Windows: add wrapper .exe to work around lack of RPATH

Windows has no concept of `RPATH`, which makes shipping binaries with
relatively-located dependencies quite challenging.  Despite
investigating many potential avenues of `RPATH` emulation, (including
but not limited to Application Manifests, PE file patching, and bundling
the true Julia executable as a resource inside of a launcher) the most
reliable (And least breaking for external workflows) was found to be to
create a launcher executable that invokes the true Julia executable from
within the `libexec` directory.

NEWS.md

@@ -104,6 +104,10 @@ Build system changes
 * The build system now contains a pure-make caching system for expanding expensive operations at the latest
   possible moment, while still expanding it only once. ([#35193])
 
+* The windows executable now uses a launcher `.exe` to transparently set up environment variables necessary
+  for the "true" Julia executable to find its dependent libraries, as they are no longer located within the
+  main `bin` directory. ([#35629])
+
 
 New library functions
 ---------------------

contrib/windows/exe_path_wrapper.c

@@ -0,0 +1,73 @@
+#include <windows.h>
+#include <tchar.h>
+#include <stdio.h>
+#include <shlwapi.h>
+#define ENVVAR_MAXLEN 32760
+
+/* PATH_ENTRIES is our simulated RPATH, usually of the form 'TEXT("../path1"), TEXT("../path2"), ...' */
+#ifndef PATH_ENTRIES
+#define PATH_ENTRIES  TEXT("")
+#endif
+
+LPWSTR pathEntries[] = {
+    PATH_ENTRIES
+};
+
+/* JULIA_EXE_PATH is the relative path to julia.exe */
+#ifndef JULIA_EXE_PATH
+#define JULIA_EXE_PATH "../libexec/julia.exe"
+#endif
+
+int wmain(int argc, wchar_t *argv[], wchar_t *envp[]) {
+    // Determine absolute path to true julia.exe sitting in `libexec/`
+    WCHAR currFileDir[MAX_PATH];
+    WCHAR juliaPath[MAX_PATH];
+    GetModuleFileName(NULL, currFileDir, MAX_PATH);
+    PathRemoveFileSpec(currFileDir);
+    PathCombine(juliaPath, currFileDir, TEXT(JULIA_EXE_PATH));
+
+    // On windows, we simulate RPATH by pushing onto PATH
+    LPWSTR pathVal = (LPWSTR) malloc(ENVVAR_MAXLEN*sizeof(WCHAR));
+    DWORD dwRet = GetEnvironmentVariable(TEXT("PATH"), pathVal, ENVVAR_MAXLEN);
+    DWORD numPathEntries = sizeof(pathEntries)/sizeof(pathEntries[0]);
+    if (dwRet == 0) {
+        // If we cannot get PATH, then our job is easy!
+        pathVal[0] = '\0';
+    } else {
+        // Otherwise, we append, if we have enough space to:
+        DWORD currFileDirLen = wcslen(currFileDir);
+        DWORD totalPathLen = dwRet + 1 + currFileDirLen;
+        for (int env_idx=0; env_idx<numPathEntries; ++env_idx) {
+            totalPathLen += 1 + currFileDirLen + 1 + wcslen(pathEntries[env_idx]);
+        }
+        if (ENVVAR_MAXLEN < totalPathLen ) {
+            fprintf(stderr, "ERROR: Cannot append entries to PATH: not enough space in environment block.  Reduce size of PATH!");
+            exit(1);
+        }
+        lstrcat(pathVal, TEXT(";"));
+    }
+    // We always add the current directory (e.g. `bin/`) to PATH so that we can find e.g. libjulia.dll
+    lstrcat(pathVal, currFileDir);
+
+    // For each entry in PATH_ENTRIES, tack it on to the end, relative to the current directory:
+    for (int env_idx=0; env_idx<numPathEntries; ++env_idx) {
+        lstrcat(pathVal, TEXT(";"));
+        lstrcat(pathVal, currFileDir);
+        lstrcat(pathVal, TEXT("\\"));
+        lstrcat(pathVal, pathEntries[env_idx]);
+    }
+    SetEnvironmentVariable(TEXT("PATH"), pathVal);
+    free(pathVal);
+
+    STARTUPINFO info;
+    PROCESS_INFORMATION processInfo;
+    DWORD exit_code = 1;
+    GetStartupInfo(&info);
+    if (CreateProcess(juliaPath, GetCommandLine(), NULL, NULL, TRUE, 0, NULL, NULL, &info, &processInfo)) {
+        WaitForSingleObject(processInfo.hProcess, INFINITE);
+        GetExitCodeProcess(processInfo.hProcess, &exit_code);
+        CloseHandle(processInfo.hProcess);
+        CloseHandle(processInfo.hThread);
+    }
+    return exit_code;
+}

