init: add wait_for_prop builtin command

There are many use cases from vendors to exec service in background and then
use a shell scriprt to wait for the command done.

This CL is to add a wait_for_prop command to suppor those use cases.

Bug: 34746108
Test: on marlin
Change-Id: Ia81290b0928f9d375710d2daa546714f0cd65b72

init/README.md

@@ -298,7 +298,8 @@ Commands
 > Fork and execute command with the given arguments. The command starts
   after "--" so that an optional security context, user, and supplementary
   groups can be provided. No other commands will be run until this one
-  finishes. _seclabel_ can be a - to denote default.
+  finishes. _seclabel_ can be a - to denote default. Properties are expanded
+  within _argument_.
 
 `export <name> <value>`
 > Set the environment variable _name_ equal to _value_ in the
@@ -412,6 +413,11 @@ Commands
   or the timeout has been reached. If timeout is not specified it
   currently defaults to five seconds.
 
+`wait_for_prop <name> <value>`
+> Wait for system property _name_ to be _value_. Properties are expanded
+  within _value_. If property _name_ is already set to _value_, continue
+  immediately.
+
 `write <path> <content>`
 > Open the file at _path_ and write a string to it with write(2).
   If the file does not exist, it will be created. If it does exist,

init/builtins.cpp

@@ -1003,6 +1003,29 @@ static int do_wait(const std::vector<std::string>& args) {
     return -1;
 }
 
+static int do_wait_for_prop(const std::vector<std::string>& args) {
+    const char* name = args[1].c_str();
+    const char* value = args[2].c_str();
+    size_t value_len = strlen(value);
+
+    if (!is_legal_property_name(name)) {
+        LOG(ERROR) << "do_wait_for_prop(\"" << name << "\", \"" << value
+                   << "\") failed: bad name";
+        return -1;
+    }
+    if (value_len >= PROP_VALUE_MAX) {
+        LOG(ERROR) << "do_wait_for_prop(\"" << name << "\", \"" << value
+                   << "\") failed: value too long";
+        return -1;
+    }
+    if (!wait_property(name, value)) {
+        LOG(ERROR) << "do_wait_for_prop(\"" << name << "\", \"" << value
+                   << "\") failed: init already in waiting";
+        return -1;
+    }
+    return 0;
+}
+
 /*
  * Callback to make a directory from the ext4 code
  */
@@ -1074,6 +1097,7 @@ BuiltinFunctionMap::Map& BuiltinFunctionMap::map() const {
         {"verity_load_state",       {0,     0,    do_verity_load_state}},
         {"verity_update_state",     {0,     0,    do_verity_update_state}},
         {"wait",                    {1,     2,    do_wait}},
+        {"wait_for_prop",           {2,     2,    do_wait_for_prop}},
         {"write",                   {2,     2,    do_write}},
     };
     return builtin_functions;

init/init.cpp

@@ -87,6 +87,10 @@ bool waiting_for_exec = false;
 
 static int epoll_fd = -1;
 
+static std::unique_ptr<Timer> waiting_for_prop(nullptr);
+static std::string wait_prop_name;
+static std::string wait_prop_value;
+
 void register_epoll_handler(int fd, void (*fn)()) {
     epoll_event ev;
     ev.events = EPOLLIN;
@@ -128,10 +132,34 @@ int add_environment(const char *key, const char *val)
     return -1;
 }
 
+bool wait_property(const char *name, const char *value)
+{
+    if (waiting_for_prop) {
+        return false;
+    }
+    if (property_get(name) != value) {
+        // Current property value is not equal to expected value
+        wait_prop_name = name;
+        wait_prop_value = value;
+        waiting_for_prop.reset(new Timer());
+    } else {
+        LOG(INFO) << "wait_property(\"" << name << "\", \"" << value << "\"): already set";
+    }
+    return true;
+}
+
 void property_changed(const char *name, const char *value)
 {
     if (property_triggers_enabled)
         ActionManager::GetInstance().QueuePropertyTrigger(name, value);
+    if (waiting_for_prop) {
+        if (wait_prop_name == name && wait_prop_value == value) {
+            wait_prop_name.clear();
+            wait_prop_value.clear();
+            LOG(INFO) << "Wait for property took " << *waiting_for_prop;
+            waiting_for_prop.reset();
+        }
+    }
 }
 
 static void restart_processes()
@@ -876,7 +904,7 @@ int main(int argc, char** argv) {
     am.QueueBuiltinAction(queue_property_triggers_action, "queue_property_triggers");
 
     while (true) {
-        if (!waiting_for_exec) {
+        if (!(waiting_for_exec || waiting_for_prop)) {
             am.ExecuteOneCommand();
             restart_processes();
         }

init/init.h

@@ -36,4 +36,6 @@ void register_epoll_handler(int fd, void (*fn)());
 
 int add_environment(const char* key, const char* val);
 
+bool wait_property(const char *name, const char *value);
+
 #endif  /* _INIT_INIT_H */