Android OTA 升级(五)：updater

一、简介

前面分析的OTA升级的各部分代码都是在搭一个舞台，而主角现在终于登场，它就是updater。Google的代码架构设计非常好，各部分尽量松耦合。前面介绍升级脚本时，可知有两种类型的脚本，amend & edify(Amend脚本<update-script>在Android 1.5中已经被废除，只保留了Edify脚本<updater-script>). 他们各自对应一个updater. 这里，我们主要关注新的edify的updater.

Updater可以作为学习解释器/编译器的同学一个很好的实例，但是我们只关心产品化相关的内容，所以并不去深究lex/yacc相关的东西。

二、入口函数main

bootable/recovery/updater/updater.c

// 这里定义脚本的位置，注释说明本updater支持edify格式的脚本。
// Where in the package we expect to find the edify script to execute.
// (Note it's "updateR-script", not the older "update-script".)
#define SCRIPT_NAME "META-INF/com/google/android/updater-script"

int main(int argc, char** argv) {
// Various things log information to stdout or stderr more or less
// at random.  The log file makes more sense if buffering is
// turned off so things appear in the right order.
setbuf(stdout, NULL);
setbuf(stderr, NULL);

if (argc != 4) {
fprintf(stderr, "unexpected number of arguments (%d)\n", argc);
return 1;
}
// 获取 version 参数。
char* version = argv[1];
if ((version[0] != '1' && version[0] != '2' && version[0] != '3') ||
version[1] != '\0') {
// We support version 1, 2, or 3.
fprintf(stderr, "wrong updater binary API; expected 1, 2, or 3; "
"got %s\n",
argv[1]);
return 2;
}

// 获取命令管道（用于图形显示等，见前篇）
// Set up the pipe for sending commands back to the parent process.
int fd = atoi(argv[2]);
FILE* cmd_pipe = fdopen(fd, "wb");
setlinebuf(cmd_pipe);

// Extract the script from the package.

char* package_data = argv[3];
ZipArchive za;
int err;
err = mzOpenZipArchive(package_data, &za);
if (err != 0) {
fprintf(stderr, "failed to open package %s: %s\n",
package_data, strerror(err));
return 3;
}

// 读入脚本 META-INF/com/google/android/updater-script
const ZipEntry* script_entry = mzFindZipEntry(&za, SCRIPT_NAME);
if (script_entry == NULL) {
fprintf(stderr, "failed to find %s in %s\n", SCRIPT_NAME, package_data);
return 4;
}

char* script = malloc(script_entry->uncompLen+1);
if (!mzReadZipEntry(&za, script_entry, script, script_entry->uncompLen)) {
fprintf(stderr, "failed to read script from package\n");
return 5;
}
script[script_entry->uncompLen] = '\0';

// Configure edify's functions.
// 注册语句处理函数
RegisterBuiltins();
RegisterInstallFunctions();
RegisterDeviceExtensions();
FinishRegistration();

// Parse the script.
// 调用yy* 库函数解析脚本。
Expr* root;
int error_count = 0;
yy_scan_string(script);
int error = yyparse(&root, &error_count);
if (error != 0 || error_count > 0) {
fprintf(stderr, "%d parse errors\n", error_count);
return 6;
}

// Evaluate the parsed script.
UpdaterInfo updater_info;
updater_info.cmd_pipe = cmd_pipe;
updater_info.package_zip = &za;
updater_info.version = atoi(version);

State state;
state.cookie = &updater_info;
state.script = script;
state.errmsg = NULL;

// 解释执行脚本。 核心函数是 Evaluate。它会调用其他callback函数，而这些callback函数
// 又会调用Evaluate去解析不同的脚本片段。从而实现一个简单的解释器
char* result = Evaluate(&state, root);
if (result == NULL) {
if (state.errmsg == NULL) {
fprintf(stderr, "script aborted (no error message)\n");
fprintf(cmd_pipe, "ui_print script aborted (no error message)\n");
} else {
fprintf(stderr, "script aborted: %s\n", state.errmsg);
char* line = strtok(state.errmsg, "\n");
while (line) {
fprintf(cmd_pipe, "ui_print %s\n", line);
line = strtok(NULL, "\n");
}
fprintf(cmd_pipe, "ui_print\n");
}
free(state.errmsg);
return 7;
} else {
fprintf(stderr, "script result was [%s]\n", result);
free(result);
}

if (updater_info.package_zip) {
mzCloseZipArchive(updater_info.package_zip);
}
free(script);

return 0;
}

还没开始，就结束了。代码非常简单，因为细节隐藏在那些callback函数里。我们看一下。

三、callback函数

1) RegisterBuiltins

void RegisterBuiltins() {
RegisterFunction("ifelse", IfElseFn);
RegisterFunction("abort", AbortFn);
RegisterFunction("assert", AssertFn);
RegisterFunction("concat", ConcatFn);
RegisterFunction("is_substring", SubstringFn);
RegisterFunction("stdout", StdoutFn);
RegisterFunction("sleep", SleepFn);

RegisterFunction("less_than_int", LessThanIntFn);
RegisterFunction("greater_than_int", GreaterThanIntFn);
}

这些语句控制执行流程。

2) RegisterInstallFunctions

void RegisterInstallFunctions() {
RegisterFunction("mount", MountFn);
RegisterFunction("is_mounted", IsMountedFn);
RegisterFunction("unmount", UnmountFn);
RegisterFunction("format", FormatFn);
RegisterFunction("show_progress", ShowProgressFn);
RegisterFunction("set_progress", SetProgressFn);
RegisterFunction("delete", DeleteFn);
RegisterFunction("delete_recursive", DeleteFn);
RegisterFunction("package_extract_dir", PackageExtractDirFn);
RegisterFunction("package_extract_file", PackageExtractFileFn);
RegisterFunction("retouch_binaries", RetouchBinariesFn);
RegisterFunction("undo_retouch_binaries", UndoRetouchBinariesFn);
RegisterFunction("symlink", SymlinkFn);
RegisterFunction("set_perm", SetPermFn);
RegisterFunction("set_perm_recursive", SetPermFn);

RegisterFunction("getprop", GetPropFn);
RegisterFunction("file_getprop", FileGetPropFn);
RegisterFunction("write_raw_image", WriteRawImageFn);
RegisterFunction("write_raw_parameter_image", WriteRawParameterImageFn);
RegisterFunction("clear_misc_command", ClearMiscCommandFn);

RegisterFunction("apply_patch", ApplyPatchFn);
RegisterFunction("apply_patch_check", ApplyPatchCheckFn);
RegisterFunction("apply_patch_space", ApplyPatchSpaceFn);

RegisterFunction("read_file", ReadFileFn);
RegisterFunction("sha1_check", Sha1CheckFn);

RegisterFunction("wipe_cache", WipeCacheFn);

RegisterFunction("ui_print", UIPrintFn);

RegisterFunction("run_program", RunProgramFn);
}

这些语句执行各种功能。基本上，我们只需要知道用法就可以了。值得注意的是，run_program原语允许我们去执行自定义程序，这应该足够满足我们的个性化需求了。