[libvirt] [PATCH] Allow non-file disk backingStores

[Adam Litke]

I am trying to use a qcow image with libvirt where the backing 'file' is a
qemu-nbd server.  Unfortunately virDomainDiskDefForeachPath() assumes that
backingStore is always a real file so something like 'nbd:0:3333' is rejected
because a file with that name cannot be accessed.  Note that I am not worried
about directly using nbd images.  That would require a new disk type with XML
markup, etc.  I only want it to be permitted as a backingStore

The following patch implements danpb's suggestion:
> I think I'm inclined to push the logic for skipping NBD one stage higher.
> I'd rather expect virStorageFileGetMetadata() to return all backing
> stores, even if not files. The virDomainDiskDefForeachPath() method
> should definitely ignore non-file backing stores though.
>
> So what I'm thinking is to extend the virStorageFileMetadata struct and
> just add a 'bool isFile' field to it. Default this field to true, unless
> you see the prefix of nbd: in which case set it to false. The
> virDomainDiskDefForeachPath() method can then skip over any backing
> store with isFile == false

Signed-off-by: Adam Litke <agl at us.ibm.com>
Cc: Daniel P. Berrange <berrange at redhat.com>
---
 src/conf/domain_conf.c  |    7 +++++++
 src/util/storage_file.c |   16 +++++++++++++++-
 src/util/storage_file.h |    1 +
 3 files changed, 23 insertions(+), 1 deletions(-)

diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index fe93711..2d11785 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -7813,6 +7813,13 @@ int virDomainDiskDefForeachPath(virDomainDiskDefPtr disk,
         depth++;
         nextpath = meta.backingStore;
 
+        /* Stop iterating if we reach a non-file backing store */
+        if (nextpath && meta.backingStoreIsFile == false) {
+            VIR_DEBUG("Stopping iteration on non-file backing store: %s",
+                      nextpath);
+            break;
+        }
+
         format = meta.backingStoreFormat;
 
         if (format == VIR_STORAGE_FILE_AUTO &&
diff --git a/src/util/storage_file.c b/src/util/storage_file.c
index 6f48b10..6e099ed 100644
--- a/src/util/storage_file.c
+++ b/src/util/storage_file.c
@@ -509,6 +509,14 @@ virStorageFileMatchesVersion(int format,
     return true;
 }
 
+static bool
+virBackingStoreIsFile(const char *backing)
+{
+    /* Backing store is a network block device */
+    if (STRPREFIX(backing, "nbd:"))
+        return false;
+    return true;
+}
 
 static int
 virStorageFileGetMetadataFromBuf(int format,
@@ -579,8 +587,14 @@ virStorageFileGetMetadataFromBuf(int format,
         if (ret == BACKING_STORE_ERROR)
             return -1;
 
+        meta->backingStoreIsFile = false;
         if (backing != NULL) {
-            meta->backingStore = absolutePathFromBaseFile(path, backing);
+            if (virBackingStoreIsFile(backing)) {
+                meta->backingStoreIsFile = true;
+                meta->backingStore = absolutePathFromBaseFile(path, backing);
+            } else {
+                meta->backingStore = strdup(backing);
+            }
             VIR_FREE(backing);
             if (meta->backingStore == NULL) {
                 virReportOOMError();
diff --git a/src/util/storage_file.h b/src/util/storage_file.h
index fb0c005..c4d4650 100644
--- a/src/util/storage_file.h
+++ b/src/util/storage_file.h
@@ -49,6 +49,7 @@ VIR_ENUM_DECL(virStorageFileFormat);
 typedef struct _virStorageFileMetadata {
     char *backingStore;
     int backingStoreFormat;
+    bool backingStoreIsFile;
     unsigned long long capacity;
     bool encrypted;
 } virStorageFileMetadata;
-- 
1.7.1


-- 
Thanks,
Adam