putty-source/windows/winhandl.c

/*
 * winhandl.c: Module to give Windows front ends the general
 * ability to deal with consoles, pipes, serial ports, or any other
 * type of data stream accessed through a Windows API HANDLE rather
 * than a WinSock SOCKET.
 *
 * We do this by spawning a subthread to continuously try to read
 * from the handle. Every time a read successfully returns some
 * data, the subthread sets an event object which is picked up by
 * the main thread, and the main thread then sets an event in
 * return to instruct the subthread to resume reading.
 * 
 * Output works precisely the other way round, in a second
 * subthread. The output subthread should not be attempting to
 * write all the time, because it hasn't always got data _to_
 * write; so the output thread waits for an event object notifying
 * it to _attempt_ a write, and then it sets an event in return
 * when one completes.
 */

#include <assert.h>

#include "putty.h"

/* ----------------------------------------------------------------------
 * Generic definitions.
 */

/*
 * Maximum amount of backlog we will allow to build up on an input
 * handle before we stop reading from it.
 */
#define MAX_BACKLOG 32768

struct handle_generic {
    /*
     * Initial fields common to both handle_input and handle_output
     * structures.
     * 
     * The three HANDLEs are set up at initialisation time and are
     * thereafter read-only to both main thread and subthread.
     * `moribund' is only used by the main thread; `done' is
     * written by the main thread before signalling to the
     * subthread. `defunct' and `busy' are used only by the main
     * thread.
     */
    HANDLE h;			       /* the handle itself */
    HANDLE ev_to_main;		       /* event used to signal main thread */
    HANDLE ev_from_main;	       /* event used to signal back to us */
    int moribund;		       /* are we going to kill this soon? */
    int done;			       /* request subthread to terminate */
    int defunct;		       /* has the subthread already gone? */
    int busy;			       /* operation currently in progress? */
    void *privdata;		       /* for client to remember who they are */
};

/* ----------------------------------------------------------------------
 * Input threads.
 */

/*
 * Data required by an input thread.
 */
struct handle_input {
    /*
     * Copy of the handle_generic structure.
     */
    HANDLE h;			       /* the handle itself */
    HANDLE ev_to_main;		       /* event used to signal main thread */
    HANDLE ev_from_main;	       /* event used to signal back to us */
    int moribund;		       /* are we going to kill this soon? */
    int done;			       /* request subthread to terminate */
    int defunct;		       /* has the subthread already gone? */
    int busy;			       /* operation currently in progress? */
    void *privdata;		       /* for client to remember who they are */

    /*
     * Data set by the input thread before signalling ev_to_main,
     * and read by the main thread after receiving that signal.
     */
    char buffer[4096];		       /* the data read from the handle */
    DWORD len;			       /* how much data that was */
    int readret;		       /* lets us know about read errors */

    /*
     * Callback function called by this module when data arrives on
     * an input handle.
     */
    handle_inputfn_t gotdata;
};

/*
 * The actual thread procedure for an input thread.
 */
static DWORD WINAPI handle_input_threadfunc(void *param)
{
    struct handle_input *ctx = (struct handle_input *) param;

    while (1) {
	ctx->readret = ReadFile(ctx->h, ctx->buffer, sizeof(ctx->buffer),
				&ctx->len, NULL);
	if (!ctx->readret)
	    ctx->len = 0;

	SetEvent(ctx->ev_to_main);

	if (!ctx->len)
	    break;

	WaitForSingleObject(ctx->ev_from_main, INFINITE);
	if (ctx->done)
	    break;		       /* main thread told us to shut down */
    }

    return 0;
}

/*
 * This is called after a succcessful read, or from the
 * `unthrottle' function. It decides whether or not to begin a new
 * read operation.
 */
static void handle_throttle(struct handle_input *ctx, int backlog)
{
    assert(!ctx->defunct);

    /*
     * If there's a read operation already in progress, do nothing:
     * when that completes, we'll come back here and be in a
     * position to make a better decision.
     */
    if (ctx->busy)
	return;

    /*
     * Otherwise, we must decide whether to start a new read based
     * on the size of the backlog.
     */
    if (backlog < MAX_BACKLOG) {
	SetEvent(ctx->ev_from_main);
	ctx->busy = TRUE;
    }
}

/* ----------------------------------------------------------------------
 * Output threads.
 */

/*
 * Data required by an output thread.
 */
struct handle_output {
    /*
     * Copy of the handle_generic structure.
     */
    HANDLE h;			       /* the handle itself */
    HANDLE ev_to_main;		       /* event used to signal main thread */
    HANDLE ev_from_main;	       /* event used to signal back to us */
    int moribund;		       /* are we going to kill this soon? */
    int done;			       /* request subthread to terminate */
    int defunct;		       /* has the subthread already gone? */
    int busy;			       /* operation currently in progress? */
    void *privdata;		       /* for client to remember who they are */

