idmap2: move commands to Result<Unit>

Change the signature of the idmap2 commands (Create, Dump, ...) to
return Result<Unit> instead of bool. This removes the need to pass in an
ostream for error messages: instead, those messages are part of the
returned Result.

Consolidate error messages: texts in Error objects should not be
prefixed with "error:", that is the responsibility of the outer-most
caller (i.e. main()).

Test: make idmap2_tests
Change-Id: I074881b3d1982ea8f4be5752161ac74b14fcba95

14 files changed, 166 insertions, 161 deletions

diff --git a/cmds/idmap2/idmap2/Commands.h b/cmds/idmap2/idmap2/Commands.h
index dcc69b30743d..718e361b38ab 100644
--- a/cmds/idmap2/idmap2/Commands.h
+++ b/cmds/idmap2/idmap2/Commands.h
@@ -20,10 +20,12 @@
 #include <string>
 #include <vector>
 
-bool Create(const std::vector<std::string>& args, std::ostream& out_error);
-bool Dump(const std::vector<std::string>& args, std::ostream& out_error);
-bool Lookup(const std::vector<std::string>& args, std::ostream& out_error);
-bool Scan(const std::vector<std::string>& args, std::ostream& out_error);
-bool Verify(const std::vector<std::string>& args, std::ostream& out_error);
+#include "idmap2/Result.h"
+
+android::idmap2::Result<android::idmap2::Unit> Create(const std::vector<std::string>& args);
+android::idmap2::Result<android::idmap2::Unit> Dump(const std::vector<std::string>& args);
+android::idmap2::Result<android::idmap2::Unit> Lookup(const std::vector<std::string>& args);
+android::idmap2::Result<android::idmap2::Unit> Scan(const std::vector<std::string>& args);
+android::idmap2::Result<android::idmap2::Unit> Verify(const std::vector<std::string>& args);
 
 #endif  // IDMAP2_IDMAP2_COMMANDS_H_
diff --git a/cmds/idmap2/idmap2/Create.cpp b/cmds/idmap2/idmap2/Create.cpp
index c416fa123b73..fdbb21044176 100644
--- a/cmds/idmap2/idmap2/Create.cpp
+++ b/cmds/idmap2/idmap2/Create.cpp
@@ -33,14 +33,17 @@
 using android::ApkAssets;
 using android::idmap2::BinaryStreamVisitor;
 using android::idmap2::CommandLineOptions;
+using android::idmap2::Error;
 using android::idmap2::Idmap;
 using android::idmap2::PoliciesToBitmask;
 using android::idmap2::PolicyBitmask;
 using android::idmap2::PolicyFlags;
+using android::idmap2::Result;
+using android::idmap2::Unit;
 using android::idmap2::utils::kIdmapFilePermissionMask;
 using android::idmap2::utils::UidHasWriteAccessToPath;
 
-bool Create(const std::vector<std::string>& args, std::ostream& out_error) {
+Result<Unit> Create(const std::vector<std::string>& args) {
   SYSTRACE << "Create " << args;
   std::string target_apk_path;
   std::string overlay_apk_path;
@@ -63,15 +66,14 @@ bool Create(const std::vector<std::string>& args, std::ostream& out_error) {
                           &policies)
           .OptionalFlag("--ignore-overlayable", "disables overlayable and policy checks",
                         &ignore_overlayable);
-  if (!opts.Parse(args, out_error)) {
-    return false;
+  const auto opts_ok = opts.Parse(args);
+  if (!opts_ok) {
+    return opts_ok.GetError();
   }
 
   const uid_t uid = getuid();
   if (!UidHasWriteAccessToPath(uid, idmap_path)) {
-    out_error << "error: uid " << uid << " does not have write access to " << idmap_path
-              << std::endl;
-    return false;
+    return Error("uid %d does not have write access to %s", uid, idmap_path.c_str());
   }
 
   PolicyBitmask fulfilled_policies = 0;
@@ -79,8 +81,7 @@ bool Create(const std::vector<std::string>& args, std::ostream& out_error) {
   if (conv_result) {
     fulfilled_policies |= *conv_result;
   } else {
-    out_error << "error: " << conv_result.GetErrorMessage() << std::endl;
-    return false;
+    return conv_result.GetError();
   }
 
   if (fulfilled_policies == 0) {
@@ -89,36 +90,33 @@ bool Create(const std::vector<std::string>& args, std::ostream& out_error) {
 
   const std::unique_ptr<const ApkAssets> target_apk = ApkAssets::Load(target_apk_path);
   if (!target_apk) {
-    out_error << "error: failed to load apk " << target_apk_path << std::endl;
-    return false;
+    return Error("failed to load apk %s", target_apk_path.c_str());
   }
 
   const std::unique_ptr<const ApkAssets> overlay_apk = ApkAssets::Load(overlay_apk_path);
   if (!overlay_apk) {
-    out_error << "error: failed to load apk " << overlay_apk_path << std::endl;
-    return false;
+    return Error("failed to load apk %s", overlay_apk_path.c_str());
   }
 
+  std::stringstream stream;
   const std::unique_ptr<const Idmap> idmap =
       Idmap::FromApkAssets(target_apk_path, *target_apk, overlay_apk_path, *overlay_apk,
-                           fulfilled_policies, !ignore_overlayable, out_error);
+                           fulfilled_policies, !ignore_overlayable, stream);
   if (!idmap) {
-    return false;
+    return Error("failed to create idmap: %s", stream.str().c_str());
   }
 
   umask(kIdmapFilePermissionMask);
   std::ofstream fout(idmap_path);
   if (fout.fail()) {
-    out_error << "failed to open idmap path " << idmap_path << std::endl;
-    return false;
+    return Error("failed to open idmap path %s", idmap_path.c_str());
   }
   BinaryStreamVisitor visitor(fout);
   idmap->accept(&visitor);
   fout.close();
   if (fout.fail()) {
-    out_error << "failed to write to idmap path " << idmap_path << std::endl;
-    return false;
+    return Error("failed to write to idmap path %s", idmap_path.c_str());
   }
 
