Constify the arguments of copydir.h functions

This makes sure that the internal logic of these functions does not
attempt to change the value of the arguments constified, and it removes
one unconstify() in basic_archive.c.

Author: Nathan Bossart
Reviewed-by: Andrew Dunstan, Peter Eisentraut
Discussion: https://postgr.es/m/20230114231126.GA2580330@nathanxps13

Author: michaelpq

contrib/basic_archive/basic_archive.c

@@ -275,7 +275,7 @@ basic_archive_file_internal(const char *file, const char *path)
 	 * Copy the file to its temporary destination.  Note that this will fail
 	 * if temp already exists.
 	 */
-	copy_file(unconstify(char *, path), temp);
+	copy_file(path, temp);
 
 	/*
 	 * Sync the temporary file to disk and move it to its final destination.

src/backend/storage/file/copydir.c

@@ -34,7 +34,7 @@
  * a directory or a regular file is ignored.
  */
 void
-copydir(char *fromdir, char *todir, bool recurse)
+copydir(const char *fromdir, const char *todir, bool recurse)
 {
 	DIR		   *xldir;
 	struct dirent *xlde;
@@ -114,7 +114,7 @@ copydir(char *fromdir, char *todir, bool recurse)
  * copy one file
  */
 void
-copy_file(char *fromfile, char *tofile)
+copy_file(const char *fromfile, const char *tofile)
 {
 	char	   *buffer;
 	int			srcfd;

src/include/storage/copydir.h

@@ -13,7 +13,7 @@
 #ifndef COPYDIR_H
 #define COPYDIR_H
 
-extern void copydir(char *fromdir, char *todir, bool recurse);
-extern void copy_file(char *fromfile, char *tofile);
+extern void copydir(const char *fromdir, const char *todir, bool recurse);
+extern void copy_file(const char *fromfile, const char *tofile);
 
 #endif							/* COPYDIR_H */