Add a Boost.Process-like environment builder

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

src/common/process.h

@@ -0,0 +1,77 @@
+// 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/>.
+
+#pragma once
+
+#include <optional>
+#include <string>
+#include <vector>
+
+// A minimal API akin to Boost.Process for launching and managing processes
+// using plain Linux APIs. Needed so we can implement our chainloader without
+// pulling in Boost.Process' Boost.Filesystem dependency (which would defeat the
+// entire purpose).
+
+/**
+ * Helper to create an `environ`-like environment object for passing to the
+ * `exec*e()` family of functions.
+ */
+class ProcessEnvironment {
+   public:
+    /**
+     * Create a new environment object based on an existing environment
+     * described by an array of null-terminated strings, terminated by a null
+     * pointer. You'll want to pass `environ` here.
+     */
+    ProcessEnvironment(char** initial_env);
+
+    /**
+     * Check if an environment variable exists within this environment. Mostly
+     * useful for debugging.
+     */
+    bool contains(const std::string_view& key) const;
+
+    /**
+     * Get the value for an environment variable, if it exists in this
+     * environment. Mostly useful for debugging.
+     */
+    std::optional<std::string_view> get(const std::string_view& key) const;
+
+    /**
+     * Add an environment variable to the environment or overwrite an
+     * existing one.
+     */
+    void insert(const std::string& key, const std::string& value);
+
+    /**
+     * Create an environ-like object from the updated environment that can be
+     * passed to the `exec*e()` functions. These pointers will be invalidated
+     * when this object changes or when gets dropped.
+     */
+    char* const* make_environ() const;
+
+   private:
+    /**
+     * All environment variables read from the constructor argument and those
+     * inserted through `insert()`. These should be in `key=value` format.
+     */
+    std::vector<std::string> variables_;
+    /**
+     * Contains pointers to the strings in `variables`, so we can return a
+     * `char**` in `make_environ()`.
+     */
+    mutable std::vector<char const*> recreated_environ_;
+};