storage: backend: Add unique id retrieval API

Different protocols have different means to uniquely identify a storage
file. This patch implements a storage driver API to retrieve a unique
string describing a volume. The current implementation works for local
storage only and returns the canonical path of the volume.

To add caching support the local filesystem driver now has a private
structure holding the cached string, which is created only when it's
initially accessed.

This patch provides the implementation for local files only for start.

src/storage/storage_backend.h

@@ -195,6 +195,8 @@ typedef ssize_t
                                    ssize_t max_len,
                                    char **buf);
 
+typedef const char *
+(*virStorageFileBackendGetUniqueIdentifier)(virStorageSourcePtr src);
 
 virStorageFileBackendPtr virStorageFileBackendForType(int type, int protocol);
 
@@ -211,6 +213,7 @@ struct _virStorageFileBackend {
     virStorageFileBackendInit backendInit;
     virStorageFileBackendDeinit backendDeinit;
     virStorageFileBackendReadHeader storageFileReadHeader;
+    virStorageFileBackendGetUniqueIdentifier storageFileGetUniqueIdentifier;
 
     /* The following group of callbacks is expected to set errno
      * and return -1 on error. No libvirt error shall be reported */

src/storage/storage_backend_fs.c

@@ -1333,6 +1333,14 @@ virStorageBackend virStorageBackendNetFileSystem = {
 };
 
 
+typedef struct _virStorageFileBackendFsPriv virStorageFileBackendFsPriv;
+typedef virStorageFileBackendFsPriv *virStorageFileBackendFsPrivPtr;
+
+struct _virStorageFileBackendFsPriv {
+    char *canonpath; /* unique file identifier (canonical path) */
+};
+
+
 static void
 virStorageFileBackendFileDeinit(virStorageSourcePtr src)
 {
@@ -1340,16 +1348,27 @@ virStorageFileBackendFileDeinit(virStorageSourcePtr src)
               virStorageTypeToString(virStorageSourceGetActualType(src)),
               src->path);
 
+    virStorageFileBackendFsPrivPtr priv = src->drv->priv;
+
+    VIR_FREE(priv->canonpath);
+    VIR_FREE(priv);
 }
 
 
 static int
 virStorageFileBackendFileInit(virStorageSourcePtr src)
 {
+    virStorageFileBackendFsPrivPtr priv = NULL;
+
     VIR_DEBUG("initializing FS storage file %p (%s:%s)", src,
               virStorageTypeToString(virStorageSourceGetActualType(src)),
               src->path);
 
+    if (VIR_ALLOC(priv) < 0)
+        return -1;
+
+    src->drv->priv = priv;
+
     return 0;
 }
 
@@ -1397,6 +1416,23 @@ virStorageFileBackendFileReadHeader(virStorageSourcePtr src,
 }
 
 
+static const char *
+virStorageFileBackendFileGetUniqueIdentifier(virStorageSourcePtr src)
+{
+    virStorageFileBackendFsPrivPtr priv = src->drv->priv;
+
+    if (!priv->canonpath) {
+        if (!(priv->canonpath = canonicalize_file_name(src->path))) {
+            virReportSystemError(errno, _("can't canonicalize path '%s'"),
+                                 src->path);
+            return NULL;
+        }
+    }
+
+    return priv->canonpath;
+}
+
+
 virStorageFileBackend virStorageFileBackendFile = {
     .type = VIR_STORAGE_TYPE_FILE,
 
@@ -1406,6 +1442,8 @@ virStorageFileBackend virStorageFileBackendFile = {
     .storageFileUnlink = virStorageFileBackendFileUnlink,
     .storageFileStat = virStorageFileBackendFileStat,
     .storageFileReadHeader = virStorageFileBackendFileReadHeader,
+
+    .storageFileGetUniqueIdentifier = virStorageFileBackendFileGetUniqueIdentifier,
 };
 
 
@@ -1417,7 +1455,18 @@ virStorageFileBackend virStorageFileBackendBlock = {
 
     .storageFileStat = virStorageFileBackendFileStat,
     .storageFileReadHeader = virStorageFileBackendFileReadHeader,
+
+    .storageFileGetUniqueIdentifier = virStorageFileBackendFileGetUniqueIdentifier,
 };
 
 
+virStorageFileBackend virStorageFileBackendDir = {
+    .type = VIR_STORAGE_TYPE_DIR,
+
+    .backendInit = virStorageFileBackendFileInit,
+    .backendDeinit = virStorageFileBackendFileDeinit,
+
+    .storageFileGetUniqueIdentifier = virStorageFileBackendFileGetUniqueIdentifier,
+};
+
 #endif /* WITH_STORAGE_FS */

src/storage/storage_driver.c

@@ -2987,3 +2987,33 @@ virStorageFileReadHeader(virStorageSourcePtr src,
 
     return ret;
 }
+
+
+/*
+ * virStorageFileGetUniqueIdentifier: Get a unique string describing the volume
+ *
+ * @src: file structure pointing to the file
+ *
+ * Returns a string uniquely describing a single volume (canonical path).
+ * The string shall not be freed and is valid until the storage file is
+ * deinitialized. Returns NULL on error and sets a libvirt error code */
+const char *
+virStorageFileGetUniqueIdentifier(virStorageSourcePtr src)
+{
+    if (!virStorageFileIsInitialized(src)) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("storage file backend not initialized"));
+        return NULL;
+    }
+
+    if (!src->drv->backend->storageFileGetUniqueIdentifier) {
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("unique storage file identifier not implemented for "
+                          "storage type %s (protocol: %s)'"),
+                       virStorageTypeToString(src->type),
+                       virStorageNetProtocolTypeToString(src->protocol));
+        return NULL;
+    }
+
+    return src->drv->backend->storageFileGetUniqueIdentifier(src);
+}

src/storage/storage_driver.h

@@ -41,6 +41,7 @@ int virStorageFileStat(virStorageSourcePtr src,
 ssize_t virStorageFileReadHeader(virStorageSourcePtr src,
                                  ssize_t max_len,
                                  char **buf);
+const char *virStorageFileGetUniqueIdentifier(virStorageSourcePtr src);
 
 int storageRegister(void);