-  return true;
+  return Unit{};
 }
diff --git a/cmds/idmap2/idmap2/Dump.cpp b/cmds/idmap2/idmap2/Dump.cpp
index 3947703fe8da..fd5822251188 100644
--- a/cmds/idmap2/idmap2/Dump.cpp
+++ b/cmds/idmap2/idmap2/Dump.cpp
@@ -17,6 +17,7 @@
 #include <fstream>
 #include <iostream>
 #include <memory>
+#include <sstream>
 #include <string>
 #include <vector>
 
@@ -24,14 +25,18 @@
 #include "idmap2/Idmap.h"
 #include "idmap2/PrettyPrintVisitor.h"
 #include "idmap2/RawPrintVisitor.h"
+#include "idmap2/Result.h"
 #include "idmap2/SysTrace.h"
 
 using android::idmap2::CommandLineOptions;
+using android::idmap2::Error;
 using android::idmap2::Idmap;
 using android::idmap2::PrettyPrintVisitor;
 using android::idmap2::RawPrintVisitor;
+using android::idmap2::Result;
+using android::idmap2::Unit;
 
-bool Dump(const std::vector<std::string>& args, std::ostream& out_error) {
+Result<Unit> Dump(const std::vector<std::string>& args) {
   SYSTRACE << "Dump " << args;
   std::string idmap_path;
   bool verbose;
@@ -40,14 +45,16 @@ bool Dump(const std::vector<std::string>& args, std::ostream& out_error) {
       CommandLineOptions("idmap2 dump")
           .MandatoryOption("--idmap-path", "input: path to idmap file to pretty-print", &idmap_path)
           .OptionalFlag("--verbose", "annotate every byte of the idmap", &verbose);
-  if (!opts.Parse(args, out_error)) {
-    return false;
+  const auto opts_ok = opts.Parse(args);
+  if (!opts_ok) {
+    return opts_ok.GetError();
   }
+  std::stringstream stream;
   std::ifstream fin(idmap_path);
-  const std::unique_ptr<const Idmap> idmap = Idmap::FromBinaryStream(fin, out_error);
+  const std::unique_ptr<const Idmap> idmap = Idmap::FromBinaryStream(fin, stream);
   fin.close();
   if (!idmap) {
-    return false;
+    return Error("failed to load idmap: %s", stream.str().c_str());
   }
 
   if (verbose) {
@@ -58,5 +65,5 @@ bool Dump(const std::vector<std::string>& args, std::ostream& out_error) {
     idmap->accept(&visitor);
   }
 
-  return true;
+  return Unit{};
 }
diff --git a/cmds/idmap2/idmap2/Lookup.cpp b/cmds/idmap2/idmap2/Lookup.cpp
index 83a40efee3f3..677c6fa155dd 100644
--- a/cmds/idmap2/idmap2/Lookup.cpp
+++ b/cmds/idmap2/idmap2/Lookup.cpp
@@ -57,6 +57,7 @@ using android::idmap2::Error;
 using android::idmap2::IdmapHeader;
 using android::idmap2::ResourceId;
 using android::idmap2::Result;
+using android::idmap2::Unit;
 using android::idmap2::Xml;
 using android::idmap2::ZipFile;
 using android::util::Utf16ToUtf8;
@@ -157,7 +158,7 @@ Result<std::string> GetTargetPackageNameFromManifest(const std::string& apk_path
 }
 }  // namespace
 
-bool Lookup(const std::vector<std::string>& args, std::ostream& out_error) {
+Result<Unit> Lookup(const std::vector<std::string>& args) {
   SYSTRACE << "Lookup " << args;
   std::vector<std::string> idmap_paths;
   std::string config_str;
@@ -172,14 +173,14 @@ bool Lookup(const std::vector<std::string>& args, std::ostream& out_error) {
                            "'[package:]type/name') to look up",
                            &resid_str);
 
-  if (!opts.Parse(args, out_error)) {
-    return false;
+  const auto opts_ok = opts.Parse(args);
+  if (!opts_ok) {
+    return opts_ok.GetError();
   }
 
   ConfigDescription config;
   if (!ConfigDescription::Parse(config_str, &config)) {
-    out_error << "error: failed to parse config" << std::endl;
-    return false;
+    return Error("failed to parse config");
   }
 
   std::vector<std::unique_ptr<const ApkAssets>> apk_assets;
@@ -191,39 +192,33 @@ bool Lookup(const std::vector<std::string>& args, std::ostream& out_error) {
     auto idmap_header = IdmapHeader::FromBinaryStream(fin);
     fin.close();
     if (!idmap_header) {
-      out_error << "error: failed to read idmap from " << idmap_path << std::endl;
-      return false;
+      return Error("failed to read idmap from %s", idmap_path.c_str());
     }
 
     if (i == 0) {
       target_path = idmap_header->GetTargetPath().to_string();
       auto target_apk = ApkAssets::Load(target_path);
       if (!target_apk) {
-        out_error << "error: failed to read target apk from " << target_path << std::endl;
-        return false;
+        return Error("failed to read target apk from %s", target_path.c_str());
       }
       apk_assets.push_back(std::move(target_apk));
 
