Remove configure probes for symlink/readlink, and dead code.

symlink() and readlink() are in SUSv2 and all targeted Unix systems have
them.  We have partial emulation on Windows.  Code that raised runtime
errors on systems without it has been dead for years, so we can remove
that and also references to such systems in the documentation.

Define HAVE_READLINK and HAVE_SYMLINK macros on Unix.  Our Windows
replacement functions based on junction points can't be used for
relative paths or for non-directories, so the macros can be used to
check for full symlink support.  The places that deal with tablespaces
can just use symlink functions without checking the macros.  (If they
did check the macros, they'd need to provide an #else branch with a
runtime or compile time error, and it'd be dead code.)

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com

src/backend/access/transam/xlog.c

@@ -8305,7 +8305,6 @@ do_pg_backup_start(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 			if (get_dirent_type(fullpath, de, false, ERROR) != PGFILETYPE_LNK)
 				continue;
 
-#if defined(HAVE_READLINK) || defined(WIN32)
 			rllen = readlink(fullpath, linkpath, sizeof(linkpath));
 			if (rllen < 0)
 			{
@@ -8358,17 +8357,6 @@ do_pg_backup_start(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 							 ti->oid, escapedpath.data);
 
 			pfree(escapedpath.data);
-#else
-
-			/*
-			 * If the platform does not have symbolic links, it should not be
-			 * possible to have tablespaces - clearly somebody else created
-			 * them. Warn about it and ignore.
-			 */
-			ereport(WARNING,
-					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("tablespaces are not supported on this platform")));
-#endif
 		}
 		FreeDir(tblspcdir);
 

src/backend/commands/tablespace.c

@@ -213,7 +213,6 @@ TablespaceCreateDbspace(Oid spcOid, Oid dbOid, bool isRedo)
 Oid
 CreateTableSpace(CreateTableSpaceStmt *stmt)
 {
-#ifdef HAVE_SYMLINK
 	Relation	rel;
 	Datum		values[Natts_pg_tablespace];
 	bool		nulls[Natts_pg_tablespace] = {0};
@@ -391,12 +390,6 @@ CreateTableSpace(CreateTableSpaceStmt *stmt)
 	table_close(rel, NoLock);
 
 	return tablespaceoid;
-#else							/* !HAVE_SYMLINK */
-	ereport(ERROR,
-			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-			 errmsg("tablespaces are not supported on this platform")));
-	return InvalidOid;			/* keep compiler quiet */
-#endif							/* HAVE_SYMLINK */
 }
 
 /*
@@ -407,7 +400,6 @@ CreateTableSpace(CreateTableSpaceStmt *stmt)
 void
 DropTableSpace(DropTableSpaceStmt *stmt)
 {
-#ifdef HAVE_SYMLINK
 	char	   *tablespacename = stmt->tablespacename;
 	TableScanDesc scandesc;
 	Relation	rel;
@@ -573,11 +565,6 @@ DropTableSpace(DropTableSpaceStmt *stmt)
 
 	/* We keep the lock on pg_tablespace until commit */
 	table_close(rel, NoLock);
-#else							/* !HAVE_SYMLINK */
-	ereport(ERROR,
-			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-			 errmsg("tablespaces are not supported on this platform")));
-#endif							/* HAVE_SYMLINK */
 }
 
 

src/backend/replication/basebackup.c

@@ -1330,7 +1330,6 @@ sendDir(bbsink *sink, const char *path, int basepathlen, bool sizeonly,
 #endif
 			)
 		{
-#if defined(HAVE_READLINK) || defined(WIN32)
 			char		linkpath[MAXPGPATH];
 			int			rllen;
 
@@ -1349,18 +1348,6 @@ sendDir(bbsink *sink, const char *path, int basepathlen, bool sizeonly,
 
 			size += _tarWriteHeader(sink, pathbuf + basepathlen + 1, linkpath,
 									&statbuf, sizeonly);
-#else
-
-			/*
-			 * If the platform does not have symbolic links, it should not be
-			 * possible to have tablespaces - clearly somebody else created
-			 * them. Warn about it and ignore.
-			 */
-			ereport(WARNING,
-					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("tablespaces are not supported on this platform")));
-			continue;
-#endif							/* HAVE_READLINK */
 		}
 		else if (S_ISDIR(statbuf.st_mode))
 		{

src/backend/utils/adt/misc.c

@@ -302,8 +302,6 @@ pg_tablespace_location(PG_FUNCTION_ARGS)
 		tablespaceOid == GLOBALTABLESPACE_OID)
 		PG_RETURN_TEXT_P(cstring_to_text(""));
 
-#if defined(HAVE_READLINK) || defined(WIN32)
-
 	/*
 	 * Find the location of the tablespace by reading the symbolic link that
 	 * is in pg_tblspc/<oid>.
@@ -349,12 +347,6 @@ pg_tablespace_location(PG_FUNCTION_ARGS)
 	targetpath[rllen] = '\0';
 
 	PG_RETURN_TEXT_P(cstring_to_text(targetpath));
-#else
-	ereport(ERROR,
-			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-			 errmsg("tablespaces are not supported on this platform")));
-	PG_RETURN_NULL();
-#endif
 }
 
 /*