exewrapper: adapt for legacy HackableMercurial

We give up using CPython's PythonXX.lib import libraries (and Python.h), and
now "manually" call the LoadLibrary() / GetProcAddress() Windows API's instead.

If there is a "hg-python" subdirectory (the canonical directory name for
HackableMercurial's private Python copy) next to the hg.exe, we load the
pythonXX.dll from there (feeding an absolute path to LoadLibrary) and we set
Py_SetPythonHome() to that directory, so that the Python libraries are used
from there as well.

If there is no "hg-python" subdir found next to the hg.exe, we do not feed an
absolute path to LoadLibrary. This continues to allow to find a globally
installed Python DLL, as before this change - that is, without having to edit,
delete, rename, or configure anything.

Note that the hg.exe built is still bound to a *specific* major version of the
pythonXX.dll (e.g. python27.dll). What version it is, is inferred from the
version of the python interpreter that was used when calling setup.py. For
example

  C:\python27_x86\python.exe setup.py build_hgexe -i --compiler=mingw32

builds a hg.exe (using the mingw32 tool chain) bound to (x86) Python 2.7. And

  C:\python27_x86\python.exe setup.py build_hgexe -i

builds the same using the Microsoft C compiler/linker. (Note that the Microsoft
toolchain combined with x64 CPython can be used to build an x64 hg.exe.)

setup.py is changed to write the name of the pythonlib into the generated header
file "mercurial/hgpythonlib.h", which is #included by exewrapper.c. For a Python
2.7 build, it for example contains:

  #define HGPYTHONLIB "python27"

exewrapper.c then uses HGPYTHONLIB for the name of the Python dll to load.

We don't want to track mercurial/hgpythonlib.h, so we add it to .hgignore.

.hgignore

@@ -32,6 +32,7 b' MANIFEST'
 MANIFEST.in
 patches
 mercurial/__version__.py
+mercurial/hgpythonlib.h
 mercurial.egg-info
 .DS_Store
 tags

mercurial/exewrapper.c

@@ -7,23 +7,31 b''
  GNU General Public License version 2 or any later version.
 */
 
-#include <Python.h>
+#include <stdio.h>
 #include <windows.h>
 
+#include "hgpythonlib.h"
 
 #ifdef __GNUC__
 int strcat_s(char *d, size_t n, const char *s)
 {
 	return !strncat(d, s, n);
 }
+int strcpy_s(char *d, size_t n, const char *s)
+{
+	return !strncpy(d, s, n);
+}
 #endif
 
 
 static char pyscript[MAX_PATH + 10];
+static char pyhome[MAX_PATH + 10];
+static char envpyhome[MAX_PATH + 10];
+static char pydllfile[MAX_PATH + 10];
 
 int main(int argc, char *argv[])
 {
-	char *dot;
+	char *p;
 	int ret;
 	int i;
 	int n;
@@ -31,6 +39,9 b' int main(int argc, char *argv[])'
 	WIN32_FIND_DATA fdata;
 	HANDLE hfind;
 	const char *err;
+	HMODULE pydll;
+	void (__cdecl *Py_SetPythonHome)(char *home);
+	int (__cdecl *Py_Main)(int argc, char *argv[]);
 
 	if (GetModuleFileName(NULL, pyscript, sizeof(pyscript)) == 0)
 	{
@@ -38,12 +49,13 b' int main(int argc, char *argv[])'
 		goto bail;
 	}
 
-	dot = strrchr(pyscript, '.');
-	if (dot == NULL) {
+	p = strrchr(pyscript, '.');
+	if (p == NULL) {
 		err = "malformed module filename";
 		goto bail;
 	}
-	*dot = 0; /* cut trailing ".exe" */
+	*p = 0; /* cut trailing ".exe" */
+	strcpy_s(pyhome, sizeof(pyhome), pyscript);
 
 	hfind = FindFirstFile(pyscript, &fdata);
 	if (hfind != INVALID_HANDLE_VALUE) {
@@ -54,6 +66,57 b' int main(int argc, char *argv[])'
 		strcat_s(pyscript, sizeof(pyscript), "exe.py");
 	}
 
+	pydll = NULL;
+	if (GetEnvironmentVariable("PYTHONHOME", envpyhome,
+				   sizeof(envpyhome)) == 0)
+	{
+		/* environment var PYTHONHOME is not set */
+
+		p = strrchr(pyhome, '\\');
+		if (p == NULL) {
+			err = "can't find backslash in module filename";
+			goto bail;
+		}
+		*p = 0; /* cut at directory */
+
+		/* check for private Python of HackableMercurial */
+		strcat_s(pyhome, sizeof(pyhome), "\\hg-python");
+
+		hfind = FindFirstFile(pyhome, &fdata);
+		if (hfind != INVALID_HANDLE_VALUE) {
+			/* path pyhome exists, let's use it */
+			FindClose(hfind);
+			strcpy_s(pydllfile, sizeof(pydllfile), pyhome);
+			strcat_s(pydllfile, sizeof(pydllfile), "\\" HGPYTHONLIB);
+			pydll = LoadLibrary(pydllfile);
+			if (pydll == NULL) {
+				err = "failed to load private Python DLL";
+				goto bail;
+			}
+			Py_SetPythonHome = (void*)GetProcAddress(pydll,
+							"Py_SetPythonHome");
+			if (Py_SetPythonHome == NULL) {
+				err = "failed to get Py_SetPythonHome";
+				goto bail;
+			}
+			Py_SetPythonHome(pyhome);
+		}
+	}
+
+	if (pydll == NULL) {
+		pydll = LoadLibrary(HGPYTHONLIB);
+		if (pydll == NULL) {
+			err = "failed to load Python DLL";
+			goto bail;
+		}
+	}
+
+	Py_Main = (void*)GetProcAddress(pydll, "Py_Main");
+	if (Py_Main == NULL) {
+		err = "failed to get Py_Main";
+		goto bail;
+	}
+
 	/*
 	Only add the pyscript to the args, if it's not already there. It may
 	already be there, if the script spawned a child process of itself, in

setup.py

@@ -344,14 +344,18 b' class buildhgexe(build_ext):'
         if isinstance(self.compiler, HackedMingw32CCompiler):
             self.compiler.compiler_so = self.compiler.compiler # no -mdll
             self.compiler.dll_libraries = [] # no -lmsrvc90
+        hv = sys.hexversion
+        pythonlib = 'python%d%d' % (hv >> 24, (hv >> 16) & 0xff)
+        f = open('mercurial/hgpythonlib.h', 'wb')
+        f.write('/* this file is autogenerated by setup.py */\n')
+        f.write('#define HGPYTHONLIB "%s"\n' % pythonlib)
+        f.close()
         objects = self.compiler.compile(['mercurial/exewrapper.c'],
                                          output_dir=self.build_temp)
         dir = os.path.dirname(self.get_ext_fullpath('dummy'))
         target = os.path.join(dir, 'hg')
-        pythonlib = ("python%d%d" %
-               (sys.hexversion >> 24, (sys.hexversion >> 16) & 0xff))
         self.compiler.link_executable(objects, target,
-                                      libraries=[pythonlib],
+                                      libraries=[],
                                       output_dir=self.build_temp)
 
 class hginstallscripts(install_scripts):