This attempts to document some of the issues recently discussed
on emacs-devel, and to indicate other such issues.  The section
could be a lot longer.
* doc/lispref/os.texi (Security Considerations):
New node.
* doc/lispref/elisp.texi (Top):
* doc/lispref/processes.texi (Shell Arguments):
* lisp/subr.el (shell-quote-argument):
* src/callproc.c (syms_of_callproc):
Reference it.

Omit ‘#include "foo.h"’ unless the file needs foo.h (Bug#21707).
In a few cases, add ‘#include "foo.h"’ if the file needs foo.h
but does not include it directly.  As a general rule, a source
file should include foo.h if it needs the interfaces that foo.h
defines.
* src/alloc.c: Don’t include process.h.  Include dispextern.h,
systime.h.
* src/atimer.c: Don’t include blockinput.h.
* src/buffer.c: Include coding.h, systime.h.  Don’t include
keyboard.h, coding.h.
* src/callint.c: Don’t include commands.h, keymap.h.
* src/callproc.c: Don’t include character.h, ccl.h, composite.h,
systty.h, termhooks.h.
* src/casetab.c: Don’t include character.h.
* src/category.c: Don’t include charset.h, keymap.h.
* src/ccl.h: Don’t include character.h.
* src/character.c: Don’t include charset.h.
* src/charset.c: Don’t include disptab.h.
* src/chartab.c: Don’t include ccl.h.
* src/cm.c: Don’t include frame.h, termhooks.h.
* src/cmds.c: Don’t include window.h, dispextern.h.
* src/coding.c: Don’t include window.h, frame.h.
* src/composite.c: Include composite.h.  Don’t include window.h,
font.h.
* src/data.c: Don’t include syssignal.h, termhooks.h, font.h.
* src/dbusbind.c: Don’t include frame.h.
* src/decompress.c: Don’t include character.h.
* src/dired.c: Don’t include character.h, commands.h, charset.h.
* src/dispnew.c: Don’t include character.h, indent.h, intervals.h,
process.h, timespec.h.  Include systime.h.
* src/doc.c: Include coding.h.  Don’t include keyboard.h.
* src/editfns.c: Include composite.h.  Don’t include frame.h.
* src/emacs.c: Include fcntl.h, coding.h.  Don’t include
commands.h, systty.h..
* src/fileio.c: Don’t include intervals.h, dispextern.h.
Include composite.h.
* src/filelock.c: Don’t include character.h, systime.h.
* src/fns.c: Don’t include time.h, commands.h, keyboard.h,
keymap.h, frame.h, blockinput.h, xterm.h.  Include composite.h.
* src/font.c: Include termhooks.h.
* src/font.h: Don’t include ccl.h, frame.h.  Add forward decls of
struct composition_it, struct face, struct glyph_string.
* src/fontset.c: Don’t include buffer.h, ccl.h, keyboard.h,
intervals.h, window.h, termhooks.h.
* src/frame.c: Don’t include character.h, commands.h, font.h.
* src/frame.h: Don’t include dispextern.h.
* src/fringe.c: Don’t include character.h.
* src/ftcrfont.c: Don’t include dispextern.h, frame.h,
character.h, charset.h, fontset.h.
* src/ftfont.c: Don’t include frame.h, blockinput.h, coding.h,
fontset.h.
* src/ftxfont.c: Don’t include dispextern.h, character.h,
charset.h, fontset.h.
* src/gfilenotify.c: Don’t include frame.h, process.h.
* src/gtkutil.c: Include dispextern.h, frame.h, systime.h.
Don’t include syssignal.h, buffer.h, charset.h, font.h.
* src/gtkutil.h: Don’t include frame.h.
