refactor: extract MakeArgv helper

The next commit will use it to create an alternative to ExecProcess that's safe to call post-fork.

Added a note that ExecProcess is not safe to call post-fork. Since it's only used by mpgen at build time, this is fine, and lets of print useful errors.

Author: Sjors

include/mp/util.h

@@ -228,6 +228,8 @@ using FdToArgsFn = std::function<std::vector<std::string>(int fd)>;
 int SpawnProcess(int& pid, FdToArgsFn&& fd_to_args);
 
 //! Call execvp with vector args.
+//! Not safe to call in a post-fork child of a multi-threaded process.
+//! Currently only used by mpgen at build time.
 void ExecProcess(const std::vector<std::string>& args);
 
 //! Wait for a process to exit and return its exit code.

src/mp/util.cpp

@@ -36,6 +36,17 @@ namespace fs = std::filesystem;
 namespace mp {
 namespace {
 
+std::vector<char*> MakeArgv(const std::vector<std::string>& args)
+{
+    std::vector<char*> argv;
+    argv.reserve(args.size() + 1);
+    for (const auto& arg : args) {
+        argv.push_back(const_cast<char*>(arg.c_str()));
+    }
+    argv.push_back(nullptr);
+    return argv;
+}
+
 //! Return highest possible file descriptor.
 size_t MaxFd()
 {
@@ -134,12 +145,7 @@ int SpawnProcess(int& pid, FdToArgsFn&& fd_to_args)
 
 void ExecProcess(const std::vector<std::string>& args)
 {
-    std::vector<char*> argv;
-    argv.reserve(args.size());
-    for (const auto& arg : args) {
-        argv.push_back(const_cast<char*>(arg.c_str()));
-    }
-    argv.push_back(nullptr);
+    const std::vector<char*> argv{MakeArgv(args)};
     if (execvp(argv[0], argv.data()) != 0) {
         perror("execvp failed");
         if (errno == ENOENT && !args.empty()) {