fuse: Allow growable exports

These will behave more like normal files in that writes beyond the EOF
will automatically grow the export size.

As an optimization, keep the RESIZE permission for growable exports so
we do not have to take it for every post-EOF write.  (This permission is
not released when the export is destroyed, because at that point the
BlockBackend is destroyed altogether anyway.)

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201027190600.192171-5-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/export/fuse.c

@@ -45,6 +45,7 @@ typedef struct FuseExport {
 
     char *mountpoint;
     bool writable;
+    bool growable;
 } FuseExport;
 
 static GHashTable *exports;
@@ -72,6 +73,19 @@ static int fuse_export_create(BlockExport *blk_exp,
 
     assert(blk_exp_args->type == BLOCK_EXPORT_TYPE_FUSE);
 
+    /* For growable exports, take the RESIZE permission */
+    if (args->growable) {
+        uint64_t blk_perm, blk_shared_perm;
+
+        blk_get_perm(exp->common.blk, &blk_perm, &blk_shared_perm);
+
+        ret = blk_set_perm(exp->common.blk, blk_perm | BLK_PERM_RESIZE,
+                           blk_shared_perm, errp);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
     init_exports_table();
 
     /*
@@ -102,6 +116,7 @@ static int fuse_export_create(BlockExport *blk_exp,
 
     exp->mountpoint = g_strdup(args->mountpoint);
     exp->writable = blk_exp_args->writable;
+    exp->growable = args->growable;
 
     ret = setup_fuse_export(exp, args->mountpoint, errp);
     if (ret < 0) {
@@ -349,19 +364,24 @@ static int fuse_do_truncate(const FuseExport *exp, int64_t size,
         truncate_flags |= BDRV_REQ_ZERO_WRITE;
     }
 
-    blk_get_perm(exp->common.blk, &blk_perm, &blk_shared_perm);
+    /* Growable exports have a permanent RESIZE permission */
+    if (!exp->growable) {
+        blk_get_perm(exp->common.blk, &blk_perm, &blk_shared_perm);
 
-    ret = blk_set_perm(exp->common.blk, blk_perm | BLK_PERM_RESIZE,
-                       blk_shared_perm, NULL);
-    if (ret < 0) {
-        return ret;
+        ret = blk_set_perm(exp->common.blk, blk_perm | BLK_PERM_RESIZE,
+                           blk_shared_perm, NULL);
+        if (ret < 0) {
+            return ret;
+        }
     }
 
     ret = blk_truncate(exp->common.blk, size, true, prealloc,
                        truncate_flags, NULL);
 
-    /* Must succeed, because we are only giving up the RESIZE permission */
-    blk_set_perm(exp->common.blk, blk_perm, blk_shared_perm, &error_abort);
+    if (!exp->growable) {
+        /* Must succeed, because we are only giving up the RESIZE permission */
+        blk_set_perm(exp->common.blk, blk_perm, blk_shared_perm, &error_abort);
+    }
 
     return ret;
 }
@@ -482,7 +502,15 @@ static void fuse_write(fuse_req_t req, fuse_ino_t inode, const char *buf,
     }
 
     if (offset + size > length) {
-        size = length - offset;
+        if (exp->growable) {
+            ret = fuse_do_truncate(exp, offset + size, true, PREALLOC_MODE_OFF);
+            if (ret < 0) {
+                fuse_reply_err(req, -ret);
+                return;
+            }
+        } else {
+            size = length - offset;
+        }
     }
 
     ret = blk_pwrite(exp->common.blk, offset, buf, size, 0);

qapi/block-export.json

@@ -129,10 +129,14 @@
 # @mountpoint: Path on which to export the block device via FUSE.
 #              This must point to an existing regular file.
 #
+# @growable: Whether writes beyond the EOF should grow the block node
+#            accordingly. (default: false)
+#
 # Since: 6.0
 ##
 { 'struct': 'BlockExportOptionsFuse',
-  'data': { 'mountpoint': 'str' },
+  'data': { 'mountpoint': 'str',
+            '*growable': 'bool' },
   'if': 'defined(CONFIG_FUSE)' }
 
 ##