hook/com-proxy.c: Add COM proxying mechanism

hook/com-proxy.h

@@ -0,0 +1,55 @@
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+#define com_proxy_downcast(self) ((struct com_proxy *) self)
+
+struct com_proxy {
+    /* Pointer to vtable filled with trampolines. Edit these as you please.
+       Each com_proxy has its own independent vtable. */
+    void *vptr;
+
+    /* Interface pointer wrapped by this proxy. */
+    void *real;
+
+    /* Context pointer for use by hook code. */
+    void *ctx;
+
+    /* Optional cleanup callback, will be called during com_proxy deallocation
+       to clean up *ctx. */
+    void (*cleanup_ctx)(void *ctx);
+
+    /* Dynamically generated x86 trampoline code. The initial vtable entries
+       all point into code located here. */
+    uint8_t *tramps;
+};
+
+/* Wrap a COM interface pointer in a proxy. This is an object that acts just
+   like the object that it wraps but has a freely editable vtable, which you
+   can modify in order to intercept a subset of the interface's method calls.
+
+   By default, all the vtable slots contain dynamically generated trampolines
+   which pass method calls onwards to the corresponding methods in the
+   underlying object's vtable.
+
+   NOTE! This does not AddRef the underlying interface.
+
+   NOTE! This function wraps COM POINTERS, not COM OBJECTS (since the latter
+   is, in general, impossible). Consequences of this distinction include the
+   following:
+
+   1. Do not wrap IUnknown pointers with this function. This will break the
+      IUnknown::QueryInterface contract. This refers to _the_ unique
+      IUnknown* for the object, not for any other interface (which necessarily
+      extends IUnknown). Wrapping the unique IUnknown* for an object will
+      cause it to no longer be unique.
+
+   2. Callers can "jailbreak" your wrapper using IUnknown::QueryInterface
+      unless you provide a custom QueryInterface implementation to prevent them
+      from doing so. */
+
+HRESULT com_proxy_wrap(
+        struct com_proxy **out,
+        void *real,
+        size_t vtbl_size);