Android OTA 升级之五：updater
前言
可以说，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. 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。
入口函数 main
(from: bootable/recovery/updater/updater.c)
62 // Where in the package we expect to find the edify script to execute.
63 // (Note it's "updateR-script", not the older "update-script".)
64 #define SCRIPT_NAME "META-INF/com/google/android/updater-script"
65
这里定义脚本的位置，注释说明本updater支持edify格式的脚本。
66 int main(int argc, char** argv) {
67 // Various things log information to stdout or stderr more or less
68 // at random. The log file makes more sense if buffering is
69 // turned off so things appear in the right order.
70 setbuf(stdout, NULL);
71 setbuf(stderr, NULL);
72
73 if (argc != 4) {
74 fprintf(stderr, "unexpected number of arguments (%d)/n", argc);
75 return 1;
76 }
77
78 char* version = argv[1];
79 if ((version[0] != '1' && version[0] != '2' && version[0] != '3') ||
80 version[1] != '/0') {
81 // We support version 1, 2, or 3.
82 fprintf(stderr, "wrong updater binary API; expected 1, 2, or 3; "
83 "got %s/n",
84 argv[1]);
85 return 2;
86 }
87
获取 version 参数。
88 // Set up the pipe for sending commands back to the parent process.
89
90 int fd = atoi(argv[2]);
91 FILE* cmd_pipe = fdopen(fd, "wb");
92 setlinebuf(cmd_pipe);
93
获取命令管道（用于图形显示等，见前篇）
94 // Extract the script from the package.
95
96 char* package_data = argv[3];
97 ZipArchive za;
98 int err;
99 err = mzOpenZipArchive(package_data, &za);
100 if (err != 0) {
101 fprintf(stderr, "failed to open package %s: %s/n",
102 package_data, strerror(err));
103 return 3;
104 }
105
106 const ZipEntry* script_entry = mzFindZipEntry(&za, SCRIPT_NAME);
107 if (script_entry == NULL) {
108 fprintf(stderr, "failed to find %s in %s/n", SCRIPT_NAME, package_data);
109 return 4;
110 }
111
112 char* script = malloc(script_entry->uncompLen+1);
113 if (!mzReadZipEntry(&za, script_entry, script, script_entry->uncompLen)) {
114 fprintf(stderr, "failed to read script from package/n");
115 return 5;
116 }
117 script[script_entry->uncompLen] = '/0';
118
读入脚本 META-INF/com/google/android/updater-script
119 // Configure edify's functions.
120
121 RegisterBuiltins();
122 RegisterInstallFunctions();
123 RegisterDeviceExtensions();
124 FinishRegistration();
125
注册语句处理函数
126 // Parse the script.
127
128 Expr* root;
129 int error_count = 0;
130 yy_scan_string(script);
131 int error = yyparse(&root, &error_count);
132 if (error != 0 || error_count > 0) {
133 fprintf(stderr, "%d parse errors/n", error_count);
134 return 6;
135 }
136
调用yy* 库函数解析脚本。
137 // Evaluate the parsed script.
138
139 UpdaterInfo updater_info;
140 updater_info.cmd_pipe = cmd_pipe;
141 updater_info.package_zip = &za;
142 updater_info.version = atoi(version);
143
144 State state;
145 state.cookie = &updater_info;
146 state.script = script;
147 state.errmsg = NULL;
148
149 char* result = Evaluate(&state, root);
150 if (result == NULL) {
151 if (state.errmsg == NULL) {
152 fprintf(stderr, "script aborted (no error message)/n");
153 fprintf(cmd_pipe, "ui_print script aborted (no error message)/n");
154 } else {
155 fprintf(stderr, "script aborted: %s/n", state.errmsg);
156 char* line = strtok(state.errmsg, "/n");
157 while (line) {
158 fprintf(cmd_pipe, "ui_print %s/n", line);
159 line = strtok(NULL, "/n");
160 }
161 fprintf(cmd_pipe, "ui_print/n");
162 }
163 free(state.errmsg);
164 return 7;
165 } else {
166 fprintf(stderr, "script result was [%s]/n", result);
167 free(result);
168 }
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。
169
170 mzCloseZipArchive(&za);
171 free(script);
172
173 return 0;
174 }
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。
RegisterBuiltins
415 void RegisterBuiltins() {
416 RegisterFunction("ifelse", IfElseFn);
417 RegisterFunction("abort", AbortFn);
418 RegisterFunction("assert", AssertFn);
419 RegisterFunction("concat", ConcatFn);
420 RegisterFunction("is_substring", SubstringFn);
421 RegisterFunction("stdout", StdoutFn);
422 RegisterFunction("sleep", SleepFn);
423
424 RegisterFunction("less_than_int", LessThanIntFn);
425 RegisterFunction("greater_than_int", GreaterThanIntFn);
426 }
这些语句控制执行流程。
RegisterInstallFunctions
1036
1037 void RegisterInstallFunctions() {
1038 RegisterFunction("mount", MountFn);
1039 RegisterFunction("is_mounted", IsMountedFn);
1040 RegisterFunction("unmount", UnmountFn);
1041 RegisterFunction("format", FormatFn);
1042 RegisterFunction("show_progress", ShowProgressFn);
1043 RegisterFunction("set_progress", SetProgressFn);
1044 RegisterFunction("delete", DeleteFn);
1045 RegisterFunction("delete_recursive", DeleteFn);
1046 RegisterFunction("package_extract_dir", PackageExtractDirFn);
1047 RegisterFunction("package_extract_file", PackageExtractFileFn);
1048 RegisterFunction("symlink", SymlinkFn);
1049 RegisterFunction("set_perm", SetPermFn);
1050 RegisterFunction("set_perm_recursive", SetPermFn);
1051
1052 RegisterFunction("getprop", GetPropFn);
1053 RegisterFunction("file_getprop", FileGetPropFn);
1054 RegisterFunction("write_raw_image", WriteRawImageFn);
1055
1056 RegisterFunction("apply_patch", ApplyPatchFn);
1057 RegisterFunction("apply_patch_check", ApplyPatchCheckFn);
1058 RegisterFunction("apply_patch_space", ApplyPatchSpaceFn);
1059
1060 RegisterFunction("read_file", ReadFileFn);
1061 RegisterFunction("sha1_check", Sha1CheckFn);
1062
1063 RegisterFunction("ui_print", UIPrintFn);
1064
1065 RegisterFunction("run_program", RunProgramFn);
1066 }