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ref: e1964307f2d3ca6cd9e31ed3299c2ab42bfb7a4e
parent: d4043e96060df9abaab72c2acee4a548f43dba4e
author: cinap_lenrek <cinap_lenrek@felloff.net>
date: Tue May 26 18:21:41 EDT 2020

devfs-posix: apply u9fs truncate support (thanks Fazlul Shahriar)

This was copied fro u9fs, with the issue filed in:

https://github.com/golang/go/issues/39238

--- a/kern/devfs-posix.c
+++ b/kern/devfs-posix.c
@@ -397,6 +397,13 @@
 	char strs[NAME_MAX*3];
 	Ufsinfo *uif;
 
+	/*
+	 * wstat is supposed to be atomic.
+	 * we check all the things we can before trying anything.
+	 * still, if we are told to truncate a file and rename it and only
+	 * one works, we're screwed.  in such cases we leave things
+	 * half broken and return an error.  it's hardly perfect.
+	 */
 	if(convM2D(buf, n, &d, strs) != n)
 		error(Ebadstat);
 	
@@ -404,6 +411,13 @@
 	if(stat(uif->path, &stbuf) < 0)
 		error(strerror(errno));
 
+	/*
+	 * try things in increasing order of harm to the file.
+	 * mtime should come after truncate so that if you
+	 * do both the mtime actually takes effect, but i'd rather
+	 * leave truncate until last.
+	 * (see above comment about atomicity).
+	 */
 	if(~d.mode != 0 && (int)(d.mode&0777) != (int)(stbuf.st_mode&0777)) {
 		if(chmod(uif->path, d.mode&0777) < 0)
 			error(strerror(errno));
@@ -450,6 +464,10 @@
 		uif->gid = p->id;
 	}
 */
+
+	if((uvlong)d.length != (uvlong)~0 && truncate(uif->path, d.length) < 0)
+		error(strerror(errno));
+
 	return n;
 }