path: root/libcr/coroutine.c

/* coroutine.c - Simple embeddable coroutine implementation
 *
 * Copyright (C) 2024  Luke T. Shumaker <lukeshu@lukeshu.com>
 * SPDX-Licence-Identifier: AGPL-3.0-or-later
 */

#include <assert.h>
#include <setjmp.h> /* for setjmp(), longjmp(), jmp_buf */
#include <stdint.h> /* for uint8_t */
#include <stdio.h>  /* for printf(), fprintf(), stderr */
#include <stdlib.h> /* for aligned_alloc(), free() */

#include <libcr/coroutine.h>

/* Configuration **************************************************************/

#define USE_CONFIG_COROUTINE
#include "config.h"

#ifndef CONFIG_COROUTINE_DEFAULT_STACK_SIZE
#	error config.h must define CONFIG_COROUTINE_DEFAULT_STACK_SIZE
#endif
#ifndef CONFIG_COROUTINE_NUM
#	error config.h must define CONFIG_COROUTINE_NUM
#endif
#ifndef CONFIG_COROUTINE_MEASURE_STACK
#	error config.h must define CONFIG_COROUTINE_MEASURE_STACK
#endif
#ifndef CONFIG_COROUTINE_PROTECT_STACK
#	error config.h must define CONFIG_COROUTINE_PROTECT_STACK
#endif
#ifndef CONFIG_COROUTINE_DEBUG
#	error config.h must define CONFIG_COROUTINE_DEBUG
#endif

/* Implementation *************************************************************/

/*
 * Portability notes:
 *
 * - It uses GCC `__attribute__`s, and the GNUC ({ ... }) statement
 *   exprs extension.
 *
 * - It has a small bit of platform-specific code in the "platform
 *   support" section.  Other than this, it should be portable to
 *   other platforms CPUs.  It currently contains implementations for
 *   __x86_64__ (assumes POSIX) and __arm__ (assumes bare-metal), and
 *   should be fairly easy to add implementations for other platforms.
 *
 * - It uses setjmp()/longjmp() in "unsafe" ways.  POSIX-2017
 *   longjmp(3p) says
 *
 *   > If the most recent invocation of setjmp() with the
 *   > corresponding jmp_buf ... or if the function containing the
 *   > invocation of setjmp() has terminated execution in the interim,
 *   > or if the invocation of setjmp() was within the scope of an
 *   > identifier with variably modified type and execution has left
 *   > that scope in the interim, the behavior is undefined.
 *
 *   We use longjmp() both of these scenarios, but make it OK by using
 *   call_with_stack() to manage the stack ourselves, assuming the
 *   sole reason that longjmp() behavior is undefined in such cases is
 *   because the stack that its saved stack-pointer points to is no
 *   longer around.  It seems absurd that an implementation would
 *   choose to do something else, but I'm calling it out here because
 *   you never know.
 *
 *   Our assumptions seem to be valid for
 *   x86_64-pc-linux-gnu/gcc-14.2.1/glibc-2.40 and
 *   arm-none-eabi/gcc-14.1.0/newlib-4.4.0.
 *
 *   Why not use <ucontext.h>, the now-deprecated (was in
 *   POSIX.1-2001, is gone in POSIX-2008) predecesor to <setjmp.h>?
 *   It would let us do this without any assembly or unsafe
 *   assumptions.  Simply: because newlib does not provide it.  But it
 *   would let us avoid having an `sp` member in `struct coroutine`...
 *   Maybe https://github.com/kaniini/libucontext ?  Or building a
 *   ucontext-lib abstraction on top of setjmp/longjmp?
 */

