Don't try to trim "../" in join_path_components().

join_path_components() tried to remove leading ".." components from its
tail argument, but it was not nearly bright enough to do so correctly
unless the head argument was (a) absolute and (b) canonicalized.
Rather than try to fix that logic, let's just get rid of it: there is no
correctness reason to remove "..", and cosmetic concerns can be taken
care of by a subsequent canonicalize_path() call.  Per bug #6715 from
Greg Davidson.

Back-patch to all supported branches.  It appears that pre-9.2, this
function is only used with absolute paths as head arguments, which is why
we'd not noticed the breakage before.  However, third-party code might be
expecting this function to work in more general cases, so it seems wise
to back-patch.

In HEAD and 9.2, also make some minor cosmetic improvements to callers.

src/port/path.c

@@ -165,6 +165,8 @@ make_native_path(char *filename)
 /*
  * join_path_components - join two path components, inserting a slash
  *
+ * We omit the slash if either given component is empty.
+ *
  * ret_path is the output area (must be of size MAXPGPATH)
  *
  * ret_path can be the same as head, but not the same as tail.
@@ -177,37 +179,22 @@ join_path_components(char *ret_path,
 		strlcpy(ret_path, head, MAXPGPATH);
 
 	/*
-	 * Remove any leading "." and ".." in the tail component, adjusting head
-	 * as needed.
+	 * Remove any leading "." in the tail component.
+	 *
+	 * Note: we used to try to remove ".." as well, but that's tricky to get
+	 * right; now we just leave it to be done by canonicalize_path() later.
 	 */
-	for (;;)
-	{
-		if (tail[0] == '.' && IS_DIR_SEP(tail[1]))
-		{
+	while (tail[0] == '.' && IS_DIR_SEP(tail[1]))
 		tail += 2;
-		}
-		else if (tail[0] == '.' && tail[1] == '\0')
-		{
-			tail += 1;
-			break;
-		}
-		else if (tail[0] == '.' && tail[1] == '.' && IS_DIR_SEP(tail[2]))
-		{
-			trim_directory(ret_path);
-			tail += 3;
-		}
-		else if (tail[0] == '.' && tail[1] == '.' && tail[2] == '\0')
-		{
-			trim_directory(ret_path);
-			tail += 2;
-			break;
-		}
-		else
-			break;
-	}
+
 	if (*tail)
+	{
+		/* only separate with slash if head wasn't empty */
 		snprintf(ret_path + strlen(ret_path), MAXPGPATH - strlen(ret_path),
-				 "/%s", tail);
+				 "%s%s",
+				 (*(skip_drive(head)) != '\0') ? "/" : "",
+				 tail);
+	}
 }
 
 
@@ -656,6 +643,15 @@ get_home_path(char *ret_path)
  *
  * Modify the given string in-place to name the parent directory of the
  * named file.
+ *
+ * If the input is just a file name with no directory part, the result is
+ * an empty string, not ".".  This is appropriate when the next step is
+ * join_path_components(), but might need special handling otherwise.
+ *
+ * Caution: this will not produce desirable results if the string ends
+ * with "..".  For most callers this is not a problem since the string
+ * is already known to name a regular file.  If in doubt, apply
+ * canonicalize_path() first.
  */
 void
 get_parent_directory(char *path)