signal.c

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/*-------------------------------------------------------------------------
 *
 * signal.c
 *    Microsoft Windows Win32 Signal Emulation Functions
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/port/win32/signal.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "libpq/pqsignal.h"

/*
 * These are exported for use by the UNBLOCKED_SIGNAL_QUEUE() macro.
 * pg_signal_queue must be volatile since it is changed by the signal
 * handling thread and inspected without any lock by the main thread.
 * pg_signal_mask is only changed by main thread so shouldn't need it.
 */
volatile int pg_signal_queue;
int         pg_signal_mask;

HANDLE      pgwin32_signal_event;
HANDLE      pgwin32_initial_signal_pipe = INVALID_HANDLE_VALUE;

/*
 * pg_signal_crit_sec is used to protect only pg_signal_queue. That is the only
 * variable that can be accessed from the signal sending threads!
 */
static CRITICAL_SECTION pg_signal_crit_sec;

/* Note that array elements 0 are unused since they correspond to signal 0 */
static pqsigfunc pg_signal_array[PG_SIGNAL_COUNT];
static pqsigfunc pg_signal_defaults[PG_SIGNAL_COUNT];


/* Signal handling thread functions */
static DWORD WINAPI pg_signal_thread(LPVOID param);
static BOOL WINAPI pg_console_handler(DWORD dwCtrlType);


/*
 * pg_usleep --- delay the specified number of microseconds, but
 * stop waiting if a signal arrives.
 *
 * This replaces the non-signal-aware version provided by src/port/pgsleep.c.
 */
void
pg_usleep(long microsec)
{
    Assert(pgwin32_signal_event != NULL);
    if (WaitForSingleObject(pgwin32_signal_event,
                            (microsec < 500 ? 1 : (microsec + 500) / 1000))
        == WAIT_OBJECT_0)
    {
        pgwin32_dispatch_queued_signals();
        errno = EINTR;
        return;
    }
}


/* Initialization */
void
pgwin32_signal_initialize(void)
{
    int         i;
    HANDLE      signal_thread_handle;

    InitializeCriticalSection(&pg_signal_crit_sec);

    for (i = 0; i < PG_SIGNAL_COUNT; i++)
    {
        pg_signal_array[i] = SIG_DFL;
        pg_signal_defaults[i] = SIG_IGN;
    }
    pg_signal_mask = 0;
    pg_signal_queue = 0;

    /* Create the global event handle used to flag signals */
    pgwin32_signal_event = CreateEvent(NULL, TRUE, FALSE, NULL);
    if (pgwin32_signal_event == NULL)
        ereport(FATAL,
                (errmsg_internal("could not create signal event: error code %lu", GetLastError())));

    /* Create thread for handling signals */
    signal_thread_handle = CreateThread(NULL, 0, pg_signal_thread, NULL, 0, NULL);
    if (signal_thread_handle == NULL)
        ereport(FATAL,
                (errmsg_internal("could not create signal handler thread")));

    /* Create console control handle to pick up Ctrl-C etc */
    if (!SetConsoleCtrlHandler(pg_console_handler, TRUE))
        ereport(FATAL,
                (errmsg_internal("could not set console control handler")));
}

/*
 * Dispatch all signals currently queued and not blocked
 * Blocked signals are ignored, and will be fired at the time of
 * the pqsigsetmask() call.
 */
void
pgwin32_dispatch_queued_signals(void)
{
    int         exec_mask;

    Assert(pgwin32_signal_event != NULL);
    EnterCriticalSection(&pg_signal_crit_sec);
    while ((exec_mask = UNBLOCKED_SIGNAL_QUEUE()) != 0)
    {
        /* One or more unblocked signals queued for execution */
        int         i;

        for (i = 1; i < PG_SIGNAL_COUNT; i++)
        {
            if (exec_mask & sigmask(i))
            {
                /* Execute this signal */
                pqsigfunc   sig = pg_signal_array[i];

                if (sig == SIG_DFL)
                    sig = pg_signal_defaults[i];
                pg_signal_queue &= ~sigmask(i);
                if (sig != SIG_ERR && sig != SIG_IGN && sig != SIG_DFL)
                {
                    LeaveCriticalSection(&pg_signal_crit_sec);
                    sig(i);
                    EnterCriticalSection(&pg_signal_crit_sec);
                    break;      /* Restart outer loop, in case signal mask or
                                 * queue has been modified inside signal
                                 * handler */
                }
            }
        }
    }
    ResetEvent(pgwin32_signal_event);
    LeaveCriticalSection(&pg_signal_crit_sec);
}

/* signal masking. Only called on main thread, no sync required */
int
pqsigsetmask(int mask)
{
    int         prevmask;

    prevmask = pg_signal_mask;
    pg_signal_mask = mask;

    /*
     * Dispatch any signals queued up right away, in case we have unblocked
     * one or more signals previously queued
     */
    pgwin32_dispatch_queued_signals();

    return prevmask;
}


/*
 * Unix-like signal handler installation
 *
 * Only called on main thread, no sync required
 */
pqsigfunc
pqsignal(int signum, pqsigfunc handler)
{
    pqsigfunc   prevfunc;

    if (signum >= PG_SIGNAL_COUNT || signum < 0)
        return SIG_ERR;
    prevfunc = pg_signal_array[signum];
    pg_signal_array[signum] = handler;
    return prevfunc;
}