/*
 * Design decisions and notes:
 *
 * - Coroutines are launched with a stack that is filled with known
 *   arbitrary values.  Because all stack variables should be
 *   initialized before they are read, this "shouldn't" make a
 *   difference, but: (1) Initializing it to a known value allows us
 *   to measure how much of the stack was written to, which is helpful
 *   to tune stack sizes.  (2) Leaving it uninitialized just gives me
 *   the willies.
 *
 * - Because embedded programs should be adverse to using the heap,
 *   CONFIG_COROUTINE_NUM is fixed, instead of having coroutine_add()
 *   dynamically grow the coroutine_table as-needed.
 *
 * - On the flip-side, coroutine stacks are allocated on the heap
 *   instead of having them be statically-allocated along with
 *   coroutine_table.  (1) This reduced the blast-area of damage for a
 *   stack-overflow; and indeed if the end of the stack alignes with a
 *   page-boundary then memory-protection can even detect the overflow
 *   for us.  (2) Having different-looking addresses for stack-area vs
 *   static-area is handy for making things jump out at you when
 *   debugging.  (3) This can likely also improve things with being
 *   page-aligned.
 *
 * - Coroutines must use cr_exit() instead of returning because if
 *   they return then they will return to call_with_stack() in
 *   coroutine_add() (not to after the longjmp() call in
 *   coroutine_main()), and besides being
 *   wrong-for-our-desired-flow-control, that's a stack location that
 *   no longer exists.
 */

#define ALWAYS_INLINE inline __attribute__((always_inline))

/* platform support ***********************************************************/

/* As part of sbc-harness, this only really needs to support ARM-32, but being
 * able to run it on my x86-64 GNU/Linux laptop is useful for debugging.  */

#define CR_PLAT_STACK_ALIGNMENT \
	({ __attribute__((aligned)) void fn(void) {}; __alignof__(fn); })

#if 0
{ /* to get Emacs indentation to work how I want */
#endif
#if __arm__ /*======================================================*/
	/* Assume bare metal ARMv6-M.  */

	#define CR_PLAT_STACK_GROWS_DOWNWARD 1

	/* Wrappers for setjmp()/longjmp() that do *not* save the
	 * interrupt mask.
	 *
	 * newlib setjmp()/longjmp() do not save the interrupt mask, *
	 * so we can use them directly.  */
	#define cr_plat_setjmp(env)             setjmp(env)
	#define cr_plat_longjmp(env, val)       longjmp(env, val)

	static ALWAYS_INLINE void cr_plat_wait_for_interrupt(void) {
		asm volatile ("wfi":::"memory");
	}
	void _cr_plat_disable_interrupts(void) {
		asm volatile ("cpsid i":::"memory");
	}
	void _cr_plat_enable_interrupts(void) {
		asm volatile ("cpsie i":::"memory");
	}

	#if CONFIG_COROUTINE_MEASURE_STACK
	static ALWAYS_INLINE uintptr_t cr_plat_get_sp(void) {
		uintptr_t sp;
		asm volatile ("mov %0, sp":"=r"(sp));
		return sp;
	}
	#endif

	static void cr_plat_call_with_stack(void *stack,
	                                    cr_fn_t fn, void *args) {
		static void *saved_sp = NULL;
		/* str/ldr can only work with a "lo" register, which
		 * sp is not, so we use r0 as an intermediate because
		 * we're going to clobber it with args anyway.  */
		asm volatile ("mov r0, sp\n\t" /* [saved_sp = sp */
		              "str r0, %0\n\t" /* ] */
		              "mov sp, %1\n\t" /* [sp = stack] */
		              "mov r0, %3\n\t" /* [arg0 = args] */
		              "blx %2\n\t"     /* [fn()] */
		              "ldr r0, %0\n\t" /* [sp = staved_sp */
		              "mov sp, r0"     /* ] */
		              :
		              : /* %0 */"m"(saved_sp),
		                /* %1 */"r"(stack),
		                /* %2 */"r"(fn),
		                /* %3 */"r"(args)
		              : "r0"
		              );
	}

#elif __x86_64__ /*=================================================*/
	/* Assume POSIX. */
	#include <signal.h> /* for sig*, SIG_* */
	#include <unistd.h> /* for pause() */