* src/image.c: Include fcntl.h and stdio.h instead of sysstdio.h.
Don’t include character.h.
* src/indent.c: Don’t include keyboard.h, termchar.h.
* src/inotify.c: Don’t include character.h, frame.h.
* src/insdel.c: Include composite.h.  Don’t include blockinput.h.
* src/intervals.c: Don’t include character.h, keyboard.h.
* src/intervals.h: Don’t include dispextern.h, composite.h.
* src/keyboard.c: Don’t include sysstdio.h, disptab.h, puresize.h.
Include coding.h.
* src/keyboard.h: Don’t incldue systime.h.
* src/keymap.c: Don’t include charset.h, frame.h.
* src/lread.c: Include dispextern.h and systime.h.
Don’t include frame.h.  Include systime.h.
* src/macros.c: Don’t include commands.h, character.h, buffer.h.
* src/menu.c: Include character.h, coding.h.  Don’t include
dispextern.h.
* src/menu.h: Don’t include systime.h.
* src/minibuf.c: Don’t include commands.h, dispextern.h, syntax.h,
intervals.h, termhooks.h.
* src/print.c: Include coding.h.  Don’t include keyboard.h,
window.h, dispextern.h, termchar.h, termhooks.h, font.h.
Add forward decl of struct terminal.
* src/process.c: Don’t include termhooks.h, commands.h,
dispextern.h, composite.h.
* src/region-cache.c: Don’t include character.h.
* src/scroll.c: Don’t include keyboard.h, window.h.
* src/search.c: Don’t include category.h, commands.h.
* src/sound.c: Don’t include dispextern.h.
* src/syntax.c: Don’t include command.h, keymap.h.
* src/sysdep.c: Don’t include window.h, dispextern.h.
* src/systime.h: Use ‘#ifdef emacs’, not ‘#ifdef EMACS_LISP_H’,
* src/term.c: Don’t include systty.h, intervals.h, xterm.h.
* src/terminal.c: Include character.h.
Don’t include charset.h, coding.h.
* src/textprop.c: Don’t include character.h.
* src/undo.c: Don’t include character.h, commands.h, window.h.
* src/unexsol.c: Don’t include character.h, charset.h.
* src/widget.c: Include widget.h.  Don’t include keyboard.h,
window.h, dispextern.h, blockinput.h, character.h, font.h.
* src/widgetprv.h: Don’t include widget.h.
* src/window.c: Don’t include character.h, menu.h, intervals.h.
* src/xdisp.c: Include composite.h, systime.h.  Don’t include
macros.h, process.h.
* src/xfaces.c: Don’t include charset.h, keyboard.h, termhooks.h,
intervals.h.
* src/xfns.c: Don’t include menu.h, character.h, intervals.h,
epaths.h, fontset.h, systime.h, atimer.h, termchar.h.
* src/xfont.c: Don’t include dispextern.h, fontset.h, ccl.h.
* src/xftfont.c: Don’t include dispextern.h, character.h, fontset.h.
* src/xgselect.c: Don’t include timespec.h, frame.h.
Include systime.h.
* src/xgselect.h: Don’t include time.h.
Use a forward decl to struct timespec instead.
* src/xmenu.c: Don’t include keymap.h, character.h, charset.h,
dispextern.h.  Include systime.h.
* src/xml.c: Don’t include character.h.
* src/xrdb.c [USE_MOTIF]: Don’t include keyboard.h.
* src/xselect.c: Don’t include dispextern.h, character.h,
buffer.h, process.h.
* src/xsmfns.c: Don’t include systime.h, sysselect.h.
* src/xterm.c: Don’t include syssignal.h, charset.h, disptab.h,
intervals.h process.h, keymap.h, xgselect.h.  Include composite.h.

