commit 4dca4b639cb20fee38f6eec0a391aecc0ad8848d
Author: Naphtali Sprei <nsprei@redhat.com>
Date:   Sun Feb 14 13:39:18 2010 +0200

    block: more read-only changes, related to backing files
    
    Open backing file read-only where possible
    Upgrade backing file to read-write during commit, back to read-only after commit
      If upgrade fail, back to read-only. If also fail, "disconnect" the drive.
    
    Signed-off-by: Naphtali Sprei <nsprei@redhat.com>
    Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

diff --git a/block.c b/block.c
index af56ea7..31d1ba4 100644
--- a/block.c
+++ b/block.c
@@ -363,6 +363,7 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
     bs->is_temporary = 0;
     bs->encrypted = 0;
     bs->valid_key = 0;
+    bs->open_flags = flags;
     /* buffer_alignment defaulted to 512, drivers can change this value */
     bs->buffer_alignment = 512;
 
@@ -450,8 +451,6 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
     if (flags & (BDRV_O_CACHE_WB|BDRV_O_NOCACHE))
         bs->enable_write_cache = 1;
 
-    bs->read_only = (flags & BDRV_O_RDWR) == 0;
-
     /*
      * Clear flags that are internal to the block layer before opening the
      * image.
@@ -472,6 +471,7 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
         goto free_and_fail;
     }
 
+    bs->keep_read_only = bs->read_only = !(open_flags & BDRV_O_RDWR);
     if (drv->bdrv_getlength) {
         bs->total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
     }
@@ -488,13 +488,22 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
                      filename, bs->backing_file);
         if (bs->backing_format[0] != '\0')
             back_drv = bdrv_find_format(bs->backing_format);
+
+        /* backing files always opened read-only */
+        open_flags &= ~BDRV_O_RDWR;
+        
         ret = bdrv_open2(bs->backing_hd, backing_filename, open_flags,
                          back_drv);
-        bs->backing_hd->read_only =  (open_flags & BDRV_O_RDWR) == 0;
         if (ret < 0) {
             bdrv_close(bs);
             return ret;
         }
+        if (bs->is_temporary) {
+            bs->backing_hd->keep_read_only = !(flags & BDRV_O_RDWR);
+        } else {
+            /* base image inherits from "parent" */
+            bs->backing_hd->keep_read_only = bs->keep_read_only;
+        }
     }
 
     if (!bdrv_key_required(bs)) {
@@ -570,19 +579,48 @@ int bdrv_commit(BlockDriverState *bs)
 {
     BlockDriver *drv = bs->drv;
     int64_t i, total_sectors;
-    int n, j;
-    int ret = 0;
+    int n, j, ro, open_flags;
+    int ret = 0, rw_ret = 0;
     unsigned char sector[512];
+    char filename[1024];
+    BlockDriverState *bs_rw, *bs_ro;
 
     if (!drv)
         return -ENOMEDIUM;
-
-    if (bs->read_only) {
-	return -EACCES;
+    
+    if (!bs->backing_hd) {
+        return -ENOTSUP;
     }
 
-    if (!bs->backing_hd) {
-	return -ENOTSUP;
+    if (bs->backing_hd->keep_read_only) {
+        return -EACCES;
+    }
+    
+    ro = bs->backing_hd->read_only;
+    strncpy(filename, bs->backing_hd->filename, sizeof(filename));
+    open_flags =  bs->backing_hd->open_flags;
+
+    if (ro) {
+        /* re-open as RW */
+        bdrv_delete(bs->backing_hd);
+        bs->backing_hd = NULL;
+        bs_rw = bdrv_new("");
+        rw_ret = bdrv_open2(bs_rw, filename, open_flags | BDRV_O_RDWR, NULL);
+        if (rw_ret < 0) {
+            bdrv_delete(bs_rw);
+            /* try to re-open read-only */
+            bs_ro = bdrv_new("");
+            ret = bdrv_open2(bs_ro, filename, open_flags & ~BDRV_O_RDWR, NULL);
+            if (ret < 0) {
+                bdrv_delete(bs_ro);
+                /* drive not functional anymore */
+                bs->drv = NULL;
+                return ret;
+            }
+            bs->backing_hd = bs_ro;
+            return rw_ret;
+        }
+        bs->backing_hd = bs_rw;
     }
 
     total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
@@ -590,11 +628,13 @@ int bdrv_commit(BlockDriverState *bs)
         if (drv->bdrv_is_allocated(bs, i, 65536, &n)) {
             for(j = 0; j < n; j++) {
                 if (bdrv_read(bs, i, sector, 1) != 0) {
-                    return -EIO;
+                    ret = -EIO;
+                    goto ro_cleanup;
                 }
 
                 if (bdrv_write(bs->backing_hd, i, sector, 1) != 0) {
-                    return -EIO;
+                    ret = -EIO;
+                    goto ro_cleanup;
                 }
                 i++;
 	    }
@@ -614,6 +654,25 @@ int bdrv_commit(BlockDriverState *bs)
      */
     if (bs->backing_hd)
         bdrv_flush(bs->backing_hd);
+
+ro_cleanup:
+
+    if (ro) {
+        /* re-open as RO */
+        bdrv_delete(bs->backing_hd);
+        bs->backing_hd = NULL;
+        bs_ro = bdrv_new("");
+        ret = bdrv_open2(bs_ro, filename, open_flags & ~BDRV_O_RDWR, NULL);
+        if (ret < 0) {
+            bdrv_delete(bs_ro);
+            /* drive not functional anymore */
+            bs->drv = NULL;
+            return ret;
+        }
+        bs->backing_hd = bs_ro;
+        bs->backing_hd->keep_read_only = 0;
+    }
+
     return ret;
 }
 
