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_malloc, json_free, json_has_prefix, json_has_suffix)
(json_make_string, json_build_string, json_encode)
(json_out_of_memory, json_parse_error)
(json_release_object, check_string_without_embedded_nulls, json_check)
(lisp_to_json, lisp_to_json_toplevel, lisp_to_json_toplevel_1)
(json_insert, json_insert_callback, json_to_lisp)
(json_read_buffer_callback, Fjson_parse_buffer, define_error): New
helper functions.
(init_json, syms_of_json): New file.

* src/lisp.h: Declaration for init_json and syms_of_json.

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

* src/eval.c (internal_catch_all, internal_catch_all_1): New helper
functions to catch all signals.
(syms_of_eval): Add uninterned symbol to signify out of memory.

* 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)
(json-parse-string/string, json-serialize/string)
(json-parse-string/incomplete, json-parse-string/trailing)
(json-parse-buffer/incomplete, json-parse-buffer/trailing): New unit
tests.

* doc/lispref/text.texi (Parsing JSON): New manual section.

5 files changed, 653 insertions, 3 deletions

diff --git a/src/Makefile.in b/src/Makefile.in
index 9a8c9c85f04..b395627893d 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -312,6 +312,10 @@ LIBGNUTLS_CFLAGS = @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=-Demacs $(MYCPPFLAGS) -I. -I$(srcdir) \
   $(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 = dispnew.o frame.o scroll.o xdisp.o menu.o $(XMENU_OBJ) window.o \
         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) $(W32_LIBS) $(LIBS_GNUSTEP) $(LIBX_BASE) $(LIBIMAGE) \
    $(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 808abcd9aa2..7c1ae1f2c5b 100644
--- a/src/emacs.c
+++ b/src/emacs.c
@@ -1262,6 +1262,10 @@ Using an Emacs configured with --with-x-toolkit=lucid does not have this problem
   running_asynch_code = 0;
   init_random ();
 
+#ifdef HAVE_JSON
+  init_json ();
+#endif
+
   no_loadup
     = argmatch (argv, argc, "-nl", "--no-loadup", 6, NULL, &skip_args);
 
@@ -1608,6 +1612,10 @@ 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/eval.c b/src/eval.c
index 47c4f17eabc..b774fd06139 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1416,6 +1416,57 @@ internal_condition_case_n (Lisp_Object (*bfun) (ptrdiff_t, Lisp_Object *),
     }
 }
 
+static Lisp_Object
+internal_catch_all_1 (Lisp_Object (*function) (void *), void *argument)
+{
+  struct handler *c = push_handler_nosignal (Qt, CATCHER_ALL);
+  if (c == NULL)
+    return Qcatch_all_memory_full;
+
+  if (sys_setjmp (c->jmp) == 0)
+    {
+      Lisp_Object val = function (argument);
+      eassert (handlerlist == c);
+      handlerlist = c->next;
+      return val;
+    }
+  else
+    {
+      eassert (handlerlist == c);
+      Lisp_Object val = c->val;
+      handlerlist = c->next;
+      Fsignal (Qno_catch, val);
+    }
+}
+
+/* Like a combination of internal_condition_case_1 and internal_catch.
+   Catches all signals and throws.  Never exits nonlocally; returns
+   Qcatch_all_memory_full if no handler could be allocated.  */
+
+Lisp_Object
+internal_catch_all (Lisp_Object (*function) (void *), void *argument,
+                    Lisp_Object (*handler) (Lisp_Object))
+{
+  struct handler *c = push_handler_nosignal (Qt, CONDITION_CASE);
+  if (c == NULL)
+    return Qcatch_all_memory_full;
+
+  if (sys_setjmp (c->jmp) == 0)
+    {
+      Lisp_Object val = internal_catch_all_1 (function, argument);
+      eassert (handlerlist == c);
+      handlerlist = c->next;
+      return val;
+    }
+  else
+    {
+      eassert (handlerlist == c);
+      Lisp_Object val = c->val;
+      handlerlist = c->next;
+      return handler (val);
+    }
+}
+
 struct handler *
 push_handler (Lisp_Object tag_ch_val, enum handlertype handlertype)
 {
@@ -4067,6 +4118,9 @@ alist of active lexical bindings.  */);
 
   inhibit_lisp_code = Qnil;
 
