Add a Boost.Process-like environment builder

We can use this with the `exec*e()` family of functions.

src/common/process.cpp

@@ -0,0 +1,70 @@
+// yabridge: a Wine VST bridge
+// Copyright (C) 2020-2022 Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+#include "process.h"
+
+#include <cassert>
+
+ProcessEnvironment::ProcessEnvironment(char** initial_env) {
+    // We'll need to read all strings from `initial_env`. They _should_ all be
+    // zero-terminated strings, with a null pointer to indicate the end of the
+    // array.
+    assert(initial_env);
+    while (*initial_env) {
+        variables_.push_back(*initial_env);
+        initial_env++;
+    }
+}
+
+bool ProcessEnvironment::contains(const std::string_view& key) const {
+    for (const auto& variable : variables_) {
+        if (variable.starts_with(key) && variable.size() > key.size() &&
+            variable[key.size()] == '=') {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+std::optional<std::string_view> ProcessEnvironment::get(
+    const std::string_view& key) const {
+    for (const auto& variable : variables_) {
+        if (variable.starts_with(key) && variable.size() > key.size() &&
+            variable[key.size()] == '=') {
+            return std::string_view(variable).substr(key.size() + 1);
+        }
+    }
+
+    return std::nullopt;
+}
+
+// NOLINTNEXTLINE(bugprone-easily-swappable-parameters)
+void ProcessEnvironment::insert(const std::string& key,
+                                const std::string& value) {
+    variables_.push_back(key + "=" + value);
+}
+
+char* const* ProcessEnvironment::make_environ() const {
+    recreated_environ_.clear();
+
+    for (const auto& variable : variables_) {
+        recreated_environ_.push_back(variable.c_str());
+    }
+    recreated_environ_.push_back(nullptr);
+
+    return const_cast<char* const*>(recreated_environ_.data());
+}