       const Result<std::string> package_name =
           GetTargetPackageNameFromManifest(idmap_header->GetOverlayPath().to_string());
       if (!package_name) {
-        out_error << "error: failed to parse android:targetPackage from overlay manifest"
-                  << std::endl;
-        return false;
+        return Error("failed to parse android:targetPackage from overlay manifest");
       }
       target_package_name = *package_name;
     } else if (target_path != idmap_header->GetTargetPath()) {
-      out_error << "error: different target APKs (expected target APK " << target_path << " but "
-                << idmap_path << " has target APK " << idmap_header->GetTargetPath() << ")"
-                << std::endl;
-      return false;
+      return Error("different target APKs (expected target APK %s but %s has target APK %s)",
+                   target_path.c_str(), idmap_path.c_str(),
+                   idmap_header->GetTargetPath().to_string().c_str());
     }
 
     auto overlay_apk = ApkAssets::LoadOverlay(idmap_path);
     if (!overlay_apk) {
-      out_error << "error: failed to read overlay apk from " << idmap_header->GetOverlayPath()
-                << std::endl;
-      return false;
+      return Error("failed to read overlay apk from %s",
+                   idmap_header->GetOverlayPath().to_string().c_str());
     }
     apk_assets.push_back(std::move(overlay_apk));
   }
@@ -238,16 +233,14 @@ bool Lookup(const std::vector<std::string>& args, std::ostream& out_error) {
 
   const Result<ResourceId> resid = ParseResReference(am, resid_str, target_package_name);
   if (!resid) {
-    out_error << "error: failed to parse resource ID" << std::endl;
-    return false;
+    return Error(resid.GetError(), "failed to parse resource ID");
   }
 
   const Result<std::string> value = GetValue(am, *resid);
   if (!value) {
-    out_error << StringPrintf("error: resource 0x%08x not found", *resid) << std::endl;
-    return false;
+    return Error(value.GetError(), "resource 0x%08x not found", *resid);
   }
   std::cout << *value << std::endl;
 
-  return true;
+  return Unit{};
 }
diff --git a/cmds/idmap2/idmap2/Main.cpp b/cmds/idmap2/idmap2/Main.cpp
index a0ffccb26dfe..d8867fe8f497 100644
--- a/cmds/idmap2/idmap2/Main.cpp
+++ b/cmds/idmap2/idmap2/Main.cpp
@@ -24,14 +24,17 @@
 #include <vector>
 
 #include "idmap2/CommandLineOptions.h"
+#include "idmap2/Result.h"
 #include "idmap2/SysTrace.h"
 
 #include "Commands.h"
 
 using android::idmap2::CommandLineOptions;
+using android::idmap2::Result;
+using android::idmap2::Unit;
 
 using NameToFunctionMap =
-    std::map<std::string, std::function<bool(const std::vector<std::string>&, std::ostream&)>>;
+    std::map<std::string, std::function<Result<Unit>(const std::vector<std::string>&)>>;
 
 namespace {
 
@@ -69,5 +72,10 @@ int main(int argc, char** argv) {
     PrintUsage(commands, std::cerr);
     return EXIT_FAILURE;
   }
-  return iter->second(*args, std::cerr) ? EXIT_SUCCESS : EXIT_FAILURE;
+  const auto result = iter->second(*args);
+  if (!result) {
+    std::cerr << "error: " << result.GetErrorMessage() << std::endl;
+    return EXIT_FAILURE;
+  }
+  return EXIT_SUCCESS;
 }
diff --git a/cmds/idmap2/idmap2/Scan.cpp b/cmds/idmap2/idmap2/Scan.cpp
index e85f132c6072..24331af9fd6d 100644
--- a/cmds/idmap2/idmap2/Scan.cpp
+++ b/cmds/idmap2/idmap2/Scan.cpp
@@ -30,6 +30,7 @@
 #include "idmap2/FileUtils.h"
 #include "idmap2/Idmap.h"
 #include "idmap2/ResourceUtils.h"
+#include "idmap2/Result.h"
 #include "idmap2/SysTrace.h"
 #include "idmap2/Xml.h"
 #include "idmap2/ZipFile.h"
@@ -37,6 +38,7 @@
 #include "Commands.h"
 
 using android::idmap2::CommandLineOptions;
+using android::idmap2::Error;
 using android::idmap2::Idmap;
 using android::idmap2::kPolicyProduct;
 using android::idmap2::kPolicyPublic;
@@ -45,6 +47,7 @@ using android::idmap2::kPolicyVendor;
 using android::idmap2::PolicyBitmask;
 using android::idmap2::PolicyFlags;
 using android::idmap2::Result;
+using android::idmap2::Unit;
 using android::idmap2::utils::ExtractOverlayManifestInfo;
 using android::idmap2::utils::FindFiles;
 using android::idmap2::utils::OverlayManifestInfo;
@@ -69,8 +72,8 @@ bool VendorIsQOrLater() {
   return version == "Q" || version == "q";
 }
 
-std::unique_ptr<std::vector<std::string>> FindApkFiles(const std::vector<std::string>& dirs,
-                                                       bool recursive, std::ostream& out_error) {
+Result<std::unique_ptr<std::vector<std::string>>> FindApkFiles(const std::vector<std::string>& dirs,
+                                                               bool recursive) {
   SYSTRACE << "FindApkFiles " << dirs << " " << recursive;
   const auto predicate = [](unsigned char type, const std::string& path) -> bool {
     static constexpr size_t kExtLen = 4;  // strlen(".apk")
@@ -82,8 +85,7 @@ std::unique_ptr<std::vector<std::string>> FindApkFiles(const std::vector<std::st
   for (const auto& dir : dirs) {
     const auto apk_paths = FindFiles(dir, recursive, predicate);
     if (!apk_paths) {
-      out_error << "error: failed to open directory " << dir << std::endl;
-      return nullptr;
+      return Error("failed to open directory %s", dir.c_str());
     }
     paths.insert(apk_paths->cbegin(), apk_paths->cend());
   }
@@ -110,7 +112,7 @@ std::vector<std::string> PoliciesForPath(const std::string& apk_path) {
 
