Rewrite realpath() wrapper.

The former implementation didn't necessarily provide all versions of
realpath() that might be invoked, and didn't necessarily invoke the
proper version.  That led to possible crashes and possible unnecessary
semantic differences.  This fixes all that, though it doesn't retrofit
the 10.5+ semantic changes to 10.4 (and neither did the earlier
implementation).

The former implementation also failed to return any internally
allocated buffer on failures, causing a memory leak.  That is now
fixed as well.

TESTED:
Builds and passes all tests (including expanded tests) on all
platforms, with all applicable SDKs.

src/realpath.c

 /*
- * Copyright (c) 2019
+ * Copyright (c) 2024 Frederick H. G. Wright II <fw@fwright.net>
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -17,38 +17,149 @@
 /* MP support header */
 #include "MacportsLegacySupport.h"
 
-/* realpath wrap */
+/* realpath wrapper */
 #if __MPLS_LIB_SUPPORT_REALPATH_WRAP__
 
-#include <limits.h>
-#include <stdlib.h>
+/* Keep stdlib from defining any version of realpath */
+#define realpath __MPLS_hide_realpath
+
 #include <dlfcn.h>
+#include <errno.h>
+#include <stddef.h>  /* For NULL */
+#include <stdlib.h>
+
+#include <sys/syslimits.h>  /* For PATH_MAX */
+
+/* Now undo our macro kludge */
+#undef realpath
+
+/*
+ * This provides a wrapper for realpath() in order to make the 10.6+
+ * optional buffer allocation available in <10.6.  At present, it does not
+ * make the optional 10.5+ semantic improvements available in 10.4.
+ *
+ * It provides wrappers for all versions of realpath(), even when targeting
+ * 10.4, since the client may be built with a later SDK.  It attempts to
+ * pass through to the same version as the one invoked (thereby maintaining
+ * the semantics), but falls back to the basic version if that fails (normally
+ * only on 10.4).  If that fails, it bails with an abort(), since that should
+ * be impossible with an uncorrupted system library.
+ *
+ * The address of the OS realpath() is cached, to avoid repeating the dlsym()
+ * lookup on every call.  In a fallback case, the address of the basic
+ * version is cached as if it were the address of the intended version, since
+ * the intended version isn't going to magically appear later.
+ */
+
+/* Function type for realpath() */
+typedef char *rp_func_t(const char * __restrict, char * __restrict);
+
+/* Macro defining all versions */
+/* Note that the UNIX2003 version never exists in 64-bit builds. */
+
+#if !__MPLS_64BIT
+
+#define RP_ALL \
+  RP_ENT(,,basic) \
+  RP_ENT($,UNIX2003,posix) \
+  RP_ENT($,DARWIN_EXTSN,darwin)
+
+#else /* 64-bit */
+
+#define RP_ALL \
+  RP_ENT(,,basic) \
+  RP_ENT($,DARWIN_EXTSN,darwin)
+
+#endif /* 64-bit */
+
+/* Table of indices of versions */
+#define RP_ENT(d,x,t) rp_##t,
+typedef enum {
+  RP_ALL
+} rp_idx_t;
+#undef RP_ENT
 
-char *realpath(const char * __restrict stringsearch, char * __restrict buffer)
+/* Table of names */
+#define RP_STR(x) #x
+#define RP_ENT(d,x,t) RP_STR(realpath##d##x),
+static const char *rp_name[] = {
+  RP_ALL
+};
+#undef RP_ENT
+
+/* Table of cached addresses */
+#define RP_ENT(d,x,t) NULL,
+static rp_func_t *rp_adr[] = {
+  RP_ALL
+};
+#undef RP_ENT
+
+/* Internal realpath(), with version as a parameter */
+static char *
+realpath_internal(const char * __restrict file_name,
+                  char * __restrict resolved_name,
+                  rp_idx_t version)
 {
-    char *(*real_realpath)(const char * __restrict, char * __restrict);
-#if (__DARWIN_UNIX03 && !defined(_POSIX_C_SOURCE)) || defined(_DARWIN_C_SOURCE) || defined(_DARWIN_BETTER_REALPATH)
-    real_realpath = dlsym(RTLD_NEXT, "realpath$DARWIN_EXTSN");
-# else
-    real_realpath = dlsym(RTLD_NEXT, "realpath");
-#endif
-    if (real_realpath == NULL) {
-	exit(EXIT_FAILURE);
-    }
+  rp_func_t *os_realpath;
+  char *buf, *result;
+  int saved_errno;
 
-    if (buffer == NULL) {
-        char *myrealpathbuf = malloc(PATH_MAX);
-        if (myrealpathbuf != NULL) {
-            return(real_realpath(stringsearch, myrealpathbuf));
-        } else {
-            return(NULL);
-        }
-    } else {
-        return(real_realpath(stringsearch, buffer));
+  /* Locate proper OS realpath(), with fallback if needed */
+  if (!(os_realpath = rp_adr[version])) {
+    os_realpath = rp_adr[version] = dlsym(RTLD_NEXT, rp_name[version]);
+    if (!os_realpath && version != rp_basic) {
+      os_realpath = rp_adr[version] = dlsym(RTLD_NEXT, rp_name[rp_basic]);
     }
+    if (!os_realpath) abort();
+  }
+
+  /* Just pass through the call if a buffer was supplied */
+  if (resolved_name) return (*os_realpath)(file_name, resolved_name);
+
+  /* Otherwise allocate a buffer and invoke it with that */
+  if (!(buf = malloc(PATH_MAX))) return NULL;
+  if ((result = (*os_realpath)(file_name, buf))) return result;
+
+  /* On failure, free the allocated buffer */
+  /* Although free() shouldn't touch errno, we preserve it just in case */
+  saved_errno = errno;
+  free(buf);
+  errno = saved_errno;
+  return NULL;
 }
 
-/* compatibility function so code does not have to be recompiled */
-char *macports_legacy_realpath(const char * __restrict stringsearch, char * __restrict buffer) { return realpath(stringsearch, buffer); }
+/* Now the various public realpath() versions */
+
+#define RP_ENT(d,x,t) \
+char * \
+realpath##d##x(const char * __restrict file_name, \
+               char * __restrict resolved_name) \
+{ \
+  return realpath_internal(file_name, resolved_name, rp_##t); \
+}
+RP_ALL
+#undef RP_ENT
+
+/*
+ * Compatibility function to avoid the need to rebuild existing binaries
+ * built with the old wrapper-macro implementation (between Jan-2019 and
+ * Apr-2022).  We have no way to determine which realpath version was
+ * used in such a build (since that information was destroyed by the wrapper
+ * macro), so we assume the usual default for the SDK matching the target
+ * OS for *this* build.  That's the best we can do under the circumstances.
+ */
+
+#if __MPLS_TARGET_OSVER < 1050
+#define RP_DEFAULT rp_basic
+#else
+#define RP_DEFAULT rp_darwin
+#endif
+
+char *
+macports_legacy_realpath(const char * __restrict file_name,
+                         char * __restrict resolved_name)
+{
+  return realpath_internal(file_name, resolved_name, RP_DEFAULT);
+}
 
 #endif /*__MPLS_LIB_SUPPORT_REALPATH_WRAP__*/