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

1 files changed, 27 insertions, 31 deletions

diff --git a/init/builtins.cpp b/init/builtins.cpp
index a1eab06f1..2d4d78bc7 100644
--- a/init/builtins.cpp
+++ b/init/builtins.cpp
@@ -151,9 +151,8 @@ static int do_class_restart(const std::vector<std::string>& args) {
 }
 
 static int do_domainname(const std::vector<std::string>& args) {
-    std::string err;
-    if (!WriteFile("/proc/sys/kernel/domainname", args[1], &err)) {
-        LOG(ERROR) << err;
+    if (auto result = WriteFile("/proc/sys/kernel/domainname", args[1]); !result) {
+        LOG(ERROR) << "Unable to write to /proc/sys/kernel/domainname: " << result.error();
         return -1;
     }
     return 0;
@@ -199,9 +198,8 @@ static int do_export(const std::vector<std::string>& args) {
 }
 
 static int do_hostname(const std::vector<std::string>& args) {
-    std::string err;
-    if (!WriteFile("/proc/sys/kernel/hostname", args[1], &err)) {
-        LOG(ERROR) << err;
+    if (auto result = WriteFile("/proc/sys/kernel/hostname", args[1]); !result) {
+        LOG(ERROR) << "Unable to write to /proc/sys/kernel/hostname: " << result.error();
         return -1;
     }
     return 0;
@@ -244,22 +242,22 @@ static int do_mkdir(const std::vector<std::string>& args) {
     }
 
     if (args.size() >= 4) {
-        uid_t uid;
-        std::string decode_uid_err;
-        if (!DecodeUid(args[3], &uid, &decode_uid_err)) {
-            LOG(ERROR) << "Unable to find UID for '" << args[3] << "': " << decode_uid_err;
+        auto uid = DecodeUid(args[3]);
+        if (!uid) {
+            LOG(ERROR) << "Unable to decode UID for '" << args[3] << "': " << uid.error();
             return -1;
         }
-        gid_t gid = -1;
+        Result<gid_t> gid = -1;
 
         if (args.size() == 5) {
-            if (!DecodeUid(args[4], &gid, &decode_uid_err)) {
-                LOG(ERROR) << "Unable to find GID for '" << args[3] << "': " << decode_uid_err;
+            gid = DecodeUid(args[4]);
+            if (!gid) {
+                LOG(ERROR) << "Unable to decode GID for '" << args[3] << "': " << gid.error();
                 return -1;
             }
         }
 
-        if (lchown(args[1].c_str(), uid, gid) == -1) {
+        if (lchown(args[1].c_str(), *uid, *gid) == -1) {
             return -errno;
         }
 
@@ -668,9 +666,8 @@ static int do_verity_update_state(const std::vector<std::string>& args) {
 }
 
 static int do_write(const std::vector<std::string>& args) {
-    std::string err;
-    if (!WriteFile(args[1], args[2], &err)) {
-        LOG(ERROR) << err;
+    if (auto result = WriteFile(args[1], args[2]); !result) {
+        LOG(ERROR) << "Unable to write to file '" << args[1] << "': " << result.error();
         return -1;
     }
     return 0;
@@ -737,39 +734,38 @@ static int do_readahead(const std::vector<std::string>& args) {
 }
 
 static int do_copy(const std::vector<std::string>& args) {
-    std::string data;
-    std::string err;
-    if (!ReadFile(args[1], &data, &err)) {
-        LOG(ERROR) << err;
+    auto file_contents = ReadFile(args[1]);
+    if (!file_contents) {
+        LOG(ERROR) << "Could not read input file '" << args[1] << "': " << file_contents.error();
         return -1;
     }
-    if (!WriteFile(args[2], data, &err)) {
-        LOG(ERROR) << err;
+    if (auto result = WriteFile(args[2], *file_contents); !result) {
+        LOG(ERROR) << "Could not write to output file '" << args[2] << "': " << result.error();
         return -1;
     }
     return 0;
 }
 
 static int do_chown(const std::vector<std::string>& args) {
-    uid_t uid;
-    std::string decode_uid_err;
-    if (!DecodeUid(args[1], &uid, &decode_uid_err)) {
-        LOG(ERROR) << "Unable to find UID for '" << args[1] << "': " << decode_uid_err;
+    auto uid = DecodeUid(args[1]);
+    if (!uid) {
+        LOG(ERROR) << "Unable to decode UID for '" << args[1] << "': " << uid.error();
         return -1;
     }
 
     // GID is optional and pushes the index of path out by one if specified.
     const std::string& path = (args.size() == 4) ? args[3] : args[2];
-    gid_t gid = -1;
+    Result<gid_t> gid = -1;
 
     if (args.size() == 4) {
-        if (!DecodeUid(args[2], &gid, &decode_uid_err)) {
-            LOG(ERROR) << "Unable to find GID for '" << args[2] << "': " << decode_uid_err;
+        gid = DecodeUid(args[2]);
+        if (!gid) {
+            LOG(ERROR) << "Unable to decode GID for '" << args[2] << "': " << gid.error();
             return -1;
         }
     }
 
-    if (lchown(path.c_str(), uid, gid) == -1) return -errno;
+    if (lchown(path.c_str(), *uid, *gid) == -1) return -errno;
 
     return 0;
 }