 }  // namespace
 
-bool Scan(const std::vector<std::string>& args, std::ostream& out_error) {
+Result<Unit> Scan(const std::vector<std::string>& args) {
   SYSTRACE << "Scan " << args;
   std::vector<std::string> input_directories;
   std::string target_package_name;
@@ -135,22 +137,22 @@ bool Scan(const std::vector<std::string>& args, std::ostream& out_error) {
               "input: an overlayable policy this overlay fulfills "
               "(if none or supplied, the overlays will not have their policies overriden",
               &override_policies);
-  if (!opts.Parse(args, out_error)) {
-    return false;
+  const auto opts_ok = opts.Parse(args);
+  if (!opts_ok) {
+    return opts_ok.GetError();
   }
 
-  const auto apk_paths = FindApkFiles(input_directories, recursive, out_error);
+  const auto apk_paths = FindApkFiles(input_directories, recursive);
   if (!apk_paths) {
-    return false;
+    return Error(apk_paths.GetError(), "failed to find apk files");
   }
 
   std::vector<InputOverlay> interesting_apks;
-  for (const std::string& path : *apk_paths) {
+  for (const std::string& path : **apk_paths) {
     Result<OverlayManifestInfo> overlay_info =
         ExtractOverlayManifestInfo(path, /* assert_overlay */ false);
     if (!overlay_info) {
-      out_error << "error: " << overlay_info.GetErrorMessage() << std::endl;
-      return false;
+      return overlay_info.GetError();
     }
 
     if (!overlay_info->is_static) {
@@ -194,16 +196,13 @@ bool Scan(const std::vector<std::string>& args, std::ostream& out_error) {
 
   std::stringstream stream;
   for (const auto& overlay : interesting_apks) {
-    // Create the idmap for the overlay if it currently does not exist or if it is not up to date.
-    std::stringstream dev_null;
-
     std::vector<std::string> verify_args = {"--idmap-path", overlay.idmap_path};
     for (const std::string& policy : overlay.policies) {
       verify_args.emplace_back("--policy");
       verify_args.emplace_back(policy);
     }
 
-    if (!Verify(std::vector<std::string>(verify_args), dev_null)) {
+    if (!Verify(std::vector<std::string>(verify_args))) {
       std::vector<std::string> create_args = {"--target-apk-path",  target_apk_path,
                                               "--overlay-apk-path", overlay.apk_path,
                                               "--idmap-path",       overlay.idmap_path};
@@ -216,8 +215,9 @@ bool Scan(const std::vector<std::string>& args, std::ostream& out_error) {
         create_args.emplace_back(policy);
       }
 
-      if (!Create(create_args, out_error)) {
-        return false;
+      const auto create_ok = Create(create_args);
+      if (!create_ok) {
+        return Error(create_ok.GetError(), "failed to create idmap");
       }
     }
 
@@ -226,5 +226,5 @@ bool Scan(const std::vector<std::string>& args, std::ostream& out_error) {
 
   std::cout << stream.str();
 
-  return true;
+  return Unit{};
 }
diff --git a/cmds/idmap2/idmap2/Verify.cpp b/cmds/idmap2/idmap2/Verify.cpp
index d8fe7aa0ed99..9cb67b33e6cf 100644
--- a/cmds/idmap2/idmap2/Verify.cpp
+++ b/cmds/idmap2/idmap2/Verify.cpp
@@ -21,29 +21,39 @@
 
 #include "idmap2/CommandLineOptions.h"
 #include "idmap2/Idmap.h"
+#include "idmap2/Result.h"
 #include "idmap2/SysTrace.h"
 
 using android::idmap2::CommandLineOptions;
+using android::idmap2::Error;
 using android::idmap2::IdmapHeader;
+using android::idmap2::Result;
+using android::idmap2::Unit;
 
-bool Verify(const std::vector<std::string>& args, std::ostream& out_error) {
+Result<Unit> Verify(const std::vector<std::string>& args) {
   SYSTRACE << "Verify " << args;
   std::string idmap_path;
 
   const CommandLineOptions opts =
       CommandLineOptions("idmap2 verify")
           .MandatoryOption("--idmap-path", "input: path to idmap file to verify", &idmap_path);
-  if (!opts.Parse(args, out_error)) {
-    return false;
+
+  const auto opts_ok = opts.Parse(args);
+  if (!opts_ok) {
+    return opts_ok.GetError();
   }
 
   std::ifstream fin(idmap_path);
   const std::unique_ptr<const IdmapHeader> header = IdmapHeader::FromBinaryStream(fin);
   fin.close();
   if (!header) {
-    out_error << "error: failed to parse idmap header" << std::endl;
-    return false;
+    return Error("failed to parse idmap header");
+  }
+
+  const auto header_ok = header->IsUpToDate();
+  if (!header_ok) {
+    return Error(header_ok.GetError(), "idmap not up to date");
   }
 
