putty-source/windows/handle-io.c

/*
 * handle-io.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.
 *
 * (It's terribly annoying having to spawn a subthread for each
 * direction of each handle. Technically it isn't necessary for
 * serial ports, since we could use overlapped I/O within the main
 * thread and wait directly on the event objects in the OVERLAPPED
 * structures. However, we can't use this trick for some types of
 * file handle at all - for some reason Windows restricts use of
 * OVERLAPPED to files which were opened with the overlapped flag -
 * and so we must use threads for those. This being the case, it's
 * simplest just to use threads for everything rather than trying
 * to keep track of multiple completely separate mechanisms.)
 */

#include <assert.h>

#include "putty.h"

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

typedef struct handle_list_node handle_list_node;
struct handle_list_node {
    handle_list_node *next, *prev;
};
static void add_to_ready_list(handle_list_node *node);

/*
 * 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_list_node ready_node;       /* for linking on to the ready list */
    HANDLE ev_from_main;               /* event used to signal back to us */
    bool moribund;                     /* are we going to kill this soon? */
    bool done;                         /* request subthread to terminate */
    bool defunct;                      /* has the subthread already gone? */
    bool busy;                         /* operation currently in progress? */
    void *privdata;                    /* for client to remember who they are */
};

typedef enum { HT_INPUT, HT_OUTPUT } HandleType;

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

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

    /*
     * Data set at initialisation and then read-only.
     */
    int flags;

    /*
     * Data set by the input thread before marking the handle ready,
     * 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 readerr;                       /* 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;
    OVERLAPPED ovl, *povl;
    HANDLE oev;
    bool readret, finished;
    int readlen;

    if (ctx->flags & HANDLE_FLAG_OVERLAPPED) {
        povl = &ovl;
        oev = CreateEvent(NULL, true, false, NULL);
    } else {
        povl = NULL;
    }

    if (ctx->flags & HANDLE_FLAG_UNITBUFFER)
        readlen = 1;
    else
        readlen = sizeof(ctx->buffer);

    while (1) {
        if (povl) {
            memset(povl, 0, sizeof(OVERLAPPED));
            povl->hEvent = oev;
        }
        readret = ReadFile(ctx->h, ctx->buffer,readlen, &ctx->len, povl);
        if (!readret)
            ctx->readerr = GetLastError();
        else
            ctx->readerr = 0;
        if (povl && !readret && ctx->readerr == ERROR_IO_PENDING) {
            WaitForSingleObject(povl->hEvent, INFINITE);
            readret = GetOverlappedResult(ctx->h, povl, &ctx->len, false);
            if (!readret)
                ctx->readerr = GetLastError();
            else
                ctx->readerr = 0;
        }

        if (!readret) {
            /*
             * Windows apparently sends ERROR_BROKEN_PIPE when a
             * pipe we're reading from is closed normally from the
             * writing end. This is ludicrous; if that situation
             * isn't a natural EOF, _nothing_ is. So if we get that
             * particular error, we pretend it's EOF.
             */
            if (ctx->readerr == ERROR_BROKEN_PIPE)
                ctx->readerr = 0;
            ctx->len = 0;
        }

        if (readret && ctx->len == 0 &&
            (ctx->flags & HANDLE_FLAG_IGNOREEOF))
            continue;

        /*
         * If we just set ctx->len to 0, that means the read operation
         * has returned end-of-file. Telling that to the main thread
         * will cause it to set its 'defunct' flag and dispose of the
         * handle structure at the next opportunity, in which case we
         * mustn't touch ctx at all after the SetEvent. (Hence we do
         * even _this_ check before the SetEvent.)
         */
        finished = (ctx->len == 0);

        add_to_ready_list(&ctx->ready_node);

        if (finished)
            break;

        WaitForSingleObject(ctx->ev_from_main, INFINITE);
        if (ctx->done) {
            /*
             * The main thread has asked us to shut down. Send back an
             * event indicating that we've done so. Hereafter we must
             * not touch ctx at all, because the main thread might
             * have freed it.
             */
            add_to_ready_list(&ctx->ready_node);
            break;
        }
    }

    if (povl)
        CloseHandle(oev);

    return 0;
}

/*
 * This is called after a successful 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)
{
    if (ctx->defunct)
        return;