+  DEFSYM (Qcatch_all_memory_full, "catch-all-memory-full");
+  Funintern (Qcatch_all_memory_full, Qnil);
+
   defsubr (&Sor);
   defsubr (&Sand);
   defsubr (&Sif);
diff --git a/src/json.c b/src/json.c
new file mode 100644
index 00000000000..dc449e43e11
--- /dev/null
+++ b/src/json.c
@@ -0,0 +1,576 @@
+/* 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 <errno.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#include <jansson.h>
+
+#include "lisp.h"
+#include "buffer.h"
+#include "coding.h"
+
+/* We install a custom allocator so that we can avoid objects larger
+   than PTRDIFF_MAX.  Such objects wouldn’t play well with the rest of
+   Emacs’s codebase, which generally uses ptrdiff_t for sizes and
+   indices.  The other functions in this file also generally assume
+   that size_t values never exceed PTRDIFF_MAX.  */
+
+static void *
+json_malloc (size_t size)
+{
+  if (size > PTRDIFF_MAX)
+    {
+      errno = ENOMEM;
+      return NULL;
+    }
+  return malloc (size);
+}
+
+static void
+json_free (void *ptr)
+{
+  free (ptr);
+}
+
+void
+init_json (void)
+{
+  json_set_alloc_funcs (json_malloc, json_free);
+}
+
+/* Return whether STRING starts with PREFIX.  */
+
+static bool
+json_has_prefix (const char *string, const char *prefix)
+{
+  size_t string_len = strlen (string);
+  size_t prefix_len = strlen (prefix);
+  return string_len >= prefix_len && memcmp (string, prefix, prefix_len) == 0;
+}
+
+/* Return whether STRING ends with SUFFIX.  */
+
+static bool
+json_has_suffix (const char *string, const char *suffix)
+{
+  size_t string_len = strlen (string);
+  size_t suffix_len = strlen (suffix);
+  return string_len >= suffix_len
+    && memcmp (string + string_len - suffix_len, suffix, suffix_len) == 0;
+}
+
+/* Create a multibyte Lisp string from the UTF-8 string in
+   [DATA, DATA + SIZE).  If the range [DATA, DATA + SIZE) does not
+   contain a valid UTF-8 string, an unspecified string is
+   returned.  */
+
+static Lisp_Object
+json_make_string (const char *data, ptrdiff_t size)
+{
+  return code_convert_string (make_specified_string (data, -1, size, false),
+                              Qutf_8_unix, Qt, false, true, true);
+}
+
+/* Create a multibyte Lisp string from the null-terminated UTF-8
+   string beginning at DATA.  If the string is not a valid UTF-8
+   string, an unspecified string is returned.  */
+
+static Lisp_Object
+json_build_string (const char *data)
+{
+  return json_make_string (data, strlen (data));
+}
+
+/* Return a unibyte string containing the sequence of UTF-8 encoding
+   units of the UTF-8 representation of STRING.  If STRING does not
+   represent a sequence of Unicode scalar values, return a string with
+   unspecified contents.  */
+
+static Lisp_Object
+json_encode (Lisp_Object string)
+{
+  return code_convert_string (string, Qutf_8_unix, Qt, true, true, true);
+}
+
+static _Noreturn void
+json_out_of_memory (void)
+{
+  xsignal0 (Qjson_out_of_memory);
+}
+
+/* Signal a Lisp error corresponding to the JSON ERROR.  */
+
+static _Noreturn void
+json_parse_error (const json_error_t *error)
+{
+  Lisp_Object symbol;
+  /* FIXME: Upstream Jansson should have a way to return error codes
+     without parsing the error messages.  See
+     https://github.com/akheron/jansson/issues/352.  */
+  if (json_has_suffix (error->text, "expected near end of file"))
+    symbol = Qjson_end_of_file;
+  else if (json_has_prefix (error->text, "end of file expected"))
+    symbol = Qjson_trailing_content;
+  else
+    symbol = Qjson_parse_error;
+  xsignal (symbol,
+           list5 (json_build_string (error->text),
+                  json_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);
+}
+
+/* Signal an error if OBJECT is not a string, or if OBJECT contains
+   embedded null characters.  */
+
+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);
+}
+
+/* Signal an error of type `json-out-of-memory' if OBJECT is
+   NULL.  */
+
+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);
+
+/* Convert a Lisp object to a toplevel JSON object (array or object).
+   This returns Lisp_Object so we can use unbind_to.  The return value
+   is always nil.  */
+
+static _GL_ARG_NONNULL ((2)) Lisp_Object
+lisp_to_json_toplevel_1 (Lisp_Object lisp, json_t **json)
+{
+  if (VECTORP (lisp))
+    {