-  return header->IsUpToDate(out_error);
+  return Unit{};
 }
diff --git a/cmds/idmap2/idmap2d/Idmap2Service.cpp b/cmds/idmap2/idmap2d/Idmap2Service.cpp
index fa944143e408..e03a9cc1032e 100644
--- a/cmds/idmap2/idmap2d/Idmap2Service.cpp
+++ b/cmds/idmap2/idmap2d/Idmap2Service.cpp
@@ -104,8 +104,7 @@ Status Idmap2Service::verifyIdmap(const std::string& overlay_apk_path,
   std::ifstream fin(idmap_path);
   const std::unique_ptr<const IdmapHeader> header = IdmapHeader::FromBinaryStream(fin);
   fin.close();
-  std::stringstream dev_null;
-  *_aidl_return = header && header->IsUpToDate(dev_null);
+  *_aidl_return = header && header->IsUpToDate();
 
   // TODO(b/119328308): Check that the set of fulfilled policies of the overlay has not changed
 
diff --git a/cmds/idmap2/include/idmap2/CommandLineOptions.h b/cmds/idmap2/include/idmap2/CommandLineOptions.h
index 6db6bf9ea8ad..52ac8181da02 100644
--- a/cmds/idmap2/include/idmap2/CommandLineOptions.h
+++ b/cmds/idmap2/include/idmap2/CommandLineOptions.h
@@ -23,6 +23,8 @@
 #include <string>
 #include <vector>
 
+#include "idmap2/Result.h"
+
 namespace android::idmap2 {
 
 /*
@@ -46,7 +48,7 @@ class CommandLineOptions {
                                      std::string* value);
   CommandLineOptions& OptionalOption(const std::string& name, const std::string& description,
                                      std::vector<std::string>* value);
-  bool Parse(const std::vector<std::string>& argv, std::ostream& outError) const;
+  Result<Unit> Parse(const std::vector<std::string>& argv) const;
   void Usage(std::ostream& out) const;
 
  private:
diff --git a/cmds/idmap2/include/idmap2/Idmap.h b/cmds/idmap2/include/idmap2/Idmap.h
index 1666dc8a3bbd..673d18d25907 100644
--- a/cmds/idmap2/include/idmap2/Idmap.h
+++ b/cmds/idmap2/include/idmap2/Idmap.h
@@ -115,7 +115,7 @@ class IdmapHeader {
   // Invariant: anytime the idmap data encoding is changed, the idmap version
   // field *must* be incremented. Because of this, we know that if the idmap
   // header is up-to-date the entire file is up-to-date.
-  bool IsUpToDate(std::ostream& out_error) const;
+  Result<Unit> IsUpToDate() const;
 
   void accept(Visitor* v) const;
 
diff --git a/cmds/idmap2/libidmap2/CommandLineOptions.cpp b/cmds/idmap2/libidmap2/CommandLineOptions.cpp
index a49a607091a4..d5fd2ce38b11 100644
--- a/cmds/idmap2/libidmap2/CommandLineOptions.cpp
+++ b/cmds/idmap2/libidmap2/CommandLineOptions.cpp
@@ -19,12 +19,14 @@
 #include <iostream>
 #include <memory>
 #include <set>
+#include <sstream>
 #include <string>
 #include <vector>
 
 #include "android-base/macros.h"
 
 #include "idmap2/CommandLineOptions.h"
+#include "idmap2/Result.h"
 
 namespace android::idmap2 {
 
@@ -77,7 +79,7 @@ CommandLineOptions& CommandLineOptions::OptionalOption(const std::string& name,
   return *this;
 }
 
-bool CommandLineOptions::Parse(const std::vector<std::string>& argv, std::ostream& outError) const {
+Result<Unit> CommandLineOptions::Parse(const std::vector<std::string>& argv) const {
   const auto pivot = std::partition(options_.begin(), options_.end(), [](const Option& opt) {
     return opt.count != Option::COUNT_OPTIONAL && opt.count != Option::COUNT_OPTIONAL_ONCE_OR_MORE;
   });
@@ -89,8 +91,9 @@ bool CommandLineOptions::Parse(const std::vector<std::string>& argv, std::ostrea
   for (size_t i = 0; i < argv_size; i++) {
     const std::string arg = argv[i];
     if ("--help" == arg || "-h" == arg) {
-      Usage(outError);
-      return false;
+      std::stringstream stream;
+      Usage(stream);
+      return Error("%s", stream.str().c_str());
     }
     bool match = false;
     for (const Option& opt : options_) {
@@ -100,9 +103,9 @@ bool CommandLineOptions::Parse(const std::vector<std::string>& argv, std::ostrea
         if (opt.argument) {
           i++;
           if (i >= argv_size) {
-            outError << "error: " << opt.name << ": missing argument" << std::endl;
-            Usage(outError);
-            return false;
+            std::stringstream stream;
+            Usage(stream);
+            return Error("%s: missing argument\n%s", opt.name.c_str(), stream.str().c_str());
           }
         }
         opt.action(argv[i]);
@@ -111,20 +114,27 @@ bool CommandLineOptions::Parse(const std::vector<std::string>& argv, std::ostrea
       }
     }
     if (!match) {
-      outError << "error: " << arg << ": unknown option" << std::endl;
-      Usage(outError);
-      return false;
+      std::stringstream stream;
+      Usage(stream);
+      return Error("%s: unknown option\n%s", arg.c_str(), stream.str().c_str());
     }
   }
 
