播放器：记录对gzip解压支持的修改

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/myvest/article/details/77746961

问题背景：
有些服务器，播放HLS时，m3u8文件经过gzip压缩，需要进行解压操作，否则无法播放。

方法1、使用curl下载

使用curl的话则十分简单，可以在下载之前设置参数，curl会自己进行解压。参考源文档的解释，参数enc可以设置“gzip”或者“”即可（“”则支持更多的压缩方式）。

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ACCEPT_ENCODING, char *enc);

例子：

CURL *curl = curl_easy_init();
if(curl) {
  curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
 
  /* enable all supported built-in compressions */
  curl_easy_setopt(curl, CURLOPT_ACCEPT_ENCODING, "");
 
  /* Perform the request */
  curl_easy_perform(curl);
}

Three encodings are supported: identity, meaning non-compressed, deflate which requests the server to compress its response using the zlib algorithm, and gzip which requests the gzip algorithm.
If a zero-length string is set like “”, then an Accept-Encoding: header containing all built-in supported encodings is sent.

参考：
https://curl.haxx.se/libcurl/c/CURLOPT_ACCEPT_ENCODING.html

方法2、下载后使用zlib库解压

需要注意的点：
1、gzip格式使用inflate函数进行解压，而不是uncompress函数（uncompress是给zlib格式使用的）。
2、初始化函数必须使用inflateInit2（stream，47）；

例子（截取一些调用代码）：

（1）初始化：
    /* 15 is the max windowBits, +32 to enable optional gzip decoding */
    if(inflateInit2(&s->b_inflate.stream, 32+15 ) != Z_OK ) {
        av_log(h, AV_LOG_ERROR, "Error during zlib initialisation: %s\n", s->b_inflate.stream.msg);
    }
    if(zlibCompileFlags() & (1<<17)) {
        av_log(h, AV_LOG_ERROR, "zlib was compiled without gzip support!\n");
    }

（2）解压：
        int ret;
        if(!s->b_inflate.p_buffer) {
            s->b_inflate.p_buffer = av_malloc(256*1024);
        }
        if(s->b_inflate.stream.avail_in == 0) {
            int i_read = http_read(h, s->b_inflate.p_buffer, 256*1024);
            if(i_read <= 0)
                return i_read;
            s->b_inflate.stream.next_in = s->b_inflate.p_buffer;
            s->b_inflate.stream.avail_in = i_read;
        }
        s->b_inflate.stream.avail_out = size;
        s->b_inflate.stream.next_out = buf;
        ret = inflate(&s->b_inflate.stream, Z_SYNC_FLUSH);

（3）结束：
    inflateEnd(&s->b_inflate.stream);
    av_free(s->b_inflate.p_buffer);

参考：
gzip格式rfc 1952 http://www.ietf.org/rfc/rfc1952.txt
deflate格式rfc 1951 http://www.ietf.org/rfc/rfc1951.txt
zlib开发库 http://www.zlib.net/manual.html