PostgreSQL has a built-in relocation code to support varied installation paths.
At runtime, lookup of extensions and other executables is performed by taking
the absolute/real path of the calling program, and then walking up the directory
structure to determine the lookup path. For example, extensions of a PostgreSQL
instance launched by /opt/postgresql/bin/postgres would be searched under
/opt/postgresql/bin/../share/postgresql/extensions.

The aforementioned relocation scheme works perfectly fine if both PostgreSQL
and all subsequent extensions are installed under the same prefix. Projects
such as GoboLinux, however, install them under different prefixes (such as
/Programs/PostgreSQL and /Programs/PostGIS), resulting in lookup problems
when the relocation code path is taken.

This patch disables the relocation code so that PostgreSQL is able to find
extensions on said projects and also introduces a GoboLinux specific change
that causes lookups on /Programs/PostgreSQL/<version> to be redirected to
the multiplexed /System/Index tree.

Signed-off-by: Lucas C. Villa Real <lucasvr@gobolinux.org>

--- postgresql-9.6.1/src/port/path.c.orig	2017-03-22 21:03:50.220969341 -0300
+++ postgresql-9.6.1/src/port/path.c	2017-03-22 22:14:35.471048984 -0300
@@ -539,6 +539,9 @@ static void
 make_relative_path(char *ret_path, const char *target_path,
 				   const char *bin_path, const char *my_exec_path)
 {
+#if 1
+	strncpy(ret_path, target_path, MAXPGPATH-1);
+#else
 	int			prefix_len;
 	int			tail_start;
 	int			tail_len;
@@ -586,6 +589,7 @@ make_relative_path(char *ret_path, const
 no_match:
 	strlcpy(ret_path, target_path, MAXPGPATH);
 	canonicalize_path(ret_path);
+#endif
 }
 
 
--- postgresql-9.6.1/src/common/exec.c.orig	2017-03-22 23:20:47.166123496 -0300
+++ postgresql-9.6.1/src/common/exec.c	2017-03-22 23:13:23.057115164 -0300
@@ -203,6 +203,39 @@ find_my_exec(const char *argv0, char *re
 	return -1;
 }
 
+/*
+ * to_system_index - converts an absolute path under /Programs/PostgreSQL
+ * into its symbolic link on /System/Index.
+ */
+static int
+to_system_index(char *path)
+{
+	if (strstr(path, "/Programs/PostgreSQL/") == path)
+	{
+		/*
+		 * Use the multiplexed /System/Index tree rather than /Programs.
+		 * This fixes access to extensions installed under other programs'
+		 * share/postgresql/extensions.
+		 */
+		size_t pgsql = strlen("/Programs/PostgreSQL/");
+		char *pstart = &path[pgsql+1];
+		char *pslash = strchr(pstart, '/');
+		if (pslash)
+		{
+			struct stat statbuf;
+			char *path_copy = strdup(path);
+			size_t sysindexlen = strlen("/System/Index");
+			sprintf(path, "/System/Index");
+			memmove(&path[sysindexlen], pslash, strlen(pslash));
+			path[sysindexlen+strlen(pslash)] = '\0';
+			if (stat(path, &statbuf) != 0)
+				/* Rollback */
+				strcpy(path, path_copy);
+			free(path_copy);
+		}
+	}
+	return 0;
+}
 
 /*
  * resolve_symlinks - resolve symlinks to the underlying file
@@ -295,7 +328,7 @@ resolve_symlinks(char *path)
 	}
 #endif   /* HAVE_READLINK */
 
-	return 0;
+	return to_system_index(path);
 }