pqexpbuffer.c

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/*-------------------------------------------------------------------------
 *
 * pqexpbuffer.c
 *
 * PQExpBuffer provides an indefinitely-extensible string data type.
 * It can be used to buffer either ordinary C strings (null-terminated text)
 * or arbitrary binary data.  All storage is allocated with malloc().
 *
 * This module is essentially the same as the backend's StringInfo data type,
 * but it is intended for use in frontend libpq and client applications.
 * Thus, it does not rely on palloc() nor elog(), nor psprintf.c which
 * will exit() on error.
 *
 * It does rely on vsnprintf(); if configure finds that libc doesn't provide
 * a usable vsnprintf(), then a copy of our own implementation of it will
 * be linked into libpq.
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/interfaces/libpq/pqexpbuffer.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres_fe.h"

#include <limits.h>

#include "pqexpbuffer.h"

#ifdef WIN32
#include "win32.h"
#endif


/* All "broken" PQExpBuffers point to this string. */
static const char oom_buffer[1] = "";

/* Need a char * for unconstify() compatibility */
static const char *oom_buffer_ptr = oom_buffer;

static bool appendPQExpBufferVA(PQExpBuffer str, const char *fmt, va_list args) pg_attribute_printf(2, 0);


/*
 * markPQExpBufferBroken
 *
 * Put a PQExpBuffer in "broken" state if it isn't already.
 */
static void
markPQExpBufferBroken(PQExpBuffer str)
{
    if (str->data != oom_buffer)
        free(str->data);

    /*
     * Casting away const here is a bit ugly, but it seems preferable to not
     * marking oom_buffer const.  We want to do that to encourage the compiler
     * to put oom_buffer in read-only storage, so that anyone who tries to
     * scribble on a broken PQExpBuffer will get a failure.
     */
    str->data = unconstify(char *, oom_buffer_ptr);
    str->len = 0;
    str->maxlen = 0;
}

/*
 * createPQExpBuffer
 *
 * Create an empty 'PQExpBufferData' & return a pointer to it.
 */
PQExpBuffer
createPQExpBuffer(void)
{
    PQExpBuffer res;

    res = (PQExpBuffer) malloc(sizeof(PQExpBufferData));
    if (res != NULL)
        initPQExpBuffer(res);

    return res;
}

/*
 * initPQExpBuffer
 *
 * Initialize a PQExpBufferData struct (with previously undefined contents)
 * to describe an empty string.
 */
void
initPQExpBuffer(PQExpBuffer str)
{
    str->data = (char *) malloc(INITIAL_EXPBUFFER_SIZE);
    if (str->data == NULL)
    {
        str->data = unconstify(char *, oom_buffer_ptr); /* see comment above */
        str->maxlen = 0;
        str->len = 0;
    }
    else
    {
        str->maxlen = INITIAL_EXPBUFFER_SIZE;
        str->len = 0;
        str->data[0] = '\0';
    }
}

/*
 * destroyPQExpBuffer(str);
 *
 *      free()s both the data buffer and the PQExpBufferData.
 *      This is the inverse of createPQExpBuffer().
 */
void
destroyPQExpBuffer(PQExpBuffer str)
{
    if (str)
    {
        termPQExpBuffer(str);
        free(str);
    }
}

/*
 * termPQExpBuffer(str)
 *      free()s the data buffer but not the PQExpBufferData itself.
 *      This is the inverse of initPQExpBuffer().
 */
void
termPQExpBuffer(PQExpBuffer str)
{
    if (str->data != oom_buffer)
        free(str->data);
    /* just for luck, make the buffer validly empty. */
    str->data = unconstify(char *, oom_buffer_ptr); /* see comment above */
    str->maxlen = 0;
    str->len = 0;
}

/*
 * resetPQExpBuffer
 *      Reset a PQExpBuffer to empty
 *
 * Note: if possible, a "broken" PQExpBuffer is returned to normal.
 */
void
resetPQExpBuffer(PQExpBuffer str)
{
    if (str)
    {
        if (str->data != oom_buffer)
        {
            str->len = 0;
            str->data[0] = '\0';
        }
        else
        {
            /* try to reinitialize to valid state */
            initPQExpBuffer(str);
        }
    }
}

/*
 * enlargePQExpBuffer
 * Make sure there is enough space for 'needed' more bytes in the buffer
 * ('needed' does not include the terminating null).
 *
 * Returns 1 if OK, 0 if failed to enlarge buffer.  (In the latter case
 * the buffer is left in "broken" state.)
 */
int
enlargePQExpBuffer(PQExpBuffer str, size_t needed)
{
    size_t      newlen;
    char       *newdata;

    if (PQExpBufferBroken(str))
        return 0;               /* already failed */

    /*
     * Guard against ridiculous "needed" values, which can occur if we're fed
     * bogus data.  Without this, we can get an overflow or infinite loop in
     * the following.
     */
    if (needed >= ((size_t) INT_MAX - str->len))
    {
        markPQExpBufferBroken(str);
        return 0;
    }

    needed += str->len + 1;     /* total space required now */

    /* Because of the above test, we now have needed <= INT_MAX */

    if (needed <= str->maxlen)
        return 1;               /* got enough space already */

    /*
     * We don't want to allocate just a little more space with each append;
     * for efficiency, double the buffer size each time it overflows.
     * Actually, we might need to more than double it if 'needed' is big...
     */
    newlen = (str->maxlen > 0) ? (2 * str->maxlen) : 64;
    while (needed > newlen)
        newlen = 2 * newlen;

