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gefs: handle full disk and other errors more gracefully

if we had a full disk, or failed upserting, we could have
double-freed disk blocks on retry. This forces us to stop
writing, eliminating the chance for retry

This should also be safe after a restart -- because sync
refuses to write to disk after a failure, we will roll
back the free operations on failure, and start from the
last successful sync.

There may still be some issues if the second superblock
fails during a disk failure, but the window for issues
is much narrower.

--- a/sys/src/cmd/gefs/fs.c

+++ b/sys/src/cmd/gefs/fs.c

@@ -88,6 +88,8 @@

 	Dlist dl;

 	int i;

+	if(agetl(&fs->rdonly))

+		error(Erdonly);

 	qlock(&fs->synclk);

 	if(waserror()){

 		fprint(2, "failed to sync: %s\n", errmsg());

@@ -110,6 +112,7 @@

          */

 	qlock(&fs->mutlk);

 	if(waserror()){

+		aincl(&fs->rdonly, 1);

 		qunlock(&fs->mutlk);

 		nexterror();

 	}

--- a/sys/src/cmd/gefs/tree.c

+++ b/sys/src/cmd/gefs/tree.c

@@ -3,6 +3,7 @@

 #include <fcall.h>

 #include <avl.h>

+#include "atomic.h"

 #include "dat.h"

 #include "fns.h"

@@ -1126,14 +1127,14 @@

 }

 static void

-freepath(Tree *t, Path *path, int npath)

+freepath(Tree *t, Path *path, int npath, int ok)

 {

 	Path *p;

 	for(p = path; p != path + npath; p++){

-		if(p->b != nil)

+		if(ok && p->b != nil)

 			freeblk(t, p->b);

-		if(p->m != nil)

+		if(ok && p->m != nil)

 			freeblk(t, p->b);

 		dropblk(p->b);

 		dropblk(p->nl);

@@ -1254,6 +1255,8 @@

 	Bptr bp;

 	assert(!canqlock(&fs->mutlk));

+	if(agetl(&fs->rdonly))

+		error(Erdonly);

 	sz = 0;

 	stablesort(msg, nmsg);

 	for(i = 0; i < nmsg; i++)

@@ -1262,6 +1265,7 @@

 Again:

 	b = getroot(t, &height);

 	if(waserror()){

+		aincl(&fs->rdonly, 1);

 		dropblk(b);

 		nexterror();

 	}

@@ -1279,7 +1283,7 @@

 		error(Enomem);

 	poperror();

 	if(waserror()){

-		freepath(t, path, height+2);	/* npath not volatile */

+		freepath(t, path, height+2, 0);	/* npath not volatile */

 		nexterror();

 	}

 	npath = 0;

@@ -1334,7 +1338,7 @@

 	unlock(&t->lk);

 	npull += rp->npull;

-	freepath(t, path, npath);

+	freepath(t, path, npath, 1);

 	poperror();

 	if(npull != nmsg){

-- 

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