	#define CR_PLAT_STACK_GROWS_DOWNWARD 1

	/* Wrappers for setjmp()/longjmp() that do *not* save the
	 * interrupt mask.
	 *
	 * POSIX leaves it implementation-defined whether they save
	 * the signal mask; while glibc does not save it, let's not
	 * rely on that.  */
	#define cr_plat_setjmp(env)             sigsetjmp(env, 0)
	#define cr_plat_longjmp(env, val)       siglongjmp(env, val)

	static inline void cr_plat_wait_for_interrupt(void) {
		pause();
	}
	void _cr_plat_disable_interrupts(void) {
		sigset_t all;
		sigfillset(&all);
		sigprocmask(SIG_BLOCK, &all, NULL);
	}
	void _cr_plat_enable_interrupts(void) {
		sigset_t all;
		sigfillset(&all);
		sigprocmask(SIG_UNBLOCK, &all, NULL);
	}

	#if CONFIG_COROUTINE_MEASURE_STACK
	static ALWAYS_INLINE uintptr_t cr_plat_get_sp(void)  {
		uintptr_t sp;
		asm volatile ("movq %%rsp, %0":"=r"(sp));
		return sp;
	}
	#endif

	static void cr_plat_call_with_stack(void *stack,
	                                    cr_fn_t fn, void *args) {
		static void *saved_sp = NULL;
		asm volatile ("movq %%rsp , %0\n\t"    /* saved_sp = sp */
		              "movq %1    , %%rsp\n\t" /* sp = stack */
		              "movq %3    , %%rdi\n\t" /* arg0 = args */
		              "call *%2\n\t"           /* fn() */
		              "movq %0    , %%rsp"     /* sp = saved_sp */
		              :
		              : /* %0 */"m"(saved_sp),
		                /* %1 */"r"(stack),
		                /* %2 */"r"(fn),
		                /* %3 */"r"(args)
		              : "rdi"
		              );
	}

#else
	#error unsupported platform

#endif
#if 0
}
#endif

/* types **********************************************************************/

enum coroutine_state {
	CR_NONE = 0,     /* this slot in the table is empty */
	CR_INITIALIZING, /* running, before cr_begin() */
	CR_RUNNING,      /* running, after cr_begin() */
	CR_PRE_RUNNABLE, /* running, after cr_unpause_from_intrhandler()
	                  *          but before cr_pause() */
	CR_RUNNABLE,     /* not running, but runnable */
	CR_PAUSED,       /* not running, and not runnable */
};

struct coroutine {
	volatile enum coroutine_state  state;
	jmp_buf                        env;
#if CONFIG_COROUTINE_MEASURE_STACK
	/* We aught to be able to get this out of .env, but libc
	 * authors insist on jmp_buf being opaque, glibc going as far
	 * as to xor it with a secret ot obfuscate it!  */
	uintptr_t                      sp;
#endif
	size_t                         stack_size;
	void                          *stack;
};

/* constants ******************************************************************/

const char *coroutine_state_strs[] = {
	[CR_NONE]         = "CR_NONE",
	[CR_INITIALIZING] = "CR_INITIALIZING",
	[CR_RUNNING]      = "CR_RUNNING",
	[CR_PRE_RUNNABLE] = "CR_PRE_RUNNABLE",
	[CR_RUNNABLE]     = "CR_RUNNABLE",
	[CR_PAUSED]       = "CR_PAUSED",
};

#if CONFIG_COROUTINE_MEASURE_STACK || CONFIG_COROUTINE_PROTECT_STACK
/* We just need a pattern that is unlikely to occur naturaly; this is
 * just a few bytes that I read from /dev/random.  */
static const uint8_t stack_pattern[] = {
	0xa1, 0x31, 0xe6, 0x07, 0x1f, 0x61, 0x20, 0x32,
	0x4b, 0x14, 0xc4, 0xe0, 0xea, 0x62, 0x25, 0x63,
};
#endif
#if CONFIG_COROUTINE_PROTECT_STACK
#	define STACK_GUARD_SIZE \
		round_up(sizeof(stack_pattern), CR_PLAT_STACK_ALIGNMENT)
#else
#	define STACK_GUARD_SIZE 0
#endif