+      ptrdiff_t size = ASIZE (lisp);
+      *json = json_check (json_array ());
+      ptrdiff_t count = SPECPDL_INDEX ();
+      record_unwind_protect_ptr (json_release_object, json);
+      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 (json_array_size (*json) == size);
+      clear_unwind_protect (count);
+      return unbind_to (count, Qnil);
+    }
+  else if (HASH_TABLE_P (lisp))
+    {
+      struct Lisp_Hash_Table *h = XHASH_TABLE (lisp);
+      *json = json_check (json_object ());
+      ptrdiff_t count = SPECPDL_INDEX ();
+      record_unwind_protect_ptr (json_release_object, *json);
+      for (ptrdiff_t i = 0; i < HASH_TABLE_SIZE (h); ++i)
+        if (!NILP (HASH_HASH (h, i)))
+          {
+            Lisp_Object key = json_encode (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 ();
+          }
+      clear_unwind_protect (count);
+      return unbind_to (count, Qnil);
+    }
+  wrong_type_argument (Qjson_value_p, lisp);
+}
+
+/* Convert LISP to a toplevel JSON object (array or object).  Signal
+   an error of type `wrong-type-argument' if LISP is not a vector or
+   hashtable.  */
+
+static json_t *
+lisp_to_json_toplevel (Lisp_Object lisp)
+{
+  if (++lisp_eval_depth > max_lisp_eval_depth)
+    xsignal0 (Qjson_object_too_deep);
+  json_t *json;
+  lisp_to_json_toplevel_1 (lisp, &json);
+  --lisp_eval_depth;
+  return json;
+}
+
+/* Convert LISP to any JSON object.  Signal an error of type
+   `wrong-type-argument' if the type of LISP can't be converted to a
+   JSON object.  */
+
+static json_t *
+lisp_to_json (Lisp_Object lisp)
+{
+  if (EQ (lisp, QCnull))
+    return json_check (json_null ());
+  else if (EQ (lisp, QCfalse))
+    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))
+    {
+      Lisp_Object encoded = json_encode (lisp);
+      ptrdiff_t size = SBYTES (encoded);
+      return json_check (json_stringn (SSDATA (encoded), size));
+    }
+
+  /* LISP now must be a vector or hashtable.  */
+  return lisp_to_json_toplevel (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 `:null', `:false', t, numbers, strings, or other vectors and
+hashtables.  `:null', `:false', and t will be converted to JSON null,
+false, and true 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_toplevel (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, json_build_string (string));
+}
+
+struct json_buffer_and_size
+{
+  const char *buffer;
+  ptrdiff_t size;
+};
+
+static Lisp_Object
+json_insert (void *data)
+{
+  struct json_buffer_and_size *buffer_and_size = data;
+  /* FIXME: This should be possible without creating an intermediate
+     string object.  */
+  Lisp_Object string
+    = json_make_string (buffer_and_size->buffer, buffer_and_size->size);
+  insert1 (string);
+  return Qnil;
+}
+
+struct json_insert_data
+{
+  /* nil if json_insert succeeded, otherwise the symbol
+     Qcatch_all_memory_full or a cons (ERROR-SYMBOL . ERROR-DATA).  */
+  Lisp_Object error;
+};
+
+/* Callback for json_dump_callback that inserts the UTF-8 string in
+   [BUFFER, BUFFER + SIZE) into the current buffer.
+   If [BUFFER, BUFFER + SIZE) does not contain a valid UTF-8 string,
+   an unspecified string is inserted into the buffer.  DATA must point
+   to a structure of type json_insert_data.  This function may not
+   exit nonlocally.  It catches all nonlocal exits and stores them in
+   data->error for reraising.  */
+
+static int
+json_insert_callback (const char *buffer, size_t size, void *data)
+{
+  struct json_insert_data *d = data;
+  struct json_buffer_and_size buffer_and_size
+    = {.buffer = buffer, .size = size};
+  d->error = internal_catch_all (json_insert, &buffer_and_size, Fidentity);
+  return NILP (d->error) ? 0 : -1;
+}
+
+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)
+    {
+      if (CONSP (data.error))
+        xsignal (XCAR (data.error), XCDR (data.error));
+      else
+        json_out_of_memory ();
+    }
+
+  return unbind_to (count, Qnil);
+}
+
+/* Convert a JSON object to a Lisp object.  */
+
+static _GL_ARG_NONNULL ((1)) Lisp_Object
+json_to_lisp (json_t *json)
+{
+  switch (json_typeof (json))
+    {
+    case JSON_NULL:
+      return QCnull;
+    case JSON_FALSE:
+      return QCfalse;
+    case JSON_TRUE:
+      return Qt;
+    case JSON_INTEGER:
+      /* Return an integer if possible, a floating-point number
