multiprocess: Add bitcoin wrapper executable

Intended to make bitcoin command line features more discoverable and allow
installing new multiprocess binaries in libexec/ instead of bin/ so they don't
cause confusion.

Idea and implementation of this were discussed in
bitcoin#30983

It's possible to debug this and see what programs it is trying to call by running:

strace -e trace=execve -s 10000 build/src/bitcoin ...

src/CMakeLists.txt

@@ -297,6 +297,9 @@ target_link_libraries(bitcoin_node
     $<TARGET_NAME_IF_EXISTS:USDT::headers>
 )
 
+# Bitcoin wrapper executable that can call other executables.
+add_executable(bitcoin bitcoin.cpp)
+target_link_libraries(bitcoin bitcoin_util)
 
 # Bitcoin Core bitcoind.
 if(BUILD_DAEMON)

src/bitcoin.cpp

@@ -0,0 +1,171 @@
+// Copyright (c) 2024 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <config/bitcoin-config.h> // IWYU pragma: keep
+
+#include <clientversion.h>
+#include <util/fs.h>
+#include <util/strencodings.h>
+
+#include <iostream>
+#include <optional>
+#include <string>
+#include <tinyformat.h>
+#include <unistd.h>
+#include <vector>
+
+extern const std::function<std::string(const char*)> G_TRANSLATION_FUN = nullptr;
+
+const char* HELP_USAGE = R"(Usage: %1$s [OPTIONS] COMMAND...
+
+Commands (run help command for more information):
+  {gui,daemon,rpc,wallet,test,help}
+
+Options:
+  -m, --multiprocess     Run multiprocess binaries bitcoin-node, bitcoin-gui.\n"
+  -M, --monolothic       Run monolithic binaries bitcoind, bitcoin-qt. (Default behavior)\n"
+  -v, --version          Show version information
+  -h, --help             Show this help message
+)";
+
+const char* HELP_COMMANDS = R"(Command overview:
+
+%1$s gui [ARGS]     Start GUI, equivalent to running 'bitcoin-qt [ARGS]' or 'bitcoin-gui [ARGS]'.
+%1$s daemon [ARGS]  Start daemon, equivalent to running 'bitcoind [ARGS]' or 'bitcoin-node [ARGS]'.
+%1$s rpc [ARGS]     Call RPC method, equivalent to running 'bitcoin-cli -named [ARGS]'.
+%1$s wallet [ARGS]  Call wallet command, equivalent to running 'bitcoin-wallet [ARGS]'.
+%1$s tx [ARGS]      Manipulate hex-encoded transactions, equivalent to running 'bitcoin-tx [ARGS]'.
+%1$s test [ARGS]    Run unit tests, equivalent to running 'test_bitcoin [ARGS]'.
+%1$s help           Show this help message.
+)";
+
+struct CommandLine {
+    bool use_multiprocess{false};
+    bool show_version{false};
+    bool show_help{false};
+    std::string_view command;
+    std::vector<std::string_view> args;
+};
+
+CommandLine ParseCommandLine(int argc, char* argv[]);
+void ExecCommand(const std::vector<std::string>& args, std::string_view argv0);
+
+int main(int argc, char* argv[])
+{
+    try {
+        CommandLine cmd{ParseCommandLine(argc, argv)};
+        if (cmd.show_version) {
+            std::cout << FormatParagraph(LicenseInfo());
+        }
+        if (cmd.show_help || cmd.command.empty()) {
+            tfm::format(std::cout, HELP_USAGE, argv[0]);
+        }
+
+        std::vector<std::string> args;
+        if (cmd.command == "gui") {
+            args.push_back(cmd.use_multiprocess ? "qt/bitcoin-gui" : "qt/bitcoin-qt");
+        } else if (cmd.command == "daemon") {
+            args.push_back(cmd.use_multiprocess ? "bitcoin-node" : "bitcoind");
+        } else if (cmd.command == "rpc") {
+            args.push_back("bitcoin-cli");
+            args.push_back("-named");
+        } else if (cmd.command == "wallet") {
+            args.push_back("bitcoin-wallet");
+        } else if (cmd.command == "tx") {
+            args.push_back("bitcoin-tx");
+        } else if (cmd.command == "test") {
+            args.push_back("test/test_bitcoin");
+        } else if (cmd.command == "help") {
+            tfm::format(std::cout, HELP_COMMANDS, argv[0]);
+        } else if (!cmd.command.empty()){
+            throw std::runtime_error(strprintf("Unrecognized command: '%s'", cmd.command));