    /*
     * 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_list_node ready_node;       /* for linking on to the ready list */
    HANDLE ev_from_main;               /* event used to signal back to us */
    bool moribund;                     /* are we going to kill this soon? */
    bool done;                         /* request subthread to terminate */
    bool defunct;                      /* has the subthread already gone? */
    bool busy;                         /* operation currently in progress? */
    void *privdata;                    /* for client to remember who they are */

    /*
     * Data set at initialisation and then read-only.
     */
    int flags;

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

    /*
     * Data set by the input thread before marking this handle as
     * ready, and read by the main thread after receiving that signal.
     */
    DWORD lenwritten;                  /* how much data we actually wrote */
    int writeerr;                      /* return value from WriteFile */

    /*
     * Data only ever read or written by the main thread.
     */
    bufchain queued_data;              /* data still waiting to be written */
    enum { EOF_NO, EOF_PENDING, EOF_SENT } outgoingeof;

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

static DWORD WINAPI handle_output_threadfunc(void *param)
{
    struct handle_output *ctx = (struct handle_output *) param;
    OVERLAPPED ovl, *povl;
    HANDLE oev;
    bool writeret;

    if (ctx->flags & HANDLE_FLAG_OVERLAPPED) {
        povl = &ovl;
        oev = CreateEvent(NULL, true, false, NULL);
    } else {
        povl = NULL;
    }

    while (1) {
        WaitForSingleObject(ctx->ev_from_main, INFINITE);
        if (ctx->done) {
            /*
             * The main thread has asked us to shut down. Send back an
             * event indicating that we've done so. Hereafter we must
             * not touch ctx at all, because the main thread might
             * have freed it.
             */
            add_to_ready_list(&ctx->ready_node);
            break;
        }
        if (povl) {
            memset(povl, 0, sizeof(OVERLAPPED));
            povl->hEvent = oev;
        }

        writeret = WriteFile(ctx->h, ctx->buffer, ctx->len,
                             &ctx->lenwritten, povl);
        if (!writeret)
            ctx->writeerr = GetLastError();
        else
            ctx->writeerr = 0;
        if (povl && !writeret && GetLastError() == ERROR_IO_PENDING) {
            writeret = GetOverlappedResult(ctx->h, povl,
                                           &ctx->lenwritten, true);
            if (!writeret)
                ctx->writeerr = GetLastError();
            else
                ctx->writeerr = 0;
        }

        add_to_ready_list(&ctx->ready_node);
        if (!writeret) {
            /*
             * The write operation has suffered an error. Telling that
             * to the main thread will cause it to set its 'defunct'
             * flag and dispose of the handle structure at the next
             * opportunity, so we must not touch ctx at all after
             * this.
             */
            break;
        }
    }

    if (povl)
        CloseHandle(oev);

    return 0;
}

static void handle_try_output(struct handle_output *ctx)
{
    if (!ctx->busy && bufchain_size(&ctx->queued_data)) {
        ptrlen data = bufchain_prefix(&ctx->queued_data);
        ctx->buffer = data.ptr;
        ctx->len = min(data.len, ~(DWORD)0);
        SetEvent(ctx->ev_from_main);
        ctx->busy = true;
    } else if (!ctx->busy && bufchain_size(&ctx->queued_data) == 0 &&
               ctx->outgoingeof == EOF_PENDING) {
        ctx->sentdata(ctx->sentdata_param, 0, 0, true);
        ctx->h = INVALID_HANDLE_VALUE;
        ctx->outgoingeof = EOF_SENT;
    }
}

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

struct handle {
    HandleType type;
    union {
        struct handle_generic g;
        struct handle_input i;
        struct handle_output o;
    } u;
};

/*
 * Linked list storing the current list of handles ready to have
 * something done to them by the main thread.
 */
static handle_list_node ready_head[1];
static CRITICAL_SECTION ready_critsec[1];

