recovery代码分析之三：try_update_binary

        OTA升级包路径META-INF\com\google\android中，存在着两个关键的文件：update-script和update-binary。在这两个脚本文件中，update-script记载着系统升级所需要执行的命令（如图1所示），而update-binary则是对于每条命令的解析。进入recovery模式后，系统将会执行文件中记载的命令，完成升级。

图1 update-script内容截图

由http://blog.youkuaiyun.com/liudekuan/article/details/8707044可知，在文件./bootable/recovery/install.c中定义了对应于每条命令的执行函数（如图2所示），即在recovery模式下

，系统会将这些命令转换为相应的函数去执行。而RegisterInstallFunctions函数在./bootable/recovery/update.c中被调用，在源码编译过程中，./bootable/recovery/updater目录下的代码被编译为可执行文件：out/target/product/sp8825ea/system/bin/updater，供recovery模式下系统使用。

图2 函数注册

static int
try_update_binary(const char *path, ZipArchive *zip, int* wipe_cache) {
    const ZipEntry* binary_entry =
            mzFindZipEntry(zip, ASSUMED_UPDATE_BINARY_NAME);
    if (binary_entry == NULL) {
        mzCloseZipArchive(zip);
        return INSTALL_CORRUPT;
    }

    char* binary = "/tmp/update_binary";
    unlink(binary);
    int fd = creat(binary, 0755);
    if (fd < 0) {
        mzCloseZipArchive(zip);
        LOGE("Can't make %s\n", binary);
        return INSTALL_ERROR;
    }
    bool ok = mzExtractZipEntryToFile(zip, binary_entry, fd);
    close(fd);
    mzCloseZipArchive(zip);

    if (!ok) {
        LOGE("Can't copy %s\n", ASSUMED_UPDATE_BINARY_NAME);
        return INSTALL_ERROR;
    }

    int pipefd[2];
    pipe(pipefd);

    // When executing the update binary contained in the package, the
    // arguments passed are:
    //
    //   - the version number for this interface
    //
    //   - an fd to which the program can write in order to update the
    //     progress bar.  The program can write single-line commands:
    //
    //        progress <frac> <secs>
    //            fill up the next <frac> part of of the progress bar
    //            over <secs> seconds.  If <secs> is zero, use
    //            set_progress commands to manually control the
    //            progress of this segment of the bar
    //
    //        set_progress <frac>
    //            <frac> should be between 0.0 and 1.0; sets the
    //            progress bar within the segment defined by the most
    //            recent progress command.
    //
    //        firmware <"hboot"|"radio"> <filename>
    //            arrange to install the contents of <filename> in the
    //            given partition on reboot.
    //
    //            (API v2: <filename> may start with "PACKAGE:" to
    //            indicate taking a file from the OTA package.)
    //
    //            (API v3: this command no longer exists.)
    //
    //        ui_print <string>
    //            display <string> on the screen.
    //
    //   - the name of the package zip file.
    //

    char** args = malloc(sizeof(char*) * 5);
    args[0] = binary;
    args[1] = EXPAND(RECOVERY_API_VERSION);   // defined in Android.mk
    args[2] = malloc(10);
    sprintf(args[2], "%d", pipefd[1]);
    args[3] = (char*)path;
    args[4] = NULL;

    pid_t pid = fork();
    if (pid == 0) {
        close(pipefd[0]);
        execv(binary, args);
        fprintf(stdout, "E:Can't run %s (%s)\n", binary, strerror(errno));
        _exit(-1);
    }
    close(pipefd[1]);

    *wipe_cache = 0;

    char buffer[1024];
    FILE* from_child = fdopen(pipefd[0], "r");
    while (fgets(buffer, sizeof(buffer), from_child) != NULL) {
        char* command = strtok(buffer, " \n");
        if (command == NULL) {
            continue;
        } else if (strcmp(command, "progress") == 0) {
            char* fraction_s = strtok(NULL, " \n");
            char* seconds_s = strtok(NULL, " \n");

            float fraction = strtof(fraction_s, NULL);
            int seconds = strtol(seconds_s, NULL, 10);

            ui_show_progress(fraction * (1-VERIFICATION_PROGRESS_FRACTION),
                             seconds);
        } else if (strcmp(command, "set_progress") == 0) {
            char* fraction_s = strtok(NULL, " \n");
            float fraction = strtof(fraction_s, NULL);
            ui_set_progress(fraction);
        } else if (strcmp(command, "ui_print") == 0) {
            char* str = strtok(NULL, "\n");
            if (str) {
                ui_print("%s", str);
            } else {
                ui_print("\n");
            }
        } else if (strcmp(command, "wipe_cache") == 0) {
            *wipe_cache = 1;
        } else {
            LOGE("unknown command [%s]\n", command);
        }
    }
    fclose(from_child);

    int status;
    waitpid(pid, &status, 0);
    if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {
        LOGE("Error in %s\n(Status %d)\n", path, WEXITSTATUS(status));
        return INSTALL_ERROR;
    }

    return INSTALL_SUCCESS;
}

代码段1 try_update_binary函数

        ./bootable/recovery/install.c中的try_update_binary函数，是真正实现读取升级包中的脚本文件并执行相应的函数的地方。在此函数中，通过调用fork函数创建出一个子进程（代码第72行），在子进程中开始读取并执行升级脚本文件（代码73-78）。在此需要注意的是函数fork的用法，fork被调用一次，将做两次返回，在父进程中返回的是子进程的进程ID，为正数；而在子进程中，则返回0。因此代码73-78事实是子进程中所进行的操作，即execv(binary, args)。子进程创建成功后，开始执行升级代码，并通过管道与父进程交互（代码27-28，创建管道，并将pipefd[1]作为参数传递给子进程，子进程则将相关信息写入到此管道描述符中）；而父进程则通过读取子进程传递过来的信息更新UI（代码84-114）。