   if (!mandatory_opts.empty()) {
+    std::stringstream stream;
+    bool separator = false;
     for (const auto& opt : mandatory_opts) {
-      outError << "error: " << opt << ": missing mandatory option" << std::endl;
+      if (separator) {
+        stream << ", ";
+      }
+      separator = true;
+      stream << opt << ": missing mandatory option";
     }
-    Usage(outError);
-    return false;
+    stream << std::endl;
+    Usage(stream);
+    return Error("%s", stream.str().c_str());
   }
-  return true;
+  return Unit{};
 }
 
 void CommandLineOptions::Usage(std::ostream& out) const {
diff --git a/cmds/idmap2/libidmap2/Idmap.cpp b/cmds/idmap2/libidmap2/Idmap.cpp
index a7d180c90307..2a39c2fd4f59 100644
--- a/cmds/idmap2/libidmap2/Idmap.cpp
+++ b/cmds/idmap2/libidmap2/Idmap.cpp
@@ -142,62 +142,46 @@ std::unique_ptr<const IdmapHeader> IdmapHeader::FromBinaryStream(std::istream& s
   return std::move(idmap_header);
 }
 
-bool IdmapHeader::IsUpToDate(std::ostream& out_error) const {
+Result<Unit> IdmapHeader::IsUpToDate() const {
   if (magic_ != kIdmapMagic) {
-    out_error << base::StringPrintf("error: bad magic: actual 0x%08x, expected 0x%08x", magic_,
-                                    kIdmapMagic)
-              << std::endl;
-    return false;
+    return Error("bad magic: actual 0x%08x, expected 0x%08x", magic_, kIdmapMagic);
   }
 
   if (version_ != kIdmapCurrentVersion) {
-    out_error << base::StringPrintf("error: bad version: actual 0x%08x, expected 0x%08x", version_,
-                                    kIdmapCurrentVersion)
-              << std::endl;
-    return false;
+    return Error("bad version: actual 0x%08x, expected 0x%08x", version_, kIdmapCurrentVersion);
   }
 
   const std::unique_ptr<const ZipFile> target_zip = ZipFile::Open(target_path_);
   if (!target_zip) {
-    out_error << "error: failed to open target " << target_path_ << std::endl;
-    return false;
+    return Error("failed to open target %s", GetTargetPath().to_string().c_str());
   }
 
   Result<uint32_t> target_crc = GetCrc(*target_zip);
   if (!target_crc) {
-    out_error << "error: failed to get target crc" << std::endl;
-    return false;
+    return Error("failed to get target crc");
   }
 
   if (target_crc_ != *target_crc) {
-    out_error << base::StringPrintf(
-                     "error: bad target crc: idmap version 0x%08x, file system version 0x%08x",
-                     target_crc_, *target_crc)
-              << std::endl;
-    return false;
+    return Error("bad target crc: idmap version 0x%08x, file system version 0x%08x", target_crc_,
+                 *target_crc);
   }
 
   const std::unique_ptr<const ZipFile> overlay_zip = ZipFile::Open(overlay_path_);
   if (!overlay_zip) {
-    out_error << "error: failed to open overlay " << overlay_path_ << std::endl;
-    return false;
+    return Error("failed to open overlay %s", GetOverlayPath().to_string().c_str());
   }
 
   Result<uint32_t> overlay_crc = GetCrc(*overlay_zip);
   if (!overlay_crc) {
-    out_error << "error: failed to get overlay crc" << std::endl;
-    return false;
+    return Error("failed to get overlay crc");
   }
 
   if (overlay_crc_ != *overlay_crc) {
-    out_error << base::StringPrintf(
-                     "error: bad overlay crc: idmap version 0x%08x, file system version 0x%08x",
-                     overlay_crc_, *overlay_crc)
-              << std::endl;
-    return false;
+    return Error("bad overlay crc: idmap version 0x%08x, file system version 0x%08x", overlay_crc_,
+                 *overlay_crc);
   }
 
-  return true;
+  return Unit{};
 }
 
 std::unique_ptr<const IdmapData::Header> IdmapData::Header::FromBinaryStream(std::istream& stream) {
diff --git a/cmds/idmap2/tests/CommandLineOptionsTests.cpp b/cmds/idmap2/tests/CommandLineOptionsTests.cpp
index 39f18d3336af..d567af64b16a 100644
--- a/cmds/idmap2/tests/CommandLineOptionsTests.cpp
+++ b/cmds/idmap2/tests/CommandLineOptionsTests.cpp
@@ -46,14 +46,13 @@ TEST(CommandLineOptionsTests, Flag) {
   CommandLineOptions opts =
       CommandLineOptions("test").OptionalFlag("--foo", "", &foo).OptionalFlag("--bar", "", &bar);
 
-  std::ostream fakeStdErr(nullptr);
-  bool success = opts.Parse({"--foo", "--bar"}, fakeStdErr);
+  auto success = opts.Parse({"--foo", "--bar"});
   ASSERT_TRUE(success);
   ASSERT_TRUE(foo);
   ASSERT_TRUE(bar);
 
   foo = bar = false;
-  success = opts.Parse({"--foo"}, fakeStdErr);
+  success = opts.Parse({"--foo"});
   ASSERT_TRUE(success);
   ASSERT_TRUE(foo);
   ASSERT_FALSE(bar);
@@ -65,21 +64,19 @@ TEST(CommandLineOptionsTests, MandatoryOption) {
   CommandLineOptions opts = CommandLineOptions("test")
                                 .MandatoryOption("--foo", "", &foo)
                                 .MandatoryOption("--bar", "", &bar);
-  std::ostream fakeStdErr(nullptr);
-  bool success = opts.Parse({"--foo", "FOO", "--bar", "BAR"}, fakeStdErr);
+  auto success = opts.Parse({"--foo", "FOO", "--bar", "BAR"});
   ASSERT_TRUE(success);
   ASSERT_EQ(foo, "FOO");
   ASSERT_EQ(bar, "BAR");
 