/* Create the signal listener pipe for specified PID */
HANDLE
pgwin32_create_signal_listener(pid_t pid)
{
    char        pipename[128];
    HANDLE      pipe;

    snprintf(pipename, sizeof(pipename), "\\\\.\\pipe\\pgsignal_%u", (int) pid);

    pipe = CreateNamedPipe(pipename, PIPE_ACCESS_DUPLEX,
                           PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE | PIPE_WAIT,
                           PIPE_UNLIMITED_INSTANCES, 16, 16, 1000, NULL);

    if (pipe == INVALID_HANDLE_VALUE)
        ereport(ERROR,
                (errmsg("could not create signal listener pipe for PID %d: error code %lu",
                        (int) pid, GetLastError())));

    return pipe;
}


/*
 * All functions below execute on the signal handler thread
 * and must be synchronized as such!
 * NOTE! The only global variable that can be used is
 * pg_signal_queue!
 */


/*
 * Queue a signal for the main thread, by setting the flag bit and event.
 */
void
pg_queue_signal(int signum)
{
    Assert(pgwin32_signal_event != NULL);
    if (signum >= PG_SIGNAL_COUNT || signum <= 0)
        return;                 /* ignore any bad signal number */

    EnterCriticalSection(&pg_signal_crit_sec);
    pg_signal_queue |= sigmask(signum);
    LeaveCriticalSection(&pg_signal_crit_sec);

    SetEvent(pgwin32_signal_event);
}

/* Signal handling thread */
static DWORD WINAPI
pg_signal_thread(LPVOID param)
{
    char        pipename[128];
    HANDLE      pipe = pgwin32_initial_signal_pipe;

    /* Set up pipe name, in case we have to re-create the pipe. */
    snprintf(pipename, sizeof(pipename), "\\\\.\\pipe\\pgsignal_%lu", GetCurrentProcessId());

    for (;;)
    {
        BOOL        fConnected;

        /* Create a new pipe instance if we don't have one. */
        if (pipe == INVALID_HANDLE_VALUE)
        {
            pipe = CreateNamedPipe(pipename, PIPE_ACCESS_DUPLEX,
                                   PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE | PIPE_WAIT,
                                   PIPE_UNLIMITED_INSTANCES, 16, 16, 1000, NULL);

            if (pipe == INVALID_HANDLE_VALUE)
            {
                write_stderr("could not create signal listener pipe: error code %lu; retrying\n", GetLastError());
                SleepEx(500, FALSE);
                continue;
            }
        }

        /*
         * Wait for a client to connect.  If something connects before we
         * reach here, we'll get back a "failure" with ERROR_PIPE_CONNECTED,
         * which is actually a success (way to go, Microsoft).
         */
        fConnected = ConnectNamedPipe(pipe, NULL) ? TRUE : (GetLastError() == ERROR_PIPE_CONNECTED);
        if (fConnected)
        {
            /*
             * We have a connection from a would-be signal sender. Process it.
             */
            BYTE        sigNum;
            DWORD       bytes;

            if (ReadFile(pipe, &sigNum, 1, &bytes, NULL) &&
                bytes == 1)
            {
                /*
                 * Queue the signal before responding to the client.  In this
                 * way, it's guaranteed that once kill() has returned in the
                 * signal sender, the next CHECK_FOR_INTERRUPTS() in the
                 * signal recipient will see the signal.  (This is a stronger
                 * guarantee than POSIX makes; maybe we don't need it?  But
                 * without it, we've seen timing bugs on Windows that do not
                 * manifest on any known Unix.)
                 */
                pg_queue_signal(sigNum);

                /*
                 * Write something back to the client, allowing its
                 * CallNamedPipe() call to terminate.
                 */
                WriteFile(pipe, &sigNum, 1, &bytes, NULL);  /* Don't care if it
                                                             * works or not */

                /*
                 * We must wait for the client to read the data before we can
                 * disconnect, else the data will be lost.  (If the WriteFile
                 * call failed, there'll be nothing in the buffer, so this
                 * shouldn't block.)
                 */
                FlushFileBuffers(pipe);
            }
            else
            {
                /*
                 * If we fail to read a byte from the client, assume it's the
                 * client's problem and do nothing.  Perhaps it'd be better to
                 * force a pipe close and reopen?
                 */
            }

            /* Disconnect from client so that we can re-use the pipe. */
            DisconnectNamedPipe(pipe);
        }
        else
        {
            /*
             * Connection failed.  Cleanup and try again.
             *
             * This should never happen.  If it does, there's a window where
             * we'll miss signals until we manage to re-create the pipe.
             * However, just trying to use the same pipe again is probably not
             * going to work, so we have little choice.
             */
            CloseHandle(pipe);
            pipe = INVALID_HANDLE_VALUE;
        }
    }
    return 0;
}


/* Console control handler will execute on a thread created
   by the OS at the time of invocation */
static BOOL WINAPI
pg_console_handler(DWORD dwCtrlType)
{
    if (dwCtrlType == CTRL_C_EVENT ||
        dwCtrlType == CTRL_BREAK_EVENT ||
        dwCtrlType == CTRL_CLOSE_EVENT ||
        dwCtrlType == CTRL_SHUTDOWN_EVENT)
    {
        pg_queue_signal(SIGINT);
        return TRUE;
    }
    return FALSE;
}