storage: make it easier to find file within chain

In order to temporarily label files read/write during a commit
operation, we need to crawl the backing chain and find the absolute
file name that needs labeling in the first place, as well as the
name of the file that owns the backing file.

* src/util/storage_file.c (virStorageFileChainLookup): New
function.
* src/util/storage_file.h: Declare it.
* src/libvirt_private.syms (storage_file.h): Export it.

src/libvirt_private.syms

@@ -1132,6 +1132,7 @@ virStorageGenerateQcowPassphrase;
 
 
 # storage_file.h
+virStorageFileChainLookup;
 virStorageFileFormatTypeFromString;
 virStorageFileFormatTypeToString;
 virStorageFileFreeMetadata;

src/util/storage_file.c

@@ -1262,3 +1262,67 @@ const char *virStorageFileGetSCSIKey(const char *path)
     return NULL;
 }
 #endif
+
+/* Given a CHAIN that starts at the named file START, return a string
+ * pointing to either START or within CHAIN that gives the preferred
+ * name for the backing file NAME within that chain.  Pass NULL for
+ * NAME to find the base of the chain.  If META is not NULL, set *META
+ * to the point in the chain that describes NAME (or to NULL if the
+ * backing element is not a file).  If PARENT is not NULL, set *PARENT
+ * to the preferred name of the parent (or to NULL if NAME matches
+ * START).  Since the results point within CHAIN, they must not be
+ * independently freed.  */
+const char *
+virStorageFileChainLookup(virStorageFileMetadataPtr chain, const char *start,
+                          const char *name, virStorageFileMetadataPtr *meta,
+                          const char **parent)
+{
+    virStorageFileMetadataPtr owner;
+    const char *tmp;
+
+    if (!parent)
+        parent = &tmp;
+
+    *parent = NULL;
+    if (name ? STREQ(start, name) || virFileLinkPointsTo(start, name) :
+        !chain->backingStore) {
+        if (meta)
+            *meta = chain;
+        return start;
+    }
+
+    owner = chain;
+    *parent = start;
+    while (owner) {
+        if (!owner->backingStore)
+            goto error;
+        if (!name) {
+            if (!owner->backingMeta ||
+                !owner->backingMeta->backingStore)
+                break;
+        } else if (STREQ_NULLABLE(name, owner->backingStoreRaw) ||
+                   STREQ(name, owner->backingStore)) {
+            break;
+        } else if (owner->backingStoreIsFile) {
+            char *absName = absolutePathFromBaseFile(*parent, name);
+            if (absName && STREQ(absName, owner->backingStore)) {
+                VIR_FREE(absName);
+                break;
+            }
+            VIR_FREE(absName);
+        }
+        *parent = owner->backingStore;
+        owner = owner->backingMeta;
+    }
+    if (!owner)
+        goto error;
+    if (meta)
+        *meta = owner->backingMeta;
+    return owner->backingStore;
+
+error:
+    *parent = NULL;
+    if (meta)
+        *meta = NULL;
+    return NULL;
+}

src/util/storage_file.h

@@ -78,6 +78,13 @@ virStorageFileMetadataPtr virStorageFileGetMetadataFromFD(const char *path,
                                                           int fd,
                                                           int format);
 
+const char *virStorageFileChainLookup(virStorageFileMetadataPtr chain,
+                                      const char *start,
+                                      const char *name,
+                                      virStorageFileMetadataPtr *meta,
+                                      const char **parent)
+    ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
+
 void virStorageFileFreeMetadata(virStorageFileMetadataPtr meta);
 
 int virStorageFileResize(const char *path, unsigned long long capacity);