signalfuncs.c

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/*-------------------------------------------------------------------------
 *
 * signalfuncs.c
 *    Functions for signaling backends
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/ipc/signalfuncs.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include <signal.h>
 
#include "catalog/pg_authid.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/syslogger.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/acl.h"
#include "utils/fmgrprotos.h"
 
 
/*
 * Send a signal to another backend.
 *
 * The signal is delivered if the user is either a superuser or the same
 * role as the backend being signaled. For "dangerous" signals, an explicit
 * check for superuser needs to be done prior to calling this function.
 *
 * Returns 0 on success, 1 on general failure, 2 on normal permission error,
 * 3 if the caller needs to be a superuser, and 4 if the caller needs to have
 * privileges of pg_signal_autovacuum_worker.
 *
 * In the event of a general failure (return code 1), a warning message will
 * be emitted. For permission errors, doing that is the responsibility of
 * the caller.
 */
#define SIGNAL_BACKEND_SUCCESS 0
#define SIGNAL_BACKEND_ERROR 1
#define SIGNAL_BACKEND_NOPERMISSION 2
#define SIGNAL_BACKEND_NOSUPERUSER 3
#define SIGNAL_BACKEND_NOAUTOVAC 4
static int
pg_signal_backend(int pid, int sig)
{
    PGPROC     *proc = BackendPidGetProc(pid);
 
    /*
     * BackendPidGetProc returns NULL if the pid isn't valid; but by the time
     * we reach kill(), a process for which we get a valid proc here might
     * have terminated on its own.  There's no way to acquire a lock on an
     * arbitrary process to prevent that. But since so far all the callers of
     * this mechanism involve some request for ending the process anyway, that
     * it might end on its own first is not a problem.
     *
     * Note that proc will also be NULL if the pid refers to an auxiliary
     * process or the postmaster (neither of which can be signaled via
     * pg_signal_backend()).
     */
    if (proc == NULL)
    {
        /*
         * This is just a warning so a loop-through-resultset will not abort
         * if one backend terminated on its own during the run.
         */
        ereport(WARNING,
                (errmsg("PID %d is not a PostgreSQL backend process", pid)));
 
        return SIGNAL_BACKEND_ERROR;
    }
 
    /*
     * Only allow superusers to signal superuser-owned backends.  Any process
     * not advertising a role might have the importance of a superuser-owned
     * backend, so treat it that way.  As an exception, we allow roles with
     * privileges of pg_signal_autovacuum_worker to signal autovacuum workers
     * (which do not advertise a role).
     *
     * Otherwise, users can signal backends for roles they have privileges of.
     */
    if (!OidIsValid(proc->roleId) || superuser_arg(proc->roleId))
    {
        ProcNumber  procNumber = GetNumberFromPGProc(proc);
        BackendType backendType = pgstat_get_backend_type_by_proc_number(procNumber);
 
        if (backendType == B_AUTOVAC_WORKER)
        {
            if (!has_privs_of_role(GetUserId(), ROLE_PG_SIGNAL_AUTOVACUUM_WORKER))
                return SIGNAL_BACKEND_NOAUTOVAC;
        }
        else if (!superuser())
            return SIGNAL_BACKEND_NOSUPERUSER;
    }
    else if (!has_privs_of_role(GetUserId(), proc->roleId) &&
             !has_privs_of_role(GetUserId(), ROLE_PG_SIGNAL_BACKEND))
        return SIGNAL_BACKEND_NOPERMISSION;
 
    /*
     * Can the process we just validated above end, followed by the pid being
     * recycled for a new process, before reaching here?  Then we'd be trying
     * to kill the wrong thing.  Seems near impossible when sequential pid
     * assignment and wraparound is used.  Perhaps it could happen on a system
     * where pid re-use is randomized.  That race condition possibility seems
     * too unlikely to worry about.
     */
 
    /* If we have setsid(), signal the backend's whole process group */
#ifdef HAVE_SETSID
    if (kill(-pid, sig))
#else
    if (kill(pid, sig))
#endif
    {
        /* Again, just a warning to allow loops */
        ereport(WARNING,
                (errmsg("could not send signal to process %d: %m", pid)));
        return SIGNAL_BACKEND_ERROR;
    }
    return SIGNAL_BACKEND_SUCCESS;
}
 
/*
 * Signal to cancel a backend process.  This is allowed if you are a member of
 * the role whose process is being canceled.
 *
 * Note that only superusers can signal superuser-owned processes.
 */
Datum
pg_cancel_backend(PG_FUNCTION_ARGS)
{
    int         r = pg_signal_backend(PG_GETARG_INT32(0), SIGINT);
 
    if (r == SIGNAL_BACKEND_NOSUPERUSER)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied to cancel query"),
                 errdetail("Only roles with the %s attribute may cancel queries of roles with the %s attribute.",
                           "SUPERUSER", "SUPERUSER")));
 
    if (r == SIGNAL_BACKEND_NOAUTOVAC)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied to cancel query"),
                 errdetail("Only roles with privileges of the \"%s\" role may cancel autovacuum workers.",
                           "pg_signal_autovacuum_worker")));
 
    if (r == SIGNAL_BACKEND_NOPERMISSION)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied to cancel query"),
                 errdetail("Only roles with privileges of the role whose query is being canceled or with privileges of the \"%s\" role may cancel this query.",
                           "pg_signal_backend")));
 
