multipart.cpp

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// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file multipart.cpp
 * @brief In-place multipart/form-data parser implementation.
 */
 
#include "multipart.h"
#include <string.h>
 
// Length-bounded, binary-safe forward search for needle[0..nlen) within hay[0..hlen).
// Unlike strstr, it does not stop at a NUL, so a body containing NUL bytes scans correctly.
static char *mem_find(char *hay, size_t hlen, const char *needle, size_t nlen)
{
    if (nlen == 0 || nlen > hlen)
        return nullptr;
    for (size_t i = 0; i + nlen <= hlen; i++)
        if (memcmp(hay + i, needle, nlen) == 0)
            return hay + i;
    return nullptr;
}
 
// Extract parameter value: search for `key="<value>"` inside `src`.
// If found, null-terminates in-place and returns pointer to the value.
// Returns nullptr if not found.
static char *extract_quoted_param(char *src, const char *key)
{
    char *p = strstr(src, key);
    if (!p)
        return nullptr;
    constexpr size_t key_max = 32; // param keys are short literals ("name=", "filename=")
    p += strnlen(key, key_max);
    if (*p != '"')
        return nullptr;
    p++; // skip opening quote
    char *end = strchr(p, '"');
    if (!end)
        return nullptr;
    *end = '\0';
    return p;
}
 
bool multipart_parse(HttpReq *req, Multipart *mp)
{
    mp->part_count = 0;
 
    const char *ct = http_get_header(req, "Content-Type");
    if (!ct)
        return false;
 
    // Extract boundary value (may be quoted or unquoted)
    const char *bsearch = strstr(ct, "boundary=");
    if (!bsearch)
        return false;
    bsearch += 9;
    if (*bsearch == '"')
        bsearch++;
 
    char bval[MAX_BOUNDARY_LEN + 1];
    size_t blen = 0;
    while (*bsearch && *bsearch != '"' && *bsearch != ';' && *bsearch != ' ' && blen < MAX_BOUNDARY_LEN)
        bval[blen++] = *bsearch++;
    bval[blen] = '\0';
 
    if (blen == 0)
        return false;
 
    // Delimiter is "--" + boundary
    char delim[MAX_BOUNDARY_LEN + 3];
    delim[0] = '-';
    delim[1] = '-';
    memcpy(delim + 2, bval, blen + 1); // includes null
    size_t dlen = blen + 2;
 
    char *body = (char *)req->body;
    char *end = body + req->body_len; // length-bounded scanning: NUL bytes in a binary part are fine
 
    // A part's data ends at the full "\r\n--boundary" delimiter (RFC 2046): matching only the
    // "--boundary" bytes would false-truncate a binary part that happens to contain them.
    char ddelim[MAX_BOUNDARY_LEN + 5];
    ddelim[0] = '\r';
    ddelim[1] = '\n';
    memcpy(ddelim + 2, delim, dlen); // "--boundary" (dlen bytes, no NUL)
    size_t ddlen = dlen + 2;
 
    // Find the first delimiter ("--boundary"; a leading CRLF / preamble is optional here).
    char *pos = mem_find(body, (size_t)(end - body), delim, dlen);
    if (!pos)
        return false;
    pos += dlen;
    if (pos + 2 <= end && pos[0] == '\r' && pos[1] == '\n')
        pos += 2;
 
    while (mp->part_count < MAX_MULTIPART_PARTS)
    {
        // "--" immediately after the delimiter marks the terminating boundary.
        if (pos + 2 <= end && pos[0] == '-' && pos[1] == '-')
            break;
 
        MultipartPart *part = &mp->parts[mp->part_count];
        part->name = nullptr;
        part->filename = nullptr;
        part->type = nullptr;
        part->data = nullptr;
        part->data_len = 0;
 
        // Parse the per-part headers (text) until the blank line.
        for (;;)
        {
            if (pos + 2 <= end && pos[0] == '\r' && pos[1] == '\n')
            {
                pos += 2; // blank line → start of data
                break;
            }
 
            char *line_end = mem_find(pos, (size_t)(end - pos), "\r\n", 2);
            if (!line_end)
                return false;
 
            *line_end = '\0'; // null-terminate header line
 
            if (strncasecmp(pos, "Content-Disposition:", 20) == 0)
            {
                char *v = pos + 20;
                while (*v == ' ')
                    v++;
                // Extract filename before name: filename= appears after name= in the
                // header, so extracting it first avoids corrupting name='s search
                // when extract_quoted_param null-terminates the value in-place.
                part->filename = extract_quoted_param(v, "filename=");
                part->name = extract_quoted_param(v, "name=");
            }
            else if (strncasecmp(pos, "Content-Type:", 13) == 0)
            {
                char *v = pos + 13;
                while (*v == ' ')
                    v++;
                part->type = v;
            }
 
            pos = line_end + 2; // next line (skip '\0' + '\n')
        }
 
        // Data runs from pos until the next "\r\n--boundary" (binary-safe, length-bounded).
        char *next = mem_find(pos, (size_t)(end - pos), ddelim, ddlen);
        if (!next)
            return false;
 
        part->data = pos;
        part->data_len = (size_t)(next - pos);
        *next = '\0'; // terminate at the CRLF so a text part is still usable as a C-string
 
        mp->part_count++;
 
        pos = next + ddlen; // past "\r\n--boundary"
        if (pos + 2 <= end && pos[0] == '\r' && pos[1] == '\n')
            pos += 2;
    }
 
    return mp->part_count > 0;
}
 
const char *multipart_get_field(const Multipart *mp, const char *field)
{
    for (int i = 0; i < mp->part_count; i++)
    {
        if (mp->parts[i].name && strcmp(mp->parts[i].name, field) == 0)
            return mp->parts[i].data;
    }
    return nullptr;
}