qemu: refactor qemuDomainGetBlockInfo to work with remote storage

The qemu block info function relied on working with local storage. Break
this assumption by adding support for remote volumes. Unfortunately we
still need to take a hybrid approach as some of the operations require a
filedescriptor.

Previously you'd get:
 $ virsh domblkinfo gl vda
 error: cannot stat file '/img10': Bad file descriptor

Now you get some stats:
 $ virsh domblkinfo gl vda
 Capacity:       10485760
 Allocation:     197120
 Physical:       197120

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1110198

src/libvirt_private.syms

@@ -1910,6 +1910,7 @@ virStorageFileGetSCSIKey;
 virStorageFileIsClusterFS;
 virStorageFileParseChainIndex;
 virStorageFileProbeFormat;
+virStorageFileProbeFormatFromBuf;
 virStorageFileResize;
 virStorageIsFile;
 virStorageNetHostDefClear;

src/qemu/qemu_driver.c

@@ -10334,11 +10334,13 @@ qemuDomainGetBlockInfo(virDomainPtr dom,
     int activeFail = false;
     virQEMUDriverConfigPtr cfg = NULL;
     char *alias = NULL;
+    char *buf = NULL;
+    ssize_t len;
 
     virCheckFlags(0, -1);
 
     if (!(vm = qemuDomObjFromDomain(dom)))
-        goto cleanup;
+        return -1;
 
     cfg = virQEMUDriverGetConfig(driver);
 
@@ -10346,8 +10348,7 @@ qemuDomainGetBlockInfo(virDomainPtr dom,
         goto cleanup;
 
     if (!path || path[0] == '\0') {
-        virReportError(VIR_ERR_INVALID_ARG,
-                       "%s", _("NULL or empty path"));
+        virReportError(VIR_ERR_INVALID_ARG, "%s", _("NULL or empty path"));
         goto cleanup;
     }
 
@@ -10357,48 +10358,68 @@ qemuDomainGetBlockInfo(virDomainPtr dom,
                        _("invalid path %s not assigned to domain"), path);
         goto cleanup;
     }
-    disk = vm->def->disks[idx];
-    path = virDomainDiskGetSource(disk);
-    if (!path) {
-        virReportError(VIR_ERR_INVALID_ARG,
-                       _("disk %s does not currently have a source assigned"),
-                       path);
-        goto cleanup;
-    }
 
-    /* The path is correct, now try to open it and get its size. */
-    fd = qemuOpenFile(driver, vm, path, O_RDONLY, NULL, NULL);
-    if (fd == -1)
-        goto cleanup;
+    disk = vm->def->disks[idx];
+
+    if (virStorageSourceIsLocalStorage(disk->src)) {
+        if (!disk->src->path) {
+            virReportError(VIR_ERR_INVALID_ARG,
+                           _("disk '%s' does not currently have a source assigned"),
+                           path);
+            goto cleanup;
+        }
+
+        if ((fd = qemuOpenFile(driver, vm, path, O_RDONLY, NULL, NULL)) == -1)
+            goto cleanup;
+
+        if (fstat(fd, &sb) < 0) {
+            virReportSystemError(errno,
+                                 _("cannot stat file '%s'"), disk->src->path);
+            goto cleanup;
+        }
+
+        if ((len = virFileReadHeaderFD(fd, VIR_STORAGE_MAX_HEADER, &buf)) < 0) {
+            virReportSystemError(errno, _("cannot read header '%s'"),
+                                 disk->src->path);
+            goto cleanup;
+        }
+    } else {
+        if (virStorageFileInitAs(disk->src, cfg->user, cfg->group) < 0)
+            goto cleanup;
+
+        if ((len = virStorageFileReadHeader(disk->src, VIR_STORAGE_MAX_HEADER,
+                                            &buf)) < 0)
+            goto cleanup;
+
+        if (virStorageFileStat(disk->src, &sb) < 0) {
+            virReportSystemError(errno, _("failed to stat remote file '%s'"),
+                                 NULLSTR(disk->src->path));
+            goto cleanup;
+        }
+    }
 
     /* Probe for magic formats */
     if (virDomainDiskGetFormat(disk)) {
         format = virDomainDiskGetFormat(disk);
     } else {
-        if (cfg->allowDiskFormatProbing) {
-            if ((format = virStorageFileProbeFormat(path,
-                                                    cfg->user,
-                                                    cfg->group)) < 0)
-                goto cleanup;
-        } else {
+        if (!cfg->allowDiskFormatProbing) {
             virReportError(VIR_ERR_INTERNAL_ERROR,
                            _("no disk format for %s and probing is disabled"),
                            path);
             goto cleanup;
         }
+
+        if ((format = virStorageFileProbeFormatFromBuf(disk->src->path,
+                                                       buf, len)) < 0)
+            goto cleanup;
     }
 
-    if (!(meta = virStorageFileGetMetadataFromFD(path, fd, format, NULL)))
+    if (!(meta = virStorageFileGetMetadataFromBuf(disk->src->path, buf, len,
+                                                  format, NULL)))
         goto cleanup;
 
     /* Get info for normal formats */
-    if (fstat(fd, &sb) < 0) {
-        virReportSystemError(errno,
-                             _("cannot stat file '%s'"), path);
-        goto cleanup;
-    }
-
-    if (S_ISREG(sb.st_mode)) {
+    if (S_ISREG(sb.st_mode) || fd == -1) {
 #ifndef WIN32
         info->physical = (unsigned long long)sb.st_blocks *
             (unsigned long long)DEV_BSIZE;
@@ -10434,7 +10455,7 @@ qemuDomainGetBlockInfo(virDomainPtr dom,
     /* ..but if guest is not using raw disk format and on a block device,
      * then query highest allocated extent from QEMU
      */
-    if (virDomainDiskGetType(disk) == VIR_STORAGE_TYPE_BLOCK &&
+    if (virStorageSourceGetActualType(disk->src) == VIR_STORAGE_TYPE_BLOCK &&
         format != VIR_STORAGE_FILE_RAW &&
         S_ISBLK(sb.st_mode)) {
         qemuDomainObjPrivatePtr priv = vm->privateData;
@@ -10475,6 +10496,8 @@ qemuDomainGetBlockInfo(virDomainPtr dom,
     VIR_FREE(alias);
     virStorageSourceFree(meta);
     VIR_FORCE_CLOSE(fd);
+    if (disk)
+        virStorageFileDeinit(disk->src);
 
     /* If we failed to get data from a domain because it's inactive and
      * it's not a persistent domain, then force failure.
@@ -10484,8 +10507,7 @@ qemuDomainGetBlockInfo(virDomainPtr dom,
                        _("domain is not running"));
         ret = -1;
     }
-    if (vm)
-        virObjectUnlock(vm);
+    virObjectUnlock(vm);
     virObjectUnref(cfg);
     return ret;
 }

src/util/virstoragefile.c

@@ -682,7 +682,7 @@ virStorageIsRelative(const char *backing)
 }
 
 
-static int
+int
 virStorageFileProbeFormatFromBuf(const char *path,
                                  char *buf,
                                  size_t buflen)

src/util/virstoragefile.h

@@ -276,6 +276,9 @@ struct _virStorageSource {
 # endif
 
 int virStorageFileProbeFormat(const char *path, uid_t uid, gid_t gid);
+int virStorageFileProbeFormatFromBuf(const char *path,
+                                     char *buf,
+                                     size_t buflen);
 
 int virStorageFileGetMetadataInternal(virStorageSourcePtr meta,
                                       char *buf,