    PG_RETURN_BOOL(r == SIGNAL_BACKEND_SUCCESS);
}
 
/*
 * Wait until there is no backend process with the given PID and return true.
 * On timeout, a warning is emitted and false is returned.
 */
static bool
pg_wait_until_termination(int pid, int64 timeout)
{
    /*
     * Wait in steps of waittime milliseconds until this function exits or
     * timeout.
     */
    int64       waittime = 100;
 
    /*
     * Initially remaining time is the entire timeout specified by the user.
     */
    int64       remainingtime = timeout;
 
    /*
     * Check existence of the backend. If the backend still exists, then wait
     * for waittime milliseconds, again check for the existence. Repeat this
     * until timeout or an error occurs or a pending interrupt such as query
     * cancel gets processed.
     */
    do
    {
        if (remainingtime < waittime)
            waittime = remainingtime;
 
        if (kill(pid, 0) == -1)
        {
            if (errno == ESRCH)
                return true;
            else
                ereport(ERROR,
                        (errcode(ERRCODE_INTERNAL_ERROR),
                         errmsg("could not check the existence of the backend with PID %d: %m",
                                pid)));
        }
 
        /* Process interrupts, if any, before waiting */
        CHECK_FOR_INTERRUPTS();
 
        (void) WaitLatch(MyLatch,
                         WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                         waittime,
                         WAIT_EVENT_BACKEND_TERMINATION);
 
        ResetLatch(MyLatch);
 
        remainingtime -= waittime;
    } while (remainingtime > 0);
 
    ereport(WARNING,
            (errmsg_plural("backend with PID %d did not terminate within %" PRId64 " millisecond",
                           "backend with PID %d did not terminate within %" PRId64 " milliseconds",
                           timeout,
                           pid, timeout)));
 
    return false;
}
 
/*
 * Send a signal to terminate a backend process. This is allowed if you are a
 * member of the role whose process is being terminated. If the timeout input
 * argument is 0, then this function just signals the backend and returns
 * true.  If timeout is nonzero, then it waits until no process has the given
 * PID; if the process ends within the timeout, true is returned, and if the
 * timeout is exceeded, a warning is emitted and false is returned.
 *
 * Note that only superusers can signal superuser-owned processes.
 */
Datum
pg_terminate_backend(PG_FUNCTION_ARGS)
{
    int         pid;
    int         r;
    int         timeout;        /* milliseconds */
 
    pid = PG_GETARG_INT32(0);
    timeout = PG_GETARG_INT64(1);
 
    if (timeout < 0)
        ereport(ERROR,
                (errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
                 errmsg("\"timeout\" must not be negative")));
 
    r = pg_signal_backend(pid, SIGTERM);
 
    if (r == SIGNAL_BACKEND_NOSUPERUSER)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied to terminate process"),
                 errdetail("Only roles with the %s attribute may terminate processes of roles with the %s attribute.",
                           "SUPERUSER", "SUPERUSER")));
 
    if (r == SIGNAL_BACKEND_NOAUTOVAC)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied to terminate process"),
                 errdetail("Only roles with privileges of the \"%s\" role may terminate autovacuum workers.",
                           "pg_signal_autovacuum_worker")));
 
    if (r == SIGNAL_BACKEND_NOPERMISSION)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied to terminate process"),
                 errdetail("Only roles with privileges of the role whose process is being terminated or with privileges of the \"%s\" role may terminate this process.",
                           "pg_signal_backend")));
 
    /* Wait only on success and if actually requested */
    if (r == SIGNAL_BACKEND_SUCCESS && timeout > 0)
        PG_RETURN_BOOL(pg_wait_until_termination(pid, timeout));
    else
        PG_RETURN_BOOL(r == SIGNAL_BACKEND_SUCCESS);
}
 
/*
 * Signal to reload the database configuration
 *
 * Permission checking for this function is managed through the normal
 * GRANT system.
 */
Datum
pg_reload_conf(PG_FUNCTION_ARGS)
{
    if (kill(PostmasterPid, SIGHUP))
    {
        ereport(WARNING,
                (errmsg("failed to send signal to postmaster: %m")));
        PG_RETURN_BOOL(false);
    }
 
    PG_RETURN_BOOL(true);
}
 
 
/*
 * Rotate log file
 *
 * Permission checking for this function is managed through the normal
 * GRANT system.
 */
Datum
pg_rotate_logfile(PG_FUNCTION_ARGS)
{
    if (!Logging_collector)
    {
        ereport(WARNING,
                (errmsg("rotation not possible because log collection not active")));
        PG_RETURN_BOOL(false);
    }
 
    SendPostmasterSignal(PMSIGNAL_ROTATE_LOGFILE);
    PG_RETURN_BOOL(true);
}