Although \( is needed in docstrings in Elisp code, it is not needed in
docstrings in C code, since C function definitiions do not start with
a parenthesis.  The backslashes made the docstrings a bit harder to
read and to format in columns.  Also, some C docstrings had ( in
column 1 and this did not appear to be causing any problems.  So,
simplify C docstrings by replacing \( with ( and \) with ).

This removes the need for GCPRO1 etc.  Suggested by Stefan Monnier in:
http://lists.gnu.org/archive/html/emacs-devel/2015-08/msg00918.html
* doc/lispref/internals.texi (Writing Emacs Primitives):
* etc/NEWS:
Document the change.
* src/alloc.c (gcprolist, dump_zombies, MAX_ZOMBIES, zombies)
(nzombies, ngcs, avg_zombies, max_live, max_zombies, avg_live)
(Fgc_status, check_gcpros, relocatable_string_data_p, gc-precise):
* src/bytecode.c (mark_byte_stack) [BYTE_MARK_STACK]:
* src/eval.c (gcpro_level) [DEBUG_GCPRO]:
* src/lisp.h (struct handler.gcpro, struct gcpro, GC_MARK_STACK)
(GC_USE_GCPROS_AS_BEFORE, GC_MAKE_GCPROS_NOOPS)
(GC_MARK_STACK_CHECK_GCPROS, GC_USE_GCPROS_CHECK_ZOMBIES)
(BYTE_MARK_STACK, GCPRO1, GCPRO2, GCPRO3, GCPRO4, GCPRO5, GCPRO6)
(GCPRO7, UNGCPRO, RETURN_UNGCPRO):
Remove.  All uses removed.  The code now assumes
GC_MARK_STACK == GC_MAKE_GCPROS_NOOPS.
* src/bytecode.c (relocate_byte_stack):
Rename from unmark_byte_stack, since it now only relocates.
All callers changed.
* src/frame.c (make_frame): Add an IF_LINT to pacify GCC 5.2
with GCPROs removed.
* src/systime.h: Use EMACS_LISP_H as the canary instead of GCPRO1.
* test/automated/finalizer-tests.el (finalizer-basic)
(finalizer-circular-reference, finalizer-cross-reference)
(finalizer-error):
* test/automated/generator-tests.el (cps-test-iter-close-finalizer):
Remove tests, as they depend on gc-precise.

* src/callproc.c (init_callproc): If running uninstalled, do not
include eventual installation libexec directory in exec-path.

* buffer.c (syms_of_buffer):
* bytecode.c (exec_byte_code):
* callint.c (Fcall_interactively):
* callproc.c (create_temp_file):
* charset.c (define_charset_internal):
* coding.c (syms_of_coding):
* editfns.c (syms_of_editfns):
* emacs.c (main):
* fns.c (syms_of_fns):
* frame.c (delete_frame, Fframe_parameters):
* keyboard.c (syms_of_keyboard):
* keymap.c (syms_of_keymap):
* minibuf.c (read_minibuf, syms_of_minibuf):
* nsfns.m (ns_cursor_type_to_lisp):
* textprop.c (syms_of_textprop):
* xdisp.c (Fformat_mode_line, syms_of_xdisp):
* xfns.c (x_create_tip_frame, Fx_select_font):
* xml.c (parse_region):
Prefer constants like 'Qfoo' to calls like 'intern ("foo")'.
* buffer.c (syms_of_buffer): OK to do (put 'erase-buffer 'disabled
t) here now ...
(keys_of_buffer): ... instead of here.
* ftfont.c (syms_of_ftfont): Move DEFSYM of Qmono from here ...
* xfns.c (syms_of_xfns): ... to here, since ftfont.c is more
optional than xfns.c.

* process.h (remove_slash_colon): New function.
* callproc.c (encode_current_directory, call_process):
* process.c (Fstart_process): Use it.

* callproc.c (encode_current_directory, Fcall_process, call_process):
* dired.c (directory_files_internal, file_name_completion):
Do not check for STRING_MULTIBYTE because encode_file_name
is a no-op for unibyte strings.

Run admin/update-copyright.

* admin/merge-gnulib (GNULIB_MODULES): Add stpcpy.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* lib/stpcpy.c, m4/stpcpy.m4: New files, from gnulib.
* lib-src/ebrowse.c (sym_scope_1, operator_name, open_file):
* lib-src/emacsclient.c (get_server_config, set_local_socket)
(start_daemon_and_retry_set_socket):
* lib-src/etags.c (main, C_entries, relative_filename):
* lib-src/pop.c (sendline):
* lib-src/update-game-score.c (main):
* lwlib/xlwmenu.c (resource_widget_value):
* src/callproc.c (child_setup):
* src/dbusbind.c (xd_signature_cat):
* src/doc.c (get_doc_string, Fsnarf_documentation):
* src/editfns.c (Fuser_full_name):
* src/frame.c (xrdb_get_resource):
* src/gtkutil.c (xg_get_file_with_chooser):
* src/tparam.c (tparam1):
* src/xfns.c (xic_create_fontsetname):
* src/xrdb.c (gethomedir, get_user_db, get_environ_db):
* src/xsmfns.c (smc_save_yourself_CB):
Rewrite to avoid the need for strcat, typically by using stpcpy
and/or lispstpcpy.  strcat tends to be part of O(N**2) algorithms.
* src/doc.c (sibling_etc):
* src/xrdb.c (xdefaults):
Now a top-level static constant.