    /*
     * Clamp to INT_MAX in case we went past it.  Note we are assuming here
     * that INT_MAX <= UINT_MAX/2, else the above loop could overflow.  We
     * will still have newlen >= needed.
     */
    if (newlen > (size_t) INT_MAX)
        newlen = (size_t) INT_MAX;

    newdata = (char *) realloc(str->data, newlen);
    if (newdata != NULL)
    {
        str->data = newdata;
        str->maxlen = newlen;
        return 1;
    }

    markPQExpBufferBroken(str);
    return 0;
}

/*
 * printfPQExpBuffer
 * Format text data under the control of fmt (an sprintf-like format string)
 * and insert it into str.  More space is allocated to str if necessary.
 * This is a convenience routine that does the same thing as
 * resetPQExpBuffer() followed by appendPQExpBuffer().
 */
void
printfPQExpBuffer(PQExpBuffer str, const char *fmt,...)
{
    int         save_errno = errno;
    va_list     args;
    bool        done;

    resetPQExpBuffer(str);

    if (PQExpBufferBroken(str))
        return;                 /* already failed */

    /* Loop in case we have to retry after enlarging the buffer. */
    do
    {
        errno = save_errno;
        va_start(args, fmt);
        done = appendPQExpBufferVA(str, fmt, args);
        va_end(args);
    } while (!done);
}

/*
 * appendPQExpBuffer
 *
 * Format text data under the control of fmt (an sprintf-like format string)
 * and append it to whatever is already in str.  More space is allocated
 * to str if necessary.  This is sort of like a combination of sprintf and
 * strcat.
 */
void
appendPQExpBuffer(PQExpBuffer str, const char *fmt,...)
{
    int         save_errno = errno;
    va_list     args;
    bool        done;

    if (PQExpBufferBroken(str))
        return;                 /* already failed */

    /* Loop in case we have to retry after enlarging the buffer. */
    do
    {
        errno = save_errno;
        va_start(args, fmt);
        done = appendPQExpBufferVA(str, fmt, args);
        va_end(args);
    } while (!done);
}

/*
 * appendPQExpBufferVA
 * Shared guts of printfPQExpBuffer/appendPQExpBuffer.
 * Attempt to format data and append it to str.  Returns true if done
 * (either successful or hard failure), false if need to retry.
 *
 * Caution: callers must be sure to preserve their entry-time errno
 * when looping, in case the fmt contains "%m".
 */
static bool
appendPQExpBufferVA(PQExpBuffer str, const char *fmt, va_list args)
{
    size_t      avail;
    size_t      needed;
    int         nprinted;

    /*
     * Try to format the given string into the available space; but if there's
     * hardly any space, don't bother trying, just enlarge the buffer first.
     */
    if (str->maxlen > str->len + 16)
    {
        avail = str->maxlen - str->len;

        nprinted = vsnprintf(str->data + str->len, avail, fmt, args);

        /*
         * If vsnprintf reports an error, fail (we assume this means there's
         * something wrong with the format string).
         */
        if (unlikely(nprinted < 0))
        {
            markPQExpBufferBroken(str);
            return true;
        }

        if ((size_t) nprinted < avail)
        {
            /* Success.  Note nprinted does not include trailing null. */
            str->len += nprinted;
            return true;
        }

        /*
         * We assume a C99-compliant vsnprintf, so believe its estimate of the
         * required space, and add one for the trailing null.  (If it's wrong,
         * the logic will still work, but we may loop multiple times.)
         *
         * Choke if the required space would exceed INT_MAX, since str->maxlen
         * can't represent more than that.
         */
        if (unlikely(nprinted > INT_MAX - 1))
        {
            markPQExpBufferBroken(str);
            return true;
        }
        needed = nprinted + 1;
    }
    else
    {
        /*
         * We have to guess at how much to enlarge, since we're skipping the
         * formatting work.  Fortunately, because of enlargePQExpBuffer's
         * preference for power-of-2 sizes, this number isn't very sensitive;
         * the net effect is that we'll double the buffer size before trying
         * to run vsnprintf, which seems sensible.
         */
        needed = 32;
    }

    /* Increase the buffer size and try again. */
    if (!enlargePQExpBuffer(str, needed))
        return true;            /* oops, out of memory */

    return false;
}

/*
 * appendPQExpBufferStr
 * Append the given string to a PQExpBuffer, allocating more space
 * if necessary.
 */
void
appendPQExpBufferStr(PQExpBuffer str, const char *data)
{
    appendBinaryPQExpBuffer(str, data, strlen(data));
}

/*
 * appendPQExpBufferChar
 * Append a single byte to str.
 * Like appendPQExpBuffer(str, "%c", ch) but much faster.
 */
void
appendPQExpBufferChar(PQExpBuffer str, char ch)
{
    /* Make more room if needed */
    if (!enlargePQExpBuffer(str, 1))
        return;

    /* OK, append the character */
    str->data[str->len] = ch;
    str->len++;
    str->data[str->len] = '\0';
}

/*
 * appendBinaryPQExpBuffer
 *
 * Append arbitrary binary data to a PQExpBuffer, allocating more space
 * if necessary.
 */
void
appendBinaryPQExpBuffer(PQExpBuffer str, const char *data, size_t datalen)
{
    /* Make more room if needed */
    if (!enlargePQExpBuffer(str, datalen))
        return;

    /* OK, append the data */
    memcpy(str->data + str->len, data, datalen);
    str->len += datalen;

    /*
     * Keep a trailing null in place, even though it's probably useless for
     * binary data...
     */
    str->data[str->len] = '\0';
}