-  success = opts.Parse({"--foo"}, fakeStdErr);
+  success = opts.Parse({"--foo"});
   ASSERT_FALSE(success);
 }
 
 TEST(CommandLineOptionsTests, MandatoryOptionMultipleArgsButExpectedOnce) {
   std::string foo;
   CommandLineOptions opts = CommandLineOptions("test").MandatoryOption("--foo", "", &foo);
-  std::ostream fakeStdErr(nullptr);
-  bool success = opts.Parse({"--foo", "FIRST", "--foo", "SECOND"}, fakeStdErr);
+  auto success = opts.Parse({"--foo", "FIRST", "--foo", "SECOND"});
   ASSERT_TRUE(success);
   ASSERT_EQ(foo, "SECOND");
 }
@@ -87,8 +84,7 @@ TEST(CommandLineOptionsTests, MandatoryOptionMultipleArgsButExpectedOnce) {
 TEST(CommandLineOptionsTests, MandatoryOptionMultipleArgsAndExpectedOnceOrMore) {
   std::vector<std::string> args;
   CommandLineOptions opts = CommandLineOptions("test").MandatoryOption("--foo", "", &args);
-  std::ostream fakeStdErr(nullptr);
-  bool success = opts.Parse({"--foo", "FOO", "--foo", "BAR"}, fakeStdErr);
+  auto success = opts.Parse({"--foo", "FOO", "--foo", "BAR"});
   ASSERT_TRUE(success);
   ASSERT_EQ(args.size(), 2U);
   ASSERT_EQ(args[0], "FOO");
@@ -101,23 +97,22 @@ TEST(CommandLineOptionsTests, OptionalOption) {
   CommandLineOptions opts = CommandLineOptions("test")
                                 .OptionalOption("--foo", "", &foo)
                                 .OptionalOption("--bar", "", &bar);
-  std::ostream fakeStdErr(nullptr);
-  bool success = opts.Parse({"--foo", "FOO", "--bar", "BAR"}, fakeStdErr);
+  auto success = opts.Parse({"--foo", "FOO", "--bar", "BAR"});
   ASSERT_TRUE(success);
   ASSERT_EQ(foo, "FOO");
   ASSERT_EQ(bar, "BAR");
 
-  success = opts.Parse({"--foo", "BAZ"}, fakeStdErr);
+  success = opts.Parse({"--foo", "BAZ"});
   ASSERT_TRUE(success);
   ASSERT_EQ(foo, "BAZ");
 
-  success = opts.Parse({"--foo"}, fakeStdErr);
+  success = opts.Parse({"--foo"});
   ASSERT_FALSE(success);
 
-  success = opts.Parse({"--foo", "--bar", "BAR"}, fakeStdErr);
+  success = opts.Parse({"--foo", "--bar", "BAR"});
   ASSERT_FALSE(success);
 
-  success = opts.Parse({"--foo", "FOO", "--bar"}, fakeStdErr);
+  success = opts.Parse({"--foo", "FOO", "--bar"});
   ASSERT_FALSE(success);
 }
 
@@ -127,8 +122,7 @@ TEST(CommandLineOptionsTests, OptionalOptionList) {
   CommandLineOptions opts = CommandLineOptions("test")
                                 .OptionalOption("--foo", "", &foo)
                                 .OptionalOption("--bar", "", &bar);
-  std::ostream fakeStdErr(nullptr);
-  bool success = opts.Parse({"--foo", "FOO", "--bar", "BAR"}, fakeStdErr);
+  auto success = opts.Parse({"--foo", "FOO", "--bar", "BAR"});
   ASSERT_TRUE(success);
   ASSERT_EQ(foo.size(), 1U);
   ASSERT_EQ(foo[0], "FOO");
@@ -137,7 +131,7 @@ TEST(CommandLineOptionsTests, OptionalOptionList) {
 
   foo.clear();
   bar.clear();
-  success = opts.Parse({"--foo", "BAZ"}, fakeStdErr);
+  success = opts.Parse({"--foo", "BAZ"});
   ASSERT_TRUE(success);
   ASSERT_EQ(foo.size(), 1U);
   ASSERT_EQ(foo[0], "BAZ");
@@ -145,8 +139,7 @@ TEST(CommandLineOptionsTests, OptionalOptionList) {
 
   foo.clear();
   bar.clear();
-  success =
-      opts.Parse({"--foo", "BAZ", "--foo", "BIZ", "--bar", "FIZ", "--bar", "FUZZ"}, fakeStdErr);
+  success = opts.Parse({"--foo", "BAZ", "--foo", "BIZ", "--bar", "FIZ", "--bar", "FUZZ"});
   ASSERT_TRUE(success);
   ASSERT_EQ(foo.size(), 2U);
   ASSERT_EQ(foo[0], "BAZ");
@@ -157,17 +150,17 @@ TEST(CommandLineOptionsTests, OptionalOptionList) {
 
   foo.clear();
   bar.clear();
-  success = opts.Parse({"--foo"}, fakeStdErr);
+  success = opts.Parse({"--foo"});
   ASSERT_FALSE(success);
 
   foo.clear();
   bar.clear();
-  success = opts.Parse({"--foo", "--bar", "BAR"}, fakeStdErr);
+  success = opts.Parse({"--foo", "--bar", "BAR"});
   ASSERT_FALSE(success);
 
   foo.clear();
   bar.clear();
-  success = opts.Parse({"--foo", "FOO", "--bar"}, fakeStdErr);
+  success = opts.Parse({"--foo", "FOO", "--bar"});
   ASSERT_FALSE(success);
 }
 
