Use truncate(2) where appropriate.

When truncating files by name, use truncate(2).  Windows hasn't got it,
so keep our previous coding based on ftruncate(2) as a fallback.

Discussion: https://postgr.es/m/16663-fe97ccf9932fc800%40postgresql.org

src/backend/storage/file/fd.c

@@ -622,6 +622,33 @@ pg_flush_data(int fd, off_t offset, off_t nbytes)
 #endif
 }
 
+/*
+ * Truncate a file to a given length by name.
+ */
+int
+pg_truncate(const char *path, off_t length)
+{
+#ifdef WIN32
+	int			save_errno;
+	int			ret;
+	int			fd;
+
+	fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
+	if (fd >= 0)
+	{
+		ret = ftruncate(fd, 0);
+		save_errno = errno;
+		CloseTransientFile(fd);
+		errno = save_errno;
+	}
+	else
+		ret = -1;
+
+	return ret;
+#else
+	return truncate(path, length);
+#endif
+}
 
 /*
  * fsync_fname -- fsync a file or directory, handling errors properly

src/backend/storage/smgr/md.c

@@ -294,19 +294,8 @@ do_truncate(const char *path)
 {
 	int			save_errno;
 	int			ret;
-	int			fd;
 
-	/* truncate(2) would be easier here, but Windows hasn't got it */
-	fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
-	if (fd >= 0)
-	{
-		ret = ftruncate(fd, 0);
-		save_errno = errno;
-		CloseTransientFile(fd);
-		errno = save_errno;
-	}
-	else
-		ret = -1;
+	ret = pg_truncate(path, 0);
 
 	/* Log a warning here to avoid repetition in callers. */
 	if (ret < 0 && errno != ENOENT)

src/include/storage/fd.h

@@ -153,6 +153,7 @@ extern int	pg_fsync_no_writethrough(int fd);
 extern int	pg_fsync_writethrough(int fd);
 extern int	pg_fdatasync(int fd);
 extern void pg_flush_data(int fd, off_t offset, off_t amount);
+extern int	pg_truncate(const char *path, off_t length);
 extern void fsync_fname(const char *fname, bool isdir);
 extern int	fsync_fname_ext(const char *fname, bool isdir, bool ignore_perm, int elevel);
 extern int	durable_rename(const char *oldfile, const char *newfile, int loglevel);