/*
 * Event object used by all subthreads to signal that they've just put
 * something on the ready list, i.e. that the ready list is non-empty.
 */
static HANDLE ready_event = INVALID_HANDLE_VALUE;

static void add_to_ready_list(handle_list_node *node)
{
    /*
     * Called from subthreads, when their handle has done something
     * that they need the main thread to respond to. We append the
     * given list node to the end of the ready list, and set
     * ready_event to signal to the main thread that the ready list is
     * now non-empty.
     */
    EnterCriticalSection(ready_critsec);
    node->next = ready_head;
    node->prev = ready_head->prev;
    node->next->prev = node->prev->next = node;
    SetEvent(ready_event);
    LeaveCriticalSection(ready_critsec);
}

static void remove_from_ready_list(handle_list_node *node)
{
    /*
     * Called from the main thread, just before destroying a 'struct
     * handle' completely: as a precaution, we make absolutely sure
     * it's not linked on the ready list, just in case somehow it
     * still was.
     */
    EnterCriticalSection(ready_critsec);
    node->next->prev = node->prev;
    node->prev->next = node->next;
    node->next = node->prev = node;
    LeaveCriticalSection(ready_critsec);
}

static void handle_ready(struct handle *h); /* process one handle (below) */

static void handle_ready_callback(void *vctx)
{
    /*
     * Called when the main thread detects ready_event, indicating
     * that at least one handle is on the ready list. We empty the
     * whole list and process the handles one by one.
     *
     * It's possible that other handles may be destroyed, and hence
     * taken _off_ the ready list, during this processing. That
     * shouldn't cause a deadlock, because according to the API docs,
     * it's safe to call EnterCriticalSection twice in the same thread
     * - the second call will return immediately because that thread
     * already owns the critsec. (And then it takes two calls to
     * LeaveCriticalSection to release it again, which is just what we
     * want here.)
     */
    EnterCriticalSection(ready_critsec);
    while (ready_head->next != ready_head) {
        handle_list_node *node = ready_head->next;
        node->prev->next = node->next;
        node->next->prev = node->prev;
        node->next = node->prev = node;
        handle_ready(container_of(node, struct handle, u.g.ready_node));
    }
    LeaveCriticalSection(ready_critsec);
}

static inline void ensure_ready_event_setup(void)
{
    if (ready_event == INVALID_HANDLE_VALUE) {
        ready_head->prev = ready_head->next = ready_head;
        InitializeCriticalSection(ready_critsec);
        ready_event = CreateEvent(NULL, false, false, NULL);
        add_handle_wait(ready_event, handle_ready_callback, NULL);
    }
}

struct handle *handle_input_new(HANDLE handle, handle_inputfn_t gotdata,
                                void *privdata, int flags)
{
    struct handle *h = snew(struct handle);
    DWORD in_threadid; /* required for Win9x */

    h->type = HT_INPUT;
    h->u.i.h = handle;
    h->u.i.ev_from_main = CreateEvent(NULL, false, false, NULL);
    h->u.i.gotdata = gotdata;
    h->u.i.defunct = false;
    h->u.i.moribund = false;
    h->u.i.done = false;
    h->u.i.privdata = privdata;
    h->u.i.flags = flags;

    ensure_ready_event_setup();
    HANDLE hThread = CreateThread(NULL, 0, handle_input_threadfunc,
                                  &h->u.i, 0, &in_threadid);
    if (hThread)
        CloseHandle(hThread);          /* we don't need the thread handle */
    h->u.i.busy = true;

    return h;
}

struct handle *handle_output_new(HANDLE handle, handle_outputfn_t sentdata,
                                 void *privdata, int flags)
{
    struct handle *h = snew(struct handle);
    DWORD out_threadid; /* required for Win9x */

    h->type = HT_OUTPUT;
    h->u.o.h = handle;
    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.outgoingeof = EOF_NO;
    h->u.o.sentdata = sentdata;
    h->u.o.sentdata_param = h;
    h->u.o.flags = flags;

