conf: tweak chain lookup internals

Thanks to the testsuite, I feel quite confident that this rewrite
is correct; it gives the same results for all cases except for one.
I can make the argument that _that_ case was a pre-existing bug:
when looking up relative names, the lookup is supposed to be
pegged to the directory that contains the parent qcow2 file.  Thus,
this resolves the fixme first mentioned in commit 367cd69d (even
though I accidentally removed the fixme comment early in 74430fe3).

* src/util/virstoragefile.c (virStorageFileChainLookup): Depend on
new rather than old fields.
* tests/virstoragetest.c (mymain): Adjust test to match fix.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/util/virstoragefile.c

@@ -1544,48 +1544,40 @@ virStorageFileChainLookup(virStorageFileMetadataPtr chain,
                           const char **parent)
 {
     const char *start = chain->canonPath;
-    virStorageFileMetadataPtr owner;
     const char *tmp;
+    const char *parentDir = ".";
+    bool nameIsFile = virStorageIsFile(name);
 
     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;
+    while (chain) {
         if (!name) {
-            if (!owner->backingMeta ||
-                !owner->backingMeta->backingStore)
+            if (!chain->backingMeta)
                 break;
-        } else if (STREQ_NULLABLE(name, owner->backingStoreRaw) ||
-                   STREQ(name, owner->backingStore)) {
-            break;
-        } else if (virStorageIsFile(owner->backingStore)) {
-            int result = virFileRelLinkPointsTo(owner->directory, name,
-                                                owner->backingStore);
-            if (result < 0)
-                goto error;
-            if (result > 0)
+        } else {
+            if (STREQ(name, chain->path))
                 break;
+            if (nameIsFile && (chain->type == VIR_STORAGE_TYPE_FILE ||
+                               chain->type == VIR_STORAGