Implement static allocator for initialization

The previous interposition logic based on plain usage of `dlsym` analogously to various online examples led to a deadlock during _neovim_ startup. This deadlock was caused by _neovim_'s custom memory allocation library _jemalloc_ because it calls `mmap` during its initialization phase. The problem with calling `mmap` during initialization is that this already leads to executing `libChangeLog`'s `mmap` version whoes static `actual_mmap` function pointer is not initialized at this point in time. This is detected and leads to a call to `dlsym` to remedy this situation. Sadly `dlsym` in turn requires memory allocation using `calloc` which leads us back to initializing _jemalloc_ and as such to a deadlock.

I first saw this as a bug in _jemalloc_ which seemed to be confirmed by a short search in my search engine of choice. This prompted me to create an appropriate [bug report](https://github.com/jemalloc/jemalloc/issues/329) which was dismissed as a problem in the way `mmap` was interposed and not as a bug in the library. Thus it seems to be accepted practice that it is not the responsibility of a custom memory allocator to cater to the initialization needs of other libraries relying on function interposition. This is of course a valid position as the whole issue is a kind of _chicken and egg_ problem where both sides can be argued.

To cut to the chase I was left with the only option of working around this deadlock by adapting `libChangeLog` to call `dlsym` without relying on the wrapped application's memory allocator of choice. The most straight forward way to do this is to provide another custom memory allocator alongside the _payload_ function interpositions of `mmap` and friends.

`init/alloc.cc` implements such a selectively transparent memory allocator that offers a small static buffer for usage in the context of executing `dlsym`.The choice between forwarding memory allocation requests to the wrapped application's allocator and using the static buffer is governed by `init::dlsymContext`. This tiny helper class maintains an `dlsym_level` counter by posing as a scope guard.

The end result of this extension to `libChangeLog` is that it now also works with applications using _jemalloc_ such as _neovim_ and should overall be much more robust during its initialization phase.

4 files changed, 122 insertions, 5 deletions

diff --git a/src/actual.h b/src/actual.h
index cb08d42..a860590 100644
--- a/src/actual.h
+++ b/src/actual.h
@@ -1,10 +1,10 @@
+#ifndef CHANGE_SRC_ACTUAL_FUNCTION_H_
+#define CHANGE_SRC_ACTUAL_FUNCTION_H_
+
 #ifndef _GNU_SOURCE
 #define _GNU_SOURCE
 #endif
 
-#ifndef CHANGE_SRC_ACTUAL_FUNCTION_H_
-#define CHANGE_SRC_ACTUAL_FUNCTION_H_
-
 #include <dlfcn.h>
 #include <sys/mman.h>
 #include <sys/uio.h>
@@ -12,6 +12,8 @@
 #include <memory>
 #include <cstring>
 
+#include "init/alloc.h"
+
 namespace actual {
 
 template <class Result, typename... Arguments>
@@ -19,6 +21,8 @@ using ptr = Result(*)(Arguments...);
 
 template <typename FunctionPtr>
 FunctionPtr get_ptr(const std::string& symbol_name) {
+	init::dlsymContext guard;
+
 	const void* symbol_address{ dlsym(RTLD_NEXT, symbol_name.c_str()) };
 
 	FunctionPtr actual_function{};
diff --git a/src/change_log.cc b/src/change_log.cc
index e0dc179..5f3b4f8 100644
--- a/src/change_log.cc
+++ b/src/change_log.cc
@@ -1,5 +1,6 @@
 #include "actual.h"
 
+#include "init/alloc.h"
 #include "utility/io.h"
 #include "utility/logger.h"
 #include "tracking/path_matcher.h"
@@ -14,8 +15,8 @@ static std::unique_ptr<utility::Logger>              logger;
 static std::unique_ptr<tracking::PathMatcher>        matcher;
 static std::unique_ptr<tracking::ChangeTracker>      tracker;
 