/* global variables ***********************************************************/

static jmp_buf coroutine_add_env;
static jmp_buf coroutine_main_env;

/*
 * Invariants (and non-invariants):
 *
 * - exactly 0 or 1 coroutines have state CR_INITIALIZING
 * - exactly 0 or 1 coroutines have state CR_RUNNING
 * - if coroutine_running is not zero, then
 *   coroutine_table[coroutine_running-1] is the currently-running
 *   coroutine
 * - the coroutine_running coroutine either has state CR_RUNNING or
 *   CR_INITIALIZNG
 * - a coroutine having state CR_RUNNING does *not* imply that
 *   coroutine_running points at that coroutine; if that coroutine is
 *   in the middle of coroutine_add(), it coroutine_running points at
 *   the CR_INITIALIZING child coroutine, while leaving the parent
 *   coroutine as CR_RUNNING.
 */

static struct coroutine coroutine_table[CONFIG_COROUTINE_NUM] = {0};
static cid_t            coroutine_running                     = 0;

/* utility functions **********************************************************/

#define errorf(...) fprintf(stderr, "error: " __VA_ARGS__)
#define infof(...) printf("info: " __VA_ARGS__)
#if CONFIG_COROUTINE_DEBUG
#	define debugf(...) printf("dbg: " __VA_ARGS__)
#else
#	define debugf(...)
#endif

#ifdef __GLIBC__
#	define assertf(expr, ...) ({                                        \
		if (!(expr)) {                                              \
			errorf("assertion: " __VA_ARGS__);                  \
			__assert_fail(#expr, __FILE__, __LINE__, __func__); \
		}                                                           \
	})
#else
#	define assertf(expr, ...) assert(expr)
#endif

/** Return `n` rounded up to the nearest multiple of `d` */
#define round_up(n, d)          ( ( ((n)+(d)-1) / (d) ) * (d) )

#define array_len(arr)          (sizeof(arr)/sizeof(arr[0]))

static inline const char* coroutine_state_str(enum coroutine_state state) {
	assert(state < array_len(coroutine_state_strs));
	return coroutine_state_strs[state];
}

static inline void assert_cid(cid_t cid) {
	assert(cid > 0);
	assert(cid <= CONFIG_COROUTINE_NUM);
#if CONFIG_COROUTINE_PROTECT_STACK
	assert(coroutine_table[cid-1].stack_size);
	uint8_t *stack = coroutine_table[cid-1].stack;
	assert(stack);
	for (size_t i = 0; i < STACK_GUARD_SIZE; i++) {
		size_t j = coroutine_table[cid-1].stack_size - (i+1);
		assert(stack[i] == stack_pattern[i%sizeof(stack_pattern)]);
		assert(stack[j] == stack_pattern[j%sizeof(stack_pattern)]);
	}
#endif
}

#define assert_cid_state(cid, expr) do {	  \
		assert_cid(cid); \
		cid_t state = coroutine_table[(cid)-1].state; \
		assert(expr); \
	} while (0)


/* coroutine_add() ************************************************************/

cid_t coroutine_add_with_stack_size(size_t stack_size, cr_fn_t fn, void *args) {
	static cid_t last_created = 0;
	cid_t parent = coroutine_running;

	if (parent)
		assert_cid_state(parent, state == CR_RUNNING);
	assert(stack_size);
	assert(fn);
	debugf("coroutine_add_with_stack_size(%zu, %p, %p)...\n",
	       stack_size, fn, args);

	cid_t child;
	{
		size_t base = last_created;
		for (size_t shift = 0; shift < CONFIG_COROUTINE_NUM; shift++) {
			child = ((base + shift) % CONFIG_COROUTINE_NUM) + 1;
			if (coroutine_table[child-1].state == CR_NONE)
				goto found;
		}
		return 0;
	found:
	}
	debugf("...child=%zu\n", child);

	last_created = child;