    ensure_ready_event_setup();
    HANDLE hThread = CreateThread(NULL, 0, handle_output_threadfunc,
                                  &h->u.o, 0, &out_threadid);
    if (hThread)
        CloseHandle(hThread);          /* we don't need the thread handle */

    return h;
}

size_t handle_write(struct handle *h, const void *data, size_t len)
{
    assert(h->type == HT_OUTPUT);
    assert(h->u.o.outgoingeof == EOF_NO);
    bufchain_add(&h->u.o.queued_data, data, len);
    handle_try_output(&h->u.o);
    return bufchain_size(&h->u.o.queued_data);
}

void handle_write_eof(struct handle *h)
{
    /*
     * This function is called when we want to proactively send an
     * end-of-file notification on the handle. We can only do this by
     * actually closing the handle - so never call this on a
     * bidirectional handle if we're still interested in its incoming
     * direction!
     */
    assert(h->type == HT_OUTPUT);
    if (h->u.o.outgoingeof == EOF_NO) {
        h->u.o.outgoingeof = EOF_PENDING;
        handle_try_output(&h->u.o);
    }
}

static void handle_destroy(struct handle *h)
{
    if (h->type == HT_OUTPUT)
        bufchain_clear(&h->u.o.queued_data);
    CloseHandle(h->u.g.ev_from_main);
    remove_from_ready_list(&h->u.g.ready_node);
    sfree(h);
}

void handle_free(struct handle *h)
{
    assert(h && !h->u.g.moribund);
    if (h->u.g.busy) {
        /*
         * If the handle is currently busy, we cannot immediately free
         * it, because its subthread is in the middle of something.
         * (Exception: foreign handles don't have a subthread.)
         *
         * 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. So
         * we 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;
        h->u.g.busy = true;
        SetEvent(h->u.g.ev_from_main);
    }
}

static void handle_ready(struct handle *h)
{
    if (h->u.g.moribund) {
        /*
         * A moribund handle is one which we have either already
         * signalled to die, or are waiting until its current I/O op
         * completes to do so. Either way, it's treated as already
         * dead from the external user's point of view, so we ignore
         * the actual I/O result. We just signal the thread to die if
         * we haven't yet done so, or destroy the handle if not.
         */
        if (h->u.g.done) {
            handle_destroy(h);
        } else {
            h->u.g.done = true;
            h->u.g.busy = true;
            SetEvent(h->u.g.ev_from_main);
        }
        return;
    }

    switch (h->type) {
        int backlog;

      case HT_INPUT:
        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.defunct = true;
            h->u.i.gotdata(h, NULL, 0, h->u.i.readerr);
        } else {
            backlog = h->u.i.gotdata(h, h->u.i.buffer, h->u.i.len, 0);
            handle_throttle(&h->u.i, backlog);
        }
        break;

      case HT_OUTPUT:
        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.writeerr) {
            /*
             * 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.defunct = true;
            h->u.o.sentdata(h, 0, h->u.o.writeerr, false);
        } else {
            bufchain_consume(&h->u.o.queued_data, h->u.o.lenwritten);
            noise_ultralight(NOISE_SOURCE_IOLEN, h->u.o.lenwritten);
            h->u.o.sentdata(h, bufchain_size(&h->u.o.queued_data), 0, false);
            handle_try_output(&h->u.o);
        }
        break;
    }
}

void handle_unthrottle(struct handle *h, size_t backlog)
{
    assert(h->type == HT_INPUT);
    handle_throttle(&h->u.i, backlog);
}

size_t handle_backlog(struct handle *h)
{
    assert(h->type == HT_OUTPUT);
    return bufchain_size(&h->u.o.queued_data);
}

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

static void handle_sink_write(BinarySink *bs, const void *data, size_t len)
{
    handle_sink *sink = BinarySink_DOWNCAST(bs, handle_sink);
    handle_write(sink->h, data, len);
}

void handle_sink_init(handle_sink *sink, struct handle *h)
{
    sink->h = h;
    BinarySink_INIT(sink, handle_sink_write);
}