init: introduce Result<T> for return values and error handling
init tries to propagate error information up to build context before
logging errors. This is a good thing, however too often init has the
overly verbose paradigm for error handling, below:
bool CalculateResult(const T& input, U* output, std::string* err)
bool CalculateAndUseResult(const T& input, std::string* err) {
U output;
std::string calculate_result_err;
if (!CalculateResult(input, &output, &calculate_result_err)) {
*err = "CalculateResult " + input + " failed: " +
calculate_result_err;
return false;
}
UseResult(output);
return true;
}
Even more common are functions that return only true/false but also
require passing a std::string* err in order to see the error message.
This change introduces a Result<T> that is use to either hold a
successful return value of type T or to hold an error message as a
std::string. If the functional only returns success or a failure with
an error message, Result<Success> may be used. The classes Error and
ErrnoError are used to indicate a failed Result<T>.
A successful Result<T> is constructed implicitly from any type that
can be implicitly converted to T or from the constructor arguments for
T. This allows you to return a type T directly from a function that
returns Result<T>.
Error and ErrnoError are used to construct a Result<T> has
failed. Each of these classes take an ostream as an input and are
implicitly cast to a Result<T> containing that failure. ErrnoError()
additionally appends ": " + strerror(errno) to the end of the failure
string to aid in interacting with C APIs.
The end result is that the above code snippet is turned into the much
clearer example below:
Result<U> CalculateResult(const T& input);
Result<Success> CalculateAndUseResult(const T& input) {
auto output = CalculateResult(input);
if (!output) {
return Error() << "CalculateResult " << input << " failed: "
<< output.error();
}
UseResult(*output);
return Success();
}
This change also makes this conversion for some of the util.cpp
functions that used the old paradigm.
Test: boot bullhead, init unit tests
Change-Id: I1e7d3a8820a79362245041251057fbeed2f7979b
diff --git a/init/property_service.cpp b/init/property_service.cpp
index bd42913..bbfb047 100644
--- a/init/property_service.cpp
+++ b/init/property_service.cpp
@@ -588,14 +588,14 @@
// "ro.foo.*" is a prefix match, and "ro.foo.bar" is an exact match.
static bool load_properties_from_file(const char* filename, const char* filter) {
Timer t;
- std::string data;
- std::string err;
- if (!ReadFile(filename, &data, &err)) {
- PLOG(WARNING) << "Couldn't load property file: " << err;
+ auto file_contents = ReadFile(filename);
+ if (!file_contents) {
+ PLOG(WARNING) << "Couldn't load property file '" << filename
+ << "': " << file_contents.error();
return false;
}
- data.push_back('\n');
- load_properties(&data[0], filter);
+ file_contents->push_back('\n');
+ load_properties(file_contents->data(), filter);
LOG(VERBOSE) << "(Loading properties from " << filename << " took " << t << ".)";
return true;
}