	coroutine_table[child-1].stack_size = stack_size;
	coroutine_table[child-1].stack =
		aligned_alloc(CR_PLAT_STACK_ALIGNMENT, stack_size);
#if CONFIG_COROUTINE_MEASURE_STACK || CONFIG_COROUTINE_PROTECT_STACK
	for (size_t i = 0; i < stack_size; i++)
		((uint8_t *)coroutine_table[child-1].stack)[i] =
			stack_pattern[i%sizeof(stack_pattern)];
#endif

	coroutine_running = child;
	coroutine_table[child-1].state = CR_INITIALIZING;
	if (!cr_plat_setjmp(coroutine_add_env)) { /* point=a */
		void *stack_base = coroutine_table[child-1].stack
#if CR_PLAT_STACK_GROWS_DOWNWARD
			+ stack_size
			- STACK_GUARD_SIZE
#else
			+ STACK_GUARD_SIZE
#endif
			;
		debugf("...stack     =%p\n", coroutine_table[child-1].stack);
		debugf("...stack_base=%p\n", stack_base);
		/* run until cr_begin() */
		cr_plat_call_with_stack(stack_base, fn, args);
		assert(false); /* should cr_begin() instead of returning */
	}
	assert_cid_state(child, state == CR_RUNNABLE);
	if (parent)
		assert_cid_state(parent, state == CR_RUNNING);
	coroutine_running = parent;

	return child;
}

cid_t coroutine_add(cr_fn_t fn, void *args) {
	return coroutine_add_with_stack_size(
		CONFIG_COROUTINE_DEFAULT_STACK_SIZE, fn, args);
}

/* coroutine_main() ***********************************************************/

static inline cid_t next_coroutine() {
	for (cid_t next = (coroutine_running%CONFIG_COROUTINE_NUM)+1;
	     next != coroutine_running;
	     next = (next%CONFIG_COROUTINE_NUM)+1) {
		if (coroutine_table[next-1].state == CR_RUNNABLE)
			return next;
	}
	return 0;
}

void coroutine_main(void) {
	debugf("coroutine_main()\n");
	cr_disable_interrupts();
	coroutine_running = 0;
	for (;;) {
		cid_t next = next_coroutine();
		if (!next) {
			cr_enable_interrupts();
			errorf("no coroutines\n");
			return;
		}
		if (!cr_plat_setjmp(coroutine_main_env)) { /* point=b */
			coroutine_running = next;
			coroutine_table[coroutine_running-1].state = CR_RUNNING;
			cr_plat_longjmp(coroutine_table[coroutine_running-1].env, 1); /* jump to point=c */
		}
		/* This is where we jump to from cr_exit(), and from
		 * nowhere else.  */
		assert_cid_state(coroutine_running, state == CR_NONE);
		free(coroutine_table[coroutine_running-1].stack);
		coroutine_table[coroutine_running-1] = (struct coroutine){0};
	}
}

/* cr_*() *********************************************************************/

void cr_begin(void) {
	debugf("cid=%zu: cr_begin()\n", coroutine_running);
	assert_cid_state(coroutine_running, state == CR_INITIALIZING);

	coroutine_table[coroutine_running-1].state = CR_RUNNABLE;
	coroutine_table[coroutine_running-1].sp = cr_plat_get_sp();
	if (!cr_plat_setjmp(coroutine_table[coroutine_running-1].env)) /* point=c1 */
		cr_plat_longjmp(coroutine_add_env, 1); /* jump to point=a */
	cr_enable_interrupts();
}

static inline void _cr_transition(enum coroutine_state state) {
	coroutine_table[coroutine_running-1].state = state;

	cid_t next;
	for (;;) {
		next = next_coroutine();
		if (next)
			/* Switch to `next`.  */
			break;
		if (coroutine_table[coroutine_running-1].state == CR_RUNNABLE) {
			/* No other coroutine is runnable, don't
			 * transition after all.  */
			coroutine_table[coroutine_running-1].state = CR_RUNNING;
			return;
		}
		/* No coroutines are runnable, wait for an interrupt
		 * to change that.  */
		cr_plat_wait_for_interrupt();
	}

