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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;

 }