+         otherwise.  This loses precision for integers with large
+         magnitude; however, such integers tend to be nonportable
+         anyway because many JSON implementations use only 64-bit
+         floating-point numbers with 53 mantissa bits.  See
+         https://tools.ietf.org/html/rfc7159#section-6 for some
+         discussion.  */
+      return make_fixnum_or_float (json_integer_value (json));
+    case JSON_REAL:
+      return make_float (json_real_value (json));
+    case JSON_STRING:
+      return json_make_string (json_string_value (json),
+                               json_string_length (json));
+    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))
+          xsignal0 (Qoverflow_error);
+        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))
+          xsignal0 (Qoverflow_error);
+        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 = json_build_string (key_str);
+            EMACS_UINT hash;
+            ptrdiff_t i = hash_lookup (h, key, &hash);
+            /* Keys in JSON objects are unique, so the key can’t be
+               present yet.  */
+            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 `:null', `:false', t, 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 ();
+  Lisp_Object encoded = json_encode (string);
+  check_string_without_embedded_nulls (encoded);
+
+  json_error_t error;
+  json_t *object = json_loads (SSDATA (encoded), 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
+{
+  /* Byte position of position to read the next chunk from.  */
+  ptrdiff_t point;
+};
+
+/* Callback for json_load_callback that reads from the current buffer.
+   DATA must point to a structure of type json_read_buffer_data.
+   data->point must point to the byte position to read from; after
+   reading, data->point is advanced accordingly.  The buffer point
+   itself is ignored.  This function may not exit nonlocally.  */
+
+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 = BUFFER_CEILING_OF (point) + 1;
+  ptrdiff_t count = end - point;
+  if (buflen < count)
+    count = buflen;
+  memcpy (buffer, BYTE_POS_ADDR (point), count);
+  d->point += count;
+  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.  */
+  point += error.position;
+  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 (QCnull, ":null");
+  DEFSYM (QCfalse, ":false");
+
+  DEFSYM (Qstring_without_embedded_nulls_p, "string-without-embedded-nulls-p");
+  DEFSYM (Qjson_value_p, "json-value-p");
+
+  DEFSYM (Qutf_8_unix, "utf-8-unix");
+
+  DEFSYM (Qjson_error, "json-error");
+  DEFSYM (Qjson_out_of_memory, "json-out-of-memory");
+  DEFSYM (Qjson_parse_error, "json-parse-error");
+  DEFSYM (Qjson_end_of_file, "json-end-of-file");
+  DEFSYM (Qjson_trailing_content, "json-trailing-content");
+  DEFSYM (Qjson_object_too_deep, "json-object-too-deep");
+  define_error (Qjson_error, "generic JSON error", Qerror);
+  define_error (Qjson_out_of_memory,
+                "not enough memory for creating JSON object", Qjson_error);
+  define_error (Qjson_parse_error, "could not parse JSON stream",
+                Qjson_error);
+  define_error (Qjson_end_of_file, "end of JSON stream", Qjson_parse_error);
+  define_error (Qjson_trailing_content, "trailing content after JSON stream",
+                Qjson_parse_error);
+  define_error (Qjson_object_too_deep,
+                "object cyclic or Lisp evaluation 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 68824d6b393..91ed14fa4c9 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3452,6 +3452,12 @@ 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 init_json (void);
+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);
@@ -3875,6 +3881,7 @@ extern Lisp_Object internal_condition_case_2 (Lisp_Object (*) (Lisp_Object, Lisp
 extern Lisp_Object internal_condition_case_n
     (Lisp_Object (*) (ptrdiff_t, Lisp_Object *), ptrdiff_t, Lisp_Object *,
      Lisp_Object, Lisp_Object (*) (Lisp_Object, ptrdiff_t, Lisp_Object *));
+extern Lisp_Object internal_catch_all (Lisp_Object (*) (void *), void *, Lisp_Object (*) (Lisp_Object));
 extern struct handler *push_handler (Lisp_Object, enum handlertype);
 extern struct handler *push_handler_nosignal (Lisp_Object, enum handlertype);
 extern void specbind (Lisp_Object, Lisp_Object);