GIT 源码阅读之 cat-file

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我是靠谱客的博主美好铃铛，这篇文章主要介绍GIT 源码阅读之 cat-file，现在分享给大家，希望可以做个参考。

说明

本文所有代码基于 GIT COMMIT e83c5163316f89bfbde7d9ab23ca2e25604af290 版本的源码，在 Ubuntu 16.04 上编译运行，部分代码有所改动。

cat-file

cat-file 是 GIT 最初版本中用于获取 SHA1 对象的内容的命令，其使用方法为 cat-file <sha1> ，该程序将获取 SHA1 对象的类型同时创建了临时文件读取原始类型（blob 类型）或对象的基本信息。其源码如下：

复制代码

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#include "cache.h"

int main(int argc, char **argv)
{
        unsigned char sha1[20];
        char type[20];
        void *buf;
        unsigned long size;
        char template[] = "temp_git_file_XXXXXX";
        int fd;

        if (argc != 2 || get_sha1_hex(argv[1], sha1))
                usage("cat-file: cat-file <sha1>");
        buf = read_sha1_file(sha1, type, &size);
        if (!buf)
                exit(1);
        fd = mkstemp(template);
        if (fd < 0)
                usage("unable to create tempfile");
        if (write(fd, buf, size) != size)
                strcpy(type, "bad");
        printf("%s: %sn", template, type);
        return 0;
}

该命令接收一个标识 SHA1 对象的 sha1 值。该命令通过 sha1 标识符获取到缓存目录中对象，并通过 read_sha1_file() 函数获取文件的类型及内容，接着创建临时文件将文件内容保存至临时文件中。read_sha1_file() 函数定义如下。

复制代码

void *read_sha1_file(unsigned char *sha1, char *type, unsigned long *size)
{
        z_stream stream;
        char buffer[8192];
        struct stat st;
        int i, fd, ret, bytes;
        void *map, *buf;
        char *filename = sha1_file_name(sha1);

fd = open(filename, O_RDONLY);
        if (fd < 0) {
                perror(filename);
                return NULL;
        }
        if (fstat(fd, &st) < 0) {
                close(fd);
                return NULL;
        }
        map = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
        close(fd);
        if (-1 == (int)(long)map)
                return NULL;

/* Get the data stream */
        memset(&stream, 0, sizeof(stream));
        stream.next_in = map;
        stream.avail_in = st.st_size;
        stream.next_out = buffer;
        stream.avail_out = sizeof(buffer);

inflateInit(&stream);
        ret = inflate(&stream, 0);
        if (sscanf(buffer, "%10s %lu", type, size) != 2)
                return NULL;
        bytes = strlen(buffer) + 1;
        buf = malloc(*size);
        if (!buf)
                return NULL;

memcpy(buf, buffer + bytes, stream.total_out - bytes);
        bytes = stream.total_out - bytes;
        if (bytes < *size && ret == Z_OK) {
                stream.next_out = buf + bytes;
                stream.avail_out = *size - bytes;
                while (inflate(&stream, Z_FINISH) == Z_OK)
                        /* nothing */;
        }
        inflateEnd(&stream);
        return buf;
}

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void *read_sha1_file(unsigned char *sha1, char *type, unsigned long *size)
{
        z_stream stream;
        char buffer[8192];
        struct stat st;
        int i, fd, ret, bytes;
        void *map, *buf;
        char *filename = sha1_file_name(sha1);

        fd = open(filename, O_RDONLY);
        if (fd < 0) {
                perror(filename);
                return NULL;
        }
        if (fstat(fd, &st) < 0) {
                close(fd);
                return NULL;
        }
        map = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
        close(fd);
        if (-1 == (int)(long)map)
                return NULL;

        /* Get the data stream */
        memset(&stream, 0, sizeof(stream));
        stream.next_in = map;
        stream.avail_in = st.st_size;
        stream.next_out = buffer;
        stream.avail_out = sizeof(buffer);

        inflateInit(&stream);
        ret = inflate(&stream, 0);
        if (sscanf(buffer, "%10s %lu", type, size) != 2)
                return NULL;
        bytes = strlen(buffer) + 1;
        buf = malloc(*size);
        if (!buf)
                return NULL;

        memcpy(buf, buffer + bytes, stream.total_out - bytes);
        bytes = stream.total_out - bytes;
        if (bytes < *size && ret == Z_OK) {
                stream.next_out = buf + bytes;
                stream.avail_out = *size - bytes;
                while (inflate(&stream, Z_FINISH) == Z_OK)
                        /* nothing */;
        }
        inflateEnd(&stream);
        return buf;
}