@@ -179,14 +172,13 @@ TEST(CommandLineOptionsTests, CornerCases) {
                                 .MandatoryOption("--foo", "", &foo)
                                 .OptionalFlag("--baz", "", &baz)
                                 .OptionalOption("--bar", "", &bar);
-  std::ostream fakeStdErr(nullptr);
-  bool success = opts.Parse({"--unexpected"}, fakeStdErr);
+  auto success = opts.Parse({"--unexpected"});
   ASSERT_FALSE(success);
 
-  success = opts.Parse({"--bar", "BAR"}, fakeStdErr);
+  success = opts.Parse({"--bar", "BAR"});
   ASSERT_FALSE(success);
 
-  success = opts.Parse({"--baz", "--foo", "FOO"}, fakeStdErr);
+  success = opts.Parse({"--baz", "--foo", "FOO"});
   ASSERT_TRUE(success);
   ASSERT_TRUE(baz);
   ASSERT_EQ(foo, "FOO");
diff --git a/cmds/idmap2/tests/IdmapTests.cpp b/cmds/idmap2/tests/IdmapTests.cpp
index 8d65428f134e..c20ae7b798a3 100644
--- a/cmds/idmap2/tests/IdmapTests.cpp
+++ b/cmds/idmap2/tests/IdmapTests.cpp
@@ -501,7 +501,7 @@ TEST(IdmapTests, IdmapHeaderIsUpToDate) {
 
   std::unique_ptr<const IdmapHeader> header = IdmapHeader::FromBinaryStream(stream);
   ASSERT_THAT(header, NotNull());
-  ASSERT_TRUE(header->IsUpToDate(error)) << error.str();
+  ASSERT_TRUE(header->IsUpToDate());
 
   // magic: bytes (0x0, 0x03)
   std::string bad_magic_string(stream.str());
@@ -514,7 +514,7 @@ TEST(IdmapTests, IdmapHeaderIsUpToDate) {
       IdmapHeader::FromBinaryStream(bad_magic_stream);
   ASSERT_THAT(bad_magic_header, NotNull());
   ASSERT_NE(header->GetMagic(), bad_magic_header->GetMagic());
-  ASSERT_FALSE(bad_magic_header->IsUpToDate(error));
+  ASSERT_FALSE(bad_magic_header->IsUpToDate());
 
   // version: bytes (0x4, 0x07)
   std::string bad_version_string(stream.str());
@@ -527,7 +527,7 @@ TEST(IdmapTests, IdmapHeaderIsUpToDate) {
       IdmapHeader::FromBinaryStream(bad_version_stream);
   ASSERT_THAT(bad_version_header, NotNull());
   ASSERT_NE(header->GetVersion(), bad_version_header->GetVersion());
-  ASSERT_FALSE(bad_version_header->IsUpToDate(error));
+  ASSERT_FALSE(bad_version_header->IsUpToDate());
 
   // target crc: bytes (0x8, 0xb)
   std::string bad_target_crc_string(stream.str());
@@ -540,7 +540,7 @@ TEST(IdmapTests, IdmapHeaderIsUpToDate) {
       IdmapHeader::FromBinaryStream(bad_target_crc_stream);
   ASSERT_THAT(bad_target_crc_header, NotNull());
   ASSERT_NE(header->GetTargetCrc(), bad_target_crc_header->GetTargetCrc());
-  ASSERT_FALSE(bad_target_crc_header->IsUpToDate(error));
+  ASSERT_FALSE(bad_target_crc_header->IsUpToDate());
 
   // overlay crc: bytes (0xc, 0xf)
   std::string bad_overlay_crc_string(stream.str());
@@ -553,7 +553,7 @@ TEST(IdmapTests, IdmapHeaderIsUpToDate) {
       IdmapHeader::FromBinaryStream(bad_overlay_crc_stream);
   ASSERT_THAT(bad_overlay_crc_header, NotNull());
   ASSERT_NE(header->GetOverlayCrc(), bad_overlay_crc_header->GetOverlayCrc());
-  ASSERT_FALSE(bad_overlay_crc_header->IsUpToDate(error));
+  ASSERT_FALSE(bad_overlay_crc_header->IsUpToDate());
 
   // target path: bytes (0x10, 0x10f)
   std::string bad_target_path_string(stream.str());
@@ -563,7 +563,7 @@ TEST(IdmapTests, IdmapHeaderIsUpToDate) {
       IdmapHeader::FromBinaryStream(bad_target_path_stream);
   ASSERT_THAT(bad_target_path_header, NotNull());
   ASSERT_NE(header->GetTargetPath(), bad_target_path_header->GetTargetPath());
-  ASSERT_FALSE(bad_target_path_header->IsUpToDate(error));
+  ASSERT_FALSE(bad_target_path_header->IsUpToDate());
 
   // overlay path: bytes (0x110, 0x20f)
   std::string bad_overlay_path_string(stream.str());
@@ -573,7 +573,7 @@ TEST(IdmapTests, IdmapHeaderIsUpToDate) {
       IdmapHeader::FromBinaryStream(bad_overlay_path_stream);
   ASSERT_THAT(bad_overlay_path_header, NotNull());
   ASSERT_NE(header->GetOverlayPath(), bad_overlay_path_header->GetOverlayPath());
-  ASSERT_FALSE(bad_overlay_path_header->IsUpToDate(error));
+  ASSERT_FALSE(bad_overlay_path_header->IsUpToDate());
 }
 
 class TestVisitor : public Visitor {