[Tests] Fix RemoteMirror test to work when DYLD_LIBRARY_PATH is set.

test/RemoteMirror/Inputs/interop.m

@@ -19,12 +19,55 @@
 #import <Foundation/Foundation.h>
 #import <mach/mach.h>
 #import <mach-o/dyld.h>
+#import <libgen.h>
+#import <stdlib.h>
+#import <string.h>
 
 #import "SwiftRemoteMirrorLegacyInterop.h"
 
+// dirname() is *not* safe - it may modify the input string; so instead just
+// copy anything up to the last slash.
+char *safe_dirname(const char *path) {
+  const char *slash = strrchr(path, '/');
+  if (!slash)
+    return strdup(".");
+  size_t len = slash - path;
+  char *result = (char *)malloc(len + 1);
+  memcpy(result, path, len);
+  result[len] = '\0';
+  return result;
+}
 
 void *Load(char *path) {
+  const char *libpath = getenv("DYLD_LIBRARY_PATH");
+  char *ourlibpath = NULL;
+  const char *oldlibpath = NULL;
+
+  if (libpath && path[0] == '/') {
+    // If DYLD_LIBRARY_PATH is set, and the path we've been given is absolute,
+    // then prepend the directory part of the path we've been given to it.
+    char *libdir = safe_dirname(path);
+    size_t dirlen = strlen(libdir);
+    if (asprintf(&ourlibpath, "%s:%s", libdir, oldlibpath) < 0) {
+      fprintf(stderr, "Unable to form new DYLD_LIBRARY_PATH!\n");
+      exit(1);
+    }
+    free(libdir);
+
+    oldlibpath = ourlibpath + dirlen + 1;
+
+    setenv("DYLD_LIBRARY_PATH", ourlibpath, 1);
+  }
+
+  // Actually open the dylib
   void *Handle = dlopen(path, RTLD_LOCAL);
+
+  if (ourlibpath) {
+    // Reset DYLD_LIBRARY_PATH, if we changed it
+    setenv("DYLD_LIBRARY_PATH", oldlibpath, 1);
+    free(ourlibpath);
+  }
+
   if (Handle == NULL) {
     fprintf(stderr, "loading %s: %s\n", path, dlerror());
     exit(1);