTRACT_WRAPV (end, point, &count);
+    eassert (!overflow);
+  }
+  if (buflen < count)
+    count = buflen;
+  memcpy (buffer, BYTE_POS_ADDR (point), count);
+  {
+    bool overflow = INT_ADD_WRAPV (d->point, count, &d->point);
+    eassert (!overflow);
+  }
+  return count;
+}
+
+DEFUN ("json-parse-buffer", Fjson_parse_buffer, Sjson_parse_buffer,
+       0, 0, NULL,
+       doc: /* Read JSON object from current buffer starting at point.
+This is similar to `json-parse-string', which see.  Move point after
+the end of the object if parsing was successful.  On error, point is
+not moved.  */)
+  (void)
+{
+  ptrdiff_t count = SPECPDL_INDEX ();
+
+  ptrdiff_t point = PT_BYTE;
+  struct json_read_buffer_data data = {.point = point};
+  json_error_t error;
+  json_t *object = json_load_callback (json_read_buffer_callback, &data,
+                                       JSON_DISABLE_EOF_CHECK, &error);
+
+  if (object == NULL)
+    json_parse_error (&error);
+
+  /* Avoid leaking the object in case of further errors.  */
+  record_unwind_protect_ptr (json_release_object, object);
+
+  /* Convert and then move point only if everything succeeded.  */
+  Lisp_Object lisp = json_to_lisp (object);
+
+  {
+    /* Adjust point by how much we just read.  Do this here because
+       tokener->char_offset becomes incorrect below.  */
+    bool overflow = INT_ADD_WRAPV (point, error.position, &point);
+    eassert (!overflow);
+    eassert (point <= ZV_BYTE);
+    SET_PT_BOTH (BYTE_TO_CHAR (point), point);
+  }
+
+  return unbind_to (count, lisp);
+}
+
+/* Simplified version of ‘define-error’ that works with pure
+   objects.  */
+
+static void
+define_error (Lisp_Object name, const char *message, Lisp_Object parent)
+{
+  eassert (SYMBOLP (name));
+  eassert (SYMBOLP (parent));
+  Lisp_Object parent_conditions = Fget (parent, Qerror_conditions);
+  eassert (CONSP (parent_conditions));
+  eassert (!NILP (Fmemq (parent, parent_conditions)));
+  eassert (NILP (Fmemq (name, parent_conditions)));
+  Fput (name, Qerror_conditions, pure_cons (name, parent_conditions));
+  Fput (name, Qerror_message, build_pure_c_string (message));
+}
+
+void
+syms_of_json (void)
+{
+  DEFSYM (QCjson_false, ":json-false");
+
+  DEFSYM (Qstring_without_embedded_nulls_p, "string-without-embedded-nulls-p");
+  DEFSYM (Qjson_value_p, "json-value-p");
+
+  DEFSYM (Qjson_error, "json-error");
+  DEFSYM (Qjson_out_of_memory, "json-out-of-memory");
+  DEFSYM (Qjson_parse_error, "json-parse-error");
+  DEFSYM (Qjson_object_too_deep, "json-object-too-deep");
+  define_error (Qjson_error, "generic JSON error", Qerror);
+  define_error (Qjson_out_of_memory, "no free memory for creating JSON object",
+                Qjson_error);
+  define_error (Qjson_parse_error, "could not parse JSON stream",
+                Qjson_error);
+  define_error (Qjson_object_too_deep, "object cyclic or too deep",
+                Qjson_error);
+
+  DEFSYM (Qpure, "pure");
+  DEFSYM (Qside_effect_free, "side-effect-free");
+
+  DEFSYM (Qjson_serialize, "json-serialize");
+  DEFSYM (Qjson_parse_string, "json-parse-string");
+  Fput (Qjson_serialize, Qpure, Qt);
+  Fput (Qjson_serialize, Qside_effect_free, Qt);
+  Fput (Qjson_parse_string, Qpure, Qt);
+  Fput (Qjson_parse_string, Qside_effect_free, Qt);
+
+  defsubr (&Sjson_serialize);
+  defsubr (&Sjson_insert);
+  defsubr (&Sjson_parse_string);
+  defsubr (&Sjson_parse_buffer);
+}
diff --git a/src/lisp.h b/src/lisp.h
index c503082442..8d485098ac 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3440,6 +3440,11 @@ extern int x_bitmap_mask (struct frame *, ptrdiff_t);
 extern void reset_image_types (void);
 extern void syms_of_image (void);
 
+#ifdef HAVE_JSON
+/* Defined in json.c.  */
+extern void syms_of_json (void);
+#endif
+
 /* Defined in insdel.c.  */
 extern void move_gap_both (ptrdiff_t, ptrdiff_t);
 extern _Noreturn void buffer_overflow (void);
diff --git a/test/src/json-tests.el b/test/src/json-tests.el
new file mode 100644
index 0000000000..1d8f9a490b
--- /dev/null
+++ b/test/src/json-tests.el
@@ -0,0 +1,61 @@
+;;; json-tests.el --- unit tests for json.c          -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2017 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Unit tests for src/json.c.
+
+;;; Code:
+
+(require 'cl-lib)
+(require 'map)
+
+(ert-deftest json-serialize/roundtrip ()
+  (let ((lisp [nil :json-false t 0 123 -456 3.75 "foo"])
+        (json "[null,false,true,0,123,-456,3.75,\"foo\"]"))
+    (should (equal (json-serialize lisp) json))
+    (with-temp-buffer
+      (json-insert lisp)
+      (should (equal (buffer-string) json))
+      (should (eobp)))
+    (should (equal (json-parse-string json) lisp))
+    (with-temp-buffer
+      (insert json)
+      (goto-char 1)
+      (should (equal (json-parse-buffer) lisp))
+      (should (eobp)))))
+
+(ert-deftest json-serialize/object ()
+  (let ((table (make-hash-table :test #'equal)))
+    (puthash "abc" [1 2 t] table)
+    (puthash "def" nil table)
+    (should (equal (json-serialize table)
+                   "{\"abc\":[1,2,true],\"def\":null}"))))
+
+(ert-deftest json-parse-string/object ()
+  (let ((actual
+         (json-parse-string
+          "{ \"abc\" : [1, 2, true], \"def\" : null, \"abc\" : [9, false] }\n")))
+    (should (hash-table-p actual))
+    (should (equal (hash-table-count actual) 2))
+    (should (equal (cl-sort (map-pairs actual) #'string< :key #'car)
+                   '(("abc" . [9 :json-false]) ("def"))))))
+
+(provide 'json-tests)
+;;; json-tests.el ends here
-- 
2.14.1