    /*
     * Data set by the main thread before signalling ev_from_main,
     * and read by the input thread after receiving that signal.
     */
    char *buffer;		       /* the data to write */
    DWORD len;			       /* how much data there is */

    /*
     * Data set by the input thread before signalling ev_to_main,
     * and read by the main thread after receiving that signal.
     */
    DWORD lenwritten;		       /* how much data we actually wrote */
    int writeret;		       /* return value from WriteFile */

    /*
     * Data only ever read or written by the main thread.
     */
    bufchain queued_data;	       /* data still waiting to be written */

    /*
     * Callback function called when the backlog in the bufchain
     * drops.
     */
    handle_outputfn_t sentdata;
};

static DWORD WINAPI handle_output_threadfunc(void *param)
{
    struct handle_output *ctx = (struct handle_output *) param;

    while (1) {
	WaitForSingleObject(ctx->ev_from_main, INFINITE);
	if (ctx->done) {
	    SetEvent(ctx->ev_to_main);
	    break;
	}
	ctx->writeret = WriteFile(ctx->h, ctx->buffer, ctx->len,
				  &ctx->lenwritten, NULL);
	SetEvent(ctx->ev_to_main);
	if (!ctx->writeret)
	    break;
    }

    return 0;
}

static void handle_try_output(struct handle_output *ctx)
{
    void *senddata;
    int sendlen;

    if (!ctx->busy && bufchain_size(&ctx->queued_data)) {
	bufchain_prefix(&ctx->queued_data, &senddata, &sendlen);
	ctx->buffer = senddata;
	ctx->len = sendlen;
	SetEvent(ctx->ev_from_main);
	ctx->busy = TRUE;
    }
}

/* ----------------------------------------------------------------------
 * Unified code handling both input and output threads.
 */

struct handle {
    int output;
    union {
	struct handle_generic g;
	struct handle_input i;
	struct handle_output o;
    } u;
};

static tree234 *handles_by_evtomain;

static int handle_cmp_evtomain(void *av, void *bv)
{
    struct handle *a = (struct handle *)av;
    struct handle *b = (struct handle *)bv;

    if ((unsigned)a->u.g.ev_to_main < (unsigned)b->u.g.ev_to_main)
	return -1;
    else if ((unsigned)a->u.g.ev_to_main > (unsigned)b->u.g.ev_to_main)
	return +1;
    else
	return 0;
}

static int handle_find_evtomain(void *av, void *bv)
{
    HANDLE *a = (HANDLE *)av;
    struct handle *b = (struct handle *)bv;

    if ((unsigned)*a < (unsigned)b->u.g.ev_to_main)
	return -1;
    else if ((unsigned)*a > (unsigned)b->u.g.ev_to_main)
	return +1;
    else
	return 0;
}

struct handle *handle_input_new(HANDLE handle, handle_inputfn_t gotdata,
				void *privdata)
{
    struct handle *h = snew(struct handle);

    h->output = FALSE;
    h->u.i.h = handle;
    h->u.i.ev_to_main = CreateEvent(NULL, FALSE, FALSE, NULL);
    h->u.i.ev_from_main = CreateEvent(NULL, FALSE, FALSE, NULL);
    h->u.i.gotdata = gotdata;
    h->u.i.busy = FALSE;
    h->u.i.defunct = FALSE;
    h->u.i.moribund = FALSE;
    h->u.i.done = FALSE;
    h->u.i.privdata = privdata;

    if (!handles_by_evtomain)
	handles_by_evtomain = newtree234(handle_cmp_evtomain);
    add234(handles_by_evtomain, h);

    CreateThread(NULL, 0, handle_input_threadfunc,
		 &h->u.i, 0, NULL);

    handle_throttle(&h->u.i, 0);       /* start first read operation */

    return h;
}

struct handle *handle_output_new(HANDLE handle, handle_outputfn_t sentdata,
				 void *privdata)
{
    struct handle *h = snew(struct handle);

    h->output = TRUE;
    h->u.o.h = handle;
    h->u.o.ev_to_main = CreateEvent(NULL, FALSE, FALSE, NULL);
    h->u.o.ev_from_main = CreateEvent(NULL, FALSE, FALSE, NULL);
    h->u.o.busy = FALSE;
    h->u.o.defunct = FALSE;
    h->u.o.moribund = FALSE;
    h->u.o.done = FALSE;
    h->u.o.privdata = privdata;
    bufchain_init(&h->u.o.queued_data);
    h->u.o.sentdata = sentdata;

    if (!handles_by_evtomain)
	handles_by_evtomain = newtree234(handle_cmp_evtomain);
    add234(handles_by_evtomain, h);