	coroutine_table[coroutine_running-1].sp = cr_plat_get_sp();
	if (!cr_plat_setjmp(coroutine_table[coroutine_running-1].env)) { /* point=c2 */
		coroutine_running = next;
		coroutine_table[coroutine_running-1].state = CR_RUNNING;
		cr_plat_longjmp(coroutine_table[coroutine_running-1].env, 1); /* jump to point=b */
	}
}

void cr_yield(void) {
	debugf("cid=%zu: cr_yield()\n", coroutine_running);
	assert_cid_state(coroutine_running, state == CR_RUNNING);

	cr_disable_interrupts();
	_cr_transition(CR_RUNNABLE);
	cr_enable_interrupts();
}

void cr_pause_and_yield(void) {
	debugf("cid=%zu: cr_pause_and_yield()\n", coroutine_running);
	assert_cid_state(coroutine_running, state == CR_RUNNING || state == CR_PRE_RUNNABLE);

	cr_disable_interrupts();
	if (coroutine_table[coroutine_running-1].state == CR_PRE_RUNNABLE)
		_cr_transition(CR_RUNNABLE);
	else
		_cr_transition(CR_PAUSED);
	cr_enable_interrupts();
}

void cr_exit(void) {
	debugf("cid=%zu: cr_exit()\n", coroutine_running);
	assert_cid_state(coroutine_running, state == CR_RUNNING);

	cr_disable_interrupts();
	coroutine_table[coroutine_running-1].state = CR_NONE;
	cr_plat_longjmp(coroutine_main_env, 1); /* jump to point=b */
}

void cr_unpause(cid_t cid) {
	debugf("cr_unpause(%zu)\n", cid);
	assert_cid_state(cid, state == CR_PAUSED);

	coroutine_table[cid-1].state = CR_RUNNABLE;
}

void cr_unpause_from_intrhandler(cid_t cid) {
	debugf("cr_unpause_from_intrhandler(%zu)\n", cid);
	assert_cid_state(cid, state == CR_RUNNING || state == CR_PAUSED);

	if (coroutine_table[cid-1].state == CR_RUNNING) {
		assert(cid == coroutine_running);
		debugf("... raced, deferring unpause\n");
		coroutine_table[cid-1].state = CR_PRE_RUNNABLE;
	} else {
		debugf("... actual unpause\n");
		coroutine_table[cid-1].state = CR_RUNNABLE;
	}
}

cid_t cr_getcid(void) {
	return coroutine_running;
}

/* cr_cid_info() **************************************************************/

#if CONFIG_COROUTINE_MEASURE_STACK

void cr_cid_info(cid_t cid, struct cr_cid_info *ret) {
	assert_cid(cid);
	assert(ret);

	/* stack_cap */
	ret->stack_cap = coroutine_table[cid-1].stack_size - 2*STACK_GUARD_SIZE;

	/* stack_max */
	ret->stack_max = ret->stack_cap;
	uint8_t *stack = (uint8_t *)coroutine_table[cid-1].stack;
	for (;;) {
		size_t i =
#if CR_PLAT_STACK_GROWS_DOWNWARD
			STACK_GUARD_SIZE + ret->stack_cap - ret->stack_max
#else
			ret->stack_max - 1 - STACK_GUARD_SIZE
#endif
			;
		if (ret->stack_max == 0 ||
		    stack[i] != stack_pattern[i%sizeof(stack_pattern)])
			break;
		ret->stack_max--;
	}

	/* stack_cur */
	uintptr_t sp;
	if (cid == coroutine_running)
		sp = cr_plat_get_sp();
	else
		sp = coroutine_table[cid-1].sp;
	assert(sp);
	uintptr_t sb = (uintptr_t)coroutine_table[cid-1].stack;
#if CR_PLAT_STACK_GROWS_DOWNWARD
	ret->stack_cur = (sb - STACK_GUARD_SIZE) - sp;
#else
	ret->stack_cur = sp - (sb + STACK_GUARD_SIZE);
#endif
}

#endif /* CONFIG_COROUTINE_MEASURE_STACK */