-void init() __attribute__ ((constructor));
-void init() {
+void initialize() __attribute__ ((constructor));
+void initialize() {
 	if ( getenv("CHANGE_LOG_TARGET") != NULL ) {
 		fd_guard = std::make_unique<utility::FileDescriptorGuard>(
 			getenv("CHANGE_LOG_TARGET")
@@ -81,6 +82,46 @@ inline void track_remove(const std::string& path) {
 	}
 }
 
+void free(void* ptr) {
+	static actual::ptr<void, void*> actual_free{};
+
+	if ( !actual_free ) {
+		actual_free = actual::get_ptr<decltype(actual_free)>("free");
+	}
+
+	if ( !init::from_static_buffer(ptr) ) {
+		actual_free(ptr);
+	}
+}
+
+void* malloc(size_t size) {
+	static actual::ptr<void*, size_t> actual_malloc{};
+
+	if ( init::dlsymContext::is_active() ) {
+		return init::static_malloc(size);
+	} else {
+		if ( !actual_malloc ) {
+			actual_malloc = actual::get_ptr<decltype(actual_malloc)>("malloc");
+		}
+
+		return actual_malloc(size);
+	}
+}
+
+void* calloc(size_t block, size_t size) {
+	static actual::ptr<void*, size_t, size_t> actual_calloc{};
+
+	if ( init::dlsymContext::is_active() ) {
+		return init::static_calloc(block, size);
+	} else {
+		if ( !actual_calloc ) {
+			actual_calloc = actual::get_ptr<decltype(actual_calloc)>("calloc");
+		}
+
+		return actual_calloc(block, size);
+	}
+}
+
 ssize_t write(int fd, const void* buffer, size_t count) {
 	static actual::ptr<ssize_t, int, const void*, size_t> actual_write{};
 
diff --git a/src/init/alloc.cc b/src/init/alloc.cc
new file mode 100644
index 0000000..d18917b
--- /dev/null
+++ b/src/init/alloc.cc
@@ -0,0 +1,50 @@
+#include "alloc.h"
+
+#include <cstdio>
+#include <cstdint>
+
+namespace init {
+
+std::uint8_t dlsym_level   = 0;
+size_t       buffer_offset = 0;
+char         buffer[4096];
+
+void* static_malloc(size_t size) {
+	if ( buffer_offset + size >= sizeof(buffer) ) {
+		std::fprintf(stderr, "static allocator out of memory: %lu\n", buffer_offset);
+		exit(1);
+	}
+
+	buffer_offset += size;
+
+	return buffer + buffer_offset;
+}
+
+void* static_calloc(size_t block, size_t size) {
+	void* const mem = static_malloc(block * size);
+
+	for ( size_t i = 0; i < (block * size); ++i ) {
+		*(static_cast<char* const>(mem) + i) = '\0';
+	}
+
+	return mem;
+}
+
+bool from_static_buffer(void* ptr) {
+	return static_cast<char*>(ptr) >= buffer
+	    && static_cast<char*>(ptr) <= buffer + sizeof(buffer);
+}
+
+dlsymContext::dlsymContext() {
+	++dlsym_level;
+}
+
+dlsymContext::~dlsymContext() {
+	--dlsym_level;
+}
+
+bool dlsymContext::is_active() {
+	return dlsym_level > 0;
+}
+
+}
diff --git a/src/init/alloc.h b/src/init/alloc.h
new file mode 100644
index 0000000..cd1228d
--- /dev/null
+++ b/src/init/alloc.h
@@ -0,0 +1,22 @@
+#ifndef CHANGE_SRC_INIT_ALLOC_H_
+#define CHANGE_SRC_INIT_ALLOC_H_
+
+#include <cstdlib>
+
+namespace init {
+
+void* static_malloc(size_t size);
+void* static_calloc(size_t block, size_t size);
+
+bool from_static_buffer(void* ptr);
+
+struct dlsymContext {
+	dlsymContext();
+	~dlsymContext();
+
+	static bool is_active();
+};
+
+}
+
+#endif  // CHANGE_SRC_INIT_ALLOC_H_