    CreateThread(NULL, 0, handle_output_threadfunc,
		 &h->u.i, 0, NULL);

    return h;
}

int handle_write(struct handle *h, const void *data, int len)
{
    assert(h->output);
    bufchain_add(&h->u.o.queued_data, data, len);
    handle_try_output(&h->u.o);
    return bufchain_size(&h->u.o.queued_data);
}

HANDLE *handle_get_events(int *nevents)
{
    HANDLE *ret;
    struct handle *h;
    int i, n, size;

    /*
     * Go through our tree counting the handle objects currently
     * engaged in useful activity.
     */
    ret = NULL;
    n = size = 0;
    if (handles_by_evtomain) {
	for (i = 0; (h = index234(handles_by_evtomain, i)) != NULL; i++) {
	    if (h->u.g.busy) {
		if (n >= size) {
		    size += 32;
		    ret = sresize(ret, size, HANDLE);
		}
		ret[n++] = h->u.g.ev_to_main;
	    }
	}
    }

    *nevents = n;
    return ret;
}

static void handle_destroy(struct handle *h)
{
    if (h->output)
	bufchain_clear(&h->u.o.queued_data);
    CloseHandle(h->u.g.ev_from_main);
    CloseHandle(h->u.g.ev_to_main);
    del234(handles_by_evtomain, h);
    sfree(h);
}

void handle_free(struct handle *h)
{
    /*
     * If the handle is currently busy, we cannot immediately free
     * it. Instead we must wait until it's finished its current
     * operation, because otherwise the subthread will write to
     * invalid memory after we free its context from under it.
     */
    assert(h && !h->u.g.moribund);
    if (h->u.g.busy) {
	/*
	 * Just set the moribund flag, which will be noticed next
	 * time an operation completes.
	 */
	h->u.g.moribund = TRUE;
    } else if (h->u.g.defunct) {
	/*
	 * There isn't even a subthread; we can go straight to
	 * handle_destroy.
	 */
	handle_destroy(h);
    } else {
	/*
	 * The subthread is alive but not busy, so we now signal it
	 * to die. Set the moribund flag to indicate that it will
	 * want destroying after that.
	 */
	h->u.g.moribund = TRUE;
	h->u.g.done = TRUE;
	SetEvent(h->u.g.ev_from_main);
    }
}

void handle_got_event(HANDLE event)
{
    struct handle *h;

    assert(handles_by_evtomain);
    h = find234(handles_by_evtomain, &event, handle_find_evtomain);
    if (!h) {
	/*
	 * This isn't an error condition. If two or more event
	 * objects were signalled during the same select operation,
	 * and processing of the first caused the second handle to
	 * be closed, then it will sometimes happen that we receive
	 * an event notification here for a handle which is already
	 * deceased. In that situation we simply do nothing.
	 */
	return;
    }

    if (h->u.g.moribund) {
	/*
	 * A moribund handle is already treated as dead from the
	 * external user's point of view, so do nothing with the
	 * actual event. Just signal the thread to die if
	 * necessary, or destroy the handle if not.
	 */
	if (h->u.g.done) {
	    handle_destroy(h);
	} else {
	    h->u.g.done = TRUE;
	    SetEvent(h->u.g.ev_from_main);
	}
	return;
    }

    if (!h->output) {
	int backlog;

	h->u.i.busy = FALSE;

	/*
	 * A signal on an input handle means data has arrived.
	 */
	if (h->u.i.len == 0) {
	    /*
	     * EOF, or (nearly equivalently) read error.
	     */
	    h->u.i.gotdata(h, NULL, (h->u.i.readret ? 0 : -1));
	    h->u.i.defunct = TRUE;
	} else {
	    backlog = h->u.i.gotdata(h, h->u.i.buffer, h->u.i.len);
	    handle_throttle(&h->u.i, backlog);
	}
    } else {
	h->u.o.busy = FALSE;

	/*
	 * A signal on an output handle means we have completed a
	 * write. Call the callback to indicate that the output
	 * buffer size has decreased, or to indicate an error.
	 */
	if (!h->u.o.writeret) {
	    /*
	     * Write error. Send a negative value to the callback,
	     * and mark the thread as defunct (because the output
	     * thread is terminating by now).
	     */
	    h->u.o.sentdata(h, -1);
	    h->u.o.defunct = TRUE;
	} else {
	    bufchain_consume(&h->u.o.queued_data, h->u.o.lenwritten);
	    h->u.o.sentdata(h, bufchain_size(&h->u.o.queued_data));
	    handle_try_output(&h->u.o);
	}
    }
}

void handle_unthrottle(struct handle *h, int backlog)
{
    assert(!h->output);
    handle_throttle(&h->u.i, backlog);
}

int handle_backlog(struct handle *h)
{
    assert(h->output);
    return bufchain_size(&h->u.o.queued_data);
}

void *handle_get_privdata(struct handle *h)
{
    return h->u.g.privdata;
}