+        }
+        if (!args.empty()) {
+            args.insert(args.end(), cmd.args.begin(), cmd.args.end());
+            ExecCommand(args, argv[0]);
+        }
+    } catch (const std::exception& e) {
+        tfm::format(std::cerr, "Error: %s\nTry '%s --help' for more information.", e.what(), argv[0]);
+        return EXIT_FAILURE;
+    }
+    return EXIT_SUCCESS;
+}
+
+CommandLine ParseCommandLine(int argc, char* argv[])
+{
+    CommandLine cmd;
+    for (int i = 1; i < argc; ++i) {
+        std::string_view arg = argv[i];
+        if (!cmd.command.empty()) {
+            cmd.args.push_back(arg);
+        } else if (arg == "-m" || arg == "--multiprocess") {
+            cmd.use_multiprocess = true;
+        } else if (arg == "-M" || arg == "--monolithic") {
+            cmd.use_multiprocess = false;
+        } else if (arg == "-v" || arg == "--version") {
+            cmd.show_version = true;
+        } else if (arg == "-h" || arg == "--help") {
+            cmd.show_help = true;
+        } else if (arg.starts_with("-")) {
+            throw std::runtime_error(strprintf("Unknown option: %s", arg));
+        } else if (!arg.empty()) {
+            cmd.command = arg;
+        }
+    }
+    return cmd;
+}
+
+void ExecCommand(const std::vector<std::string>& args, std::string_view argv0)
+{
+    // Construct argument string for execvp
+    std::vector<const char*> cstr_args{};
+    for (const auto& arg : args) {
+        cstr_args.push_back(arg.c_str());
+    }
+    cstr_args.push_back(nullptr);
+
+    // Try to call execvp with given exe path.
+    auto try_exec = [&](fs::path exe_path, bool allow_notfound = true) {
+        std::string exe_path_str{exe_path};
+        cstr_args[0] = exe_path_str.c_str();
+        if (execvp(cstr_args[0], (char*const*)cstr_args.data()) == -1) {
+            if (allow_notfound && errno == ENOENT) return false;
+            throw std::system_error(errno, std::system_category(), strprintf("execvp failed to execute '%s'", cstr_args[0]));
+        }
+        return true; // Will not actually be reached if execvp succeeds
+    };
+
+    // Try to figure out where current executable is located. This is a
+    // simplified search that won't work perfectly on every platform and doesn't
+    // need to, as it is only trying to prioritize locally built or installed
+    // executables over system executables. We may want to add options to
+    // override this behavior in the future, though.
+    fs::path exe_dir(argv0);
+    std::error_code ec;
+    if (argv0.find('/') == std::string_view::npos) {
+        if (const char* path_env = std::getenv("PATH")) {
+            size_t start{0}, end{0};
+            for (std::string_view paths{path_env}; end != std::string_view::npos; start = end + 1) {
+                end = paths.find(':', start);
+                fs::path candidate = fs::path(paths.substr(start, end - start)) / exe_dir;
+                if (std::filesystem::exists(candidate, ec) && fs::is_regular_file(candidate, ec)) {
+                    exe_dir = candidate;
+                    break;
+                }
+            }
+        }
+    }
+    exe_dir = fs::weakly_canonical(exe_dir, ec).parent_path();
+
+    // If exe is in a CMake build tree, first look for target executable
+    // relative to it.
+    (exe_dir.filename() == "src" && try_exec(exe_dir / fs::PathFromString(args[0]))) ||
+    // Otherwise if exe is installed in a bin/ directory, first look for target
+    // executable in libexec/
+    (exe_dir.filename() == "bin" && try_exec(fs::path{exe_dir.parent_path()} / "libexec" / fs::PathFromString(args[0]).filename())) ||
+    // Otherwise look for target executable next to current exe
+    try_exec(exe_dir / fs::PathFromString(args[0]).filename()) ||
+    // Otherwise just look on the system path.
+    try_exec(fs::PathFromString(args[0]).filename());
+};