[vcpkg] Enable reentrant vcpkg calls (#13751)

* [vcpkg] Enable recursive vcpkg calls

Via envvars VCPKG_COMMAND and VCPKG_X_RECURSIVE_DATA. Child processes can call vcpkg via "$VCPKG_COMMAND <args>" in limited internal circumstances.

* [vcpkg] Address CR comments

* [vcpkg] Do not move through Optional<&> into Optional<T>

Co-authored-by: Robert Schumacher <roschuma@microsoft.com>

3 files changed, 23 insertions, 0 deletions

diff --git a/toolsrc/include/vcpkg/base/optional.h b/toolsrc/include/vcpkg/base/optional.h
index 54e370dff..caacf815e 100644
--- a/toolsrc/include/vcpkg/base/optional.h
+++ b/toolsrc/include/vcpkg/base/optional.h
@@ -29,6 +29,24 @@ namespace vcpkg
             constexpr OptionalStorage() noexcept : m_is_present(false), m_inactive() { }
             constexpr OptionalStorage(const T& t) : m_is_present(true), m_t(t) { }
             constexpr OptionalStorage(T&& t) : m_is_present(true), m_t(std::move(t)) { }
+            template<class U, class = std::enable_if_t<!std::is_reference<U>::value>>
+            constexpr explicit OptionalStorage(Optional<U>&& t) : m_is_present(false), m_inactive()
+            {
+                if (auto p = t.get())
+                {
+                    m_is_present = true;
+                    new (&m_t) T(std::move(*p));
+                }
+            }
+            template<class U>
+            constexpr explicit OptionalStorage(const Optional<U>& t) : m_is_present(false), m_inactive()
+            {
+                if (auto p = t.get())
+                {
+                    m_is_present = true;
+                    new (&m_t) T(*p);
+                }
+            }
 
             ~OptionalStorage() noexcept
             {
diff --git a/toolsrc/include/vcpkg/base/system.h b/toolsrc/include/vcpkg/base/system.h
index 2340728fd..8e45f6538 100644
--- a/toolsrc/include/vcpkg/base/system.h
+++ b/toolsrc/include/vcpkg/base/system.h
@@ -8,6 +8,7 @@
 namespace vcpkg::System
 {
     Optional<std::string> get_environment_variable(ZStringView varname) noexcept;
+    void set_environment_variable(ZStringView varname, Optional<ZStringView> value) noexcept;
 
     const ExpectedS<fs::path>& get_home_dir() noexcept;
 
diff --git a/toolsrc/include/vcpkg/vcpkgcmdarguments.h b/toolsrc/include/vcpkg/vcpkgcmdarguments.h
index 3901000e3..eaf5cc3eb 100644
--- a/toolsrc/include/vcpkg/vcpkgcmdarguments.h
+++ b/toolsrc/include/vcpkg/vcpkgcmdarguments.h
@@ -174,10 +174,13 @@ namespace vcpkg
         constexpr static StringLiteral REGISTRIES_FEATURE = "registries";
         Optional<bool> registries_feature = nullopt;
 
+        constexpr static StringLiteral RECURSIVE_DATA_ENV = "VCPKG_X_RECURSIVE_DATA";
+
         bool binary_caching_enabled() const { return binary_caching.value_or(true); }
         bool compiler_tracking_enabled() const { return compiler_tracking.value_or(true); }
         bool registries_enabled() const { return registries_feature.value_or(false); }
         bool output_json() const { return json.value_or(false); }
+        bool is_recursive_invocation() const { return m_is_recursive_invocation; }
 
         std::string command;
         std::vector<std::string> command_arguments;
@@ -192,6 +195,7 @@ namespace vcpkg
         void track_feature_flag_metrics() const;
 
     private:
+        bool m_is_recursive_invocation = false;
         std::unordered_set<std::string> command_switches;
         std::unordered_map<std::string, std::vector<std::string>> command_options;
     };