ui/Makefile

@@ -45,6 +45,8 @@ $(BUILDDIR)/%.dbg.obj: $(SRCDIR)/%.c $(HEADERS)
 	@$(call PRINT_CC, $(CC) $(CPPFLAGS) $(CFLAGS) $(DEBUGFLAGS) -c $< -o $@)
 
 ifeq ($(OS),WINNT)
+JLDFLAGS += -municode
+
 ifneq ($(USEMSVC), 1)
 $(BUILDDIR)/julia_res.o: $(JULIAHOME)/contrib/windows/julia.rc $(JULIAHOME)/VERSION
 	JLVER=`cat $(JULIAHOME)/VERSION` && \
@@ -82,10 +84,36 @@ else
 CXXLD := $(LD)
 endif
 
+# Add `.exe` wrapper generation to properly setup `PATH` on windows to simulate `RPATH`
+ifeq ($(OS),WINNT)
+define rel_path_entry
+TEXT(\"$$(echo $(call rel_path,$(build_libexecdir),$(1)) | sed -e 's_/_\\\\_g')\")
+endef
+CPPFLAGS_PATH = -DPATH_ENTRIES="$(call rel_path_entry,$(CSL_SHLIBDIR)), $(call rel_path_entry,$(LIBLLVM_SHLIBDIR))"
+define julia_exe_path
+-DJULIA_EXE_PATH="\"$$(echo '$(libexecdir_rel)$(PATHSEP)$(1)' | sed -e 's_\\_\\\\_g')\""
+endef
+define gen_wrapper_exe
+	-mv $(build_bindir)/$(1) $(build_libexecdir)/$(1)
+	@$(call PRINT_CC, $(CC) $(CPPFLAGS) $(CFLAGS) $(SHIPFLAGS) $(call julia_exe_path,$(1)) $(CPPFLAGS_PATH) $(JULIAHOME)/contrib/windows/exe_path_wrapper.c -o $(build_bindir)/$(1) $(JLDFLAGS) -lshlwapi)
+endef
+
+$(build_bindir)/julia$(EXE):  $(JULIAHOME)/contrib/windows/exe_path_wrapper.c
+$(build_bindir)/julia-debug$(EXE):  $(JULIAHOME)/contrib/windows/exe_path_wrapper.c
+endif
+
+
 $(build_bindir)/julia$(EXE): $(OBJS)
 	@$(call PRINT_LINK, $(CXXLD) $(CXXFLAGS) $(CXXLDFLAGS) $(LINK_FLAGS) $(SHIPFLAGS) $(OBJS) -o $@ -L$(build_private_libdir) -L$(build_libdir) -L$(build_shlibdir) -ljulia $(JLDFLAGS) $(CXXLDFLAGS))
+ifeq ($(OS),WINNT)
+	@$(call gen_wrapper_exe,julia$(EXE))
+endif
+
 $(build_bindir)/julia-debug$(EXE): $(DOBJS)
 	@$(call PRINT_LINK, $(CXXLD) $(CXXFLAGS) $(CXXLDFLAGS) $(LINK_FLAGS) $(DEBUGFLAGS) $(DOBJS) -o $@ -L$(build_private_libdir) -L$(build_libdir) -L$(build_shlibdir) -ljulia-debug $(JLDFLAGS) $(CXXLDFLAGS))
+ifeq ($(OS),WINNT)
+	@$(call gen_wrapper_exe,julia-debug$(EXE))
+endif
 
 clean: | $(CLEAN_TARGETS)
 	rm -f *.o *.dbg.obj