exewrapper: find the proper python3X.dll in the registry

Previously, we relied on the default library lookup[1], which for us is
essentially to look on `PATH`.  That has issues- the Python installations are
not necessarily on `PATH`, so I started copying the DLLs locally in 2960b7fac966
and ed286d150aa8 during the build to work around that.  However, it's been
discovered that causes `python3.dll` and `python3X.dll` to get slipped into the
wheel that gets distributed on PyPI.  Additionally, Mercurial would fail to run
in a venv if the Python environment that created it isn't on `PATH`, because
venv creation doesn't copy the DLLs locally.

The logic here is inspired by the `py.exe` launcher[2], though this is simpler
because we don't care about the architecture- if this is a 32 bit process
running on Win64, the registry reflection will redirect to where the 32 bit
Python process wrote its keys.  A nice unintended side effect is to also make
venvs that don't have their root Python on `PATH` work without all of the code
required to read `pyvenv.cfg`[3].  I don't see any reasonable way to create a
venv without Python being installed (other than maybe building Python from
source?), so punt on trying to read that file for now and save a bunch of string
manipulation code.

I somehow managed to corrupt my Windows user profile, and that makes the
Microsoft Store python not run (even loading the DLL gives an access error), so
I'm giving priority to both global and user specific python.org installations.

Loading python3.dll is new, but when I went down the rabbit hole of implementing
`pyvenv.cfg` support, I saw a comment[4] that led me to think we could have
trouble if we don't.  The comment in ed286d150aa8 confirms this, so we should
probably bail out completely if Python3 can't be loaded from the registry,
rather than getting something random on `PATH`.  But I'll leave that for the
default branch.

[1] https://docs.microsoft.com/en-us/windows/win32/Dlls/dynamic-link-library-search-order#standard-search-order-for-desktop-applications
[2] https://github.com/python/cpython/blob/adcd2205565f91c6719f4141ab4e1da6d7086126/PC/launcher.c#L249
[3] https://github.com/python/cpython/blob/bb3e0c240bc60fe08d332ff5955d54197f79751c/PC/getpathp.c#L707
[4] https://github.com/python/cpython/blob/bb3e0c240bc60fe08d332ff5955d54197f79751c/PC/getpathp.c#L1098

Differential Revision: https://phab.mercurial-scm.org/D11454

mercurial/exewrapper.c

@@ -29,6 +29,134 b' int strcpy_s(char *d, size_t n, const ch'
 #define _countof(array) (sizeof(array) / sizeof(array[0]))
 #endif
 
+#if PY_MAJOR_VERSION >= 3
+
+#pragma comment(lib, "Advapi32.lib")
+
+/* python.org installations */
+#define CORE_PATH L"SOFTWARE\\Python\\PythonCore"
+
+/* Microsoft Store installations */
+#define LOOKASIDE_PATH                                                         \
+	L"SOFTWARE\\Microsoft\\AppModel\\Lookaside\\user\\Software\\Python\\"  \
+	L"PythonCore"
+
+static wchar_t *_locate_python_for_key(HKEY root, LPCWSTR subkey, size_t *size)
+{
+	wchar_t installPathKey[512];
+	wchar_t *executable = NULL;
+	DWORD type;
+	DWORD sz = 0;
+	HKEY ip_key;
+	LSTATUS status;
+
+	_snwprintf_s(installPathKey, sizeof(installPathKey), _TRUNCATE,
+	             L"%ls\\%d.%d\\InstallPath", subkey, PY_MAJOR_VERSION,
+	             PY_MINOR_VERSION);
+
+	status =
+	    RegOpenKeyExW(root, installPathKey, 0, KEY_QUERY_VALUE, &ip_key);
+
+	if (status != ERROR_SUCCESS)
+		return NULL;
+
+	status = RegQueryValueExW(ip_key, L"ExecutablePath", NULL, &type,
+	                          (LPBYTE)executable, &sz);
+	if (status == ERROR_SUCCESS) {
+		/* Allocate extra space so path\to\python.exe can be converted
+		 * to path\to\python39.dll + NUL.
+		 */
+		*size = sz + sizeof(_T(HGPYTHONLIB ".dll")) + sizeof(wchar_t);
+		executable = malloc(*size);
+
+		if (executable) {
+			status =
+			    RegQueryValueExW(ip_key, L"ExecutablePath", NULL,
+			                     &type, (LPBYTE)executable, &sz);
+
+			if (status != ERROR_SUCCESS) {
+				free(executable);
+				executable = NULL;
+			} else {
+				/* Not all values are stored NUL terminated */
+				executable[sz] = L'\0';
+			}
+		}
+	}
+
+	RegCloseKey(ip_key);
+	return executable;
+}
+
+static HMODULE load_system_py3(void)
+{
+	wchar_t *subkeys[] = {CORE_PATH, LOOKASIDE_PATH};
+	HKEY roots[] = {HKEY_LOCAL_MACHINE, HKEY_CURRENT_USER};
+
+	/* Give priority to python.org installs, because MS Store installs can
+	 * break with user profile corruption, and also use some NTFS feature
+	 * that MSYS doesn't understand.
+	 */
+	for (int s = 0; s < _countof(subkeys); s++) {
+		for (int r = 0; r < _countof(roots); r++) {
+			size_t size = 0;
+			wchar_t *py =
+			    _locate_python_for_key(roots[r], subkeys[s], &size);
+			wchar_t *cut = NULL;
+			HMODULE pydll;
+
+			if (!py) {
+				continue;
+			}
+
+			/* Cut off the python executable component */
+			cut = wcsrchr(py, L'\\');
+			if (cut == NULL) {
+				free(py);
+				continue;
+			}
+			*cut = 0;
+
+			wcscat_s(py, size, _T("\\" HGPYTHONLIB ".dll"));
+
+			pydll = LoadLibrary(py);
+
+			/* Also load python3.dll, so we don't pick up a random
+			 * one on PATH. We don't search {sys.prefix}\DLLs like
+			 * python.exe because this is commented as "not been a
+			 * normal install layout for a while", and don't search
+			 * LOAD_LIBRARY_SEARCH_APPLICATION_DIR because it's not
+			 * clear what the use case is.
+			 */
+			if (pydll != NULL) {
+				HANDLE py3dll = NULL;
+
+				*cut = 0;
+				wcscat_s(py, size, L"\\python3.dll");
+
+				py3dll = LoadLibrary(py);
+				if (py3dll == NULL) {
+					const char *err;
+					err = "failed to load python3.dll "
+					      "for " HGPYTHONLIB ".dll";
+					fprintf(stderr, "abort: %s (0x%X)\n",
+					        err, GetLastError());
+					exit(255);
+				}
+			}
+
+			free(py);
+
+			if (pydll != NULL) {
+				return pydll;
+			}
+		}
+	}
+
+	return NULL;
+}
+#endif
+
 static TCHAR pyscript[MAX_PATH + 10];
 static TCHAR pyhome[MAX_PATH + 10];
 static TCHAR pydllfile[MAX_PATH + 10];
@@ -108,6 +236,12 b' int _tmain(int argc, TCHAR *argv[])'
 		Py_SetPythonHome(pyhome);
 	}
 
+#if PY_MAJOR_VERSION >= 3
+	if (pydll == NULL) {
+		pydll = load_system_py3();
+	}
+#endif
+
 	if (pydll == NULL) {
 		pydll = LoadLibrary(_T(HGPYTHONLIB) _T(".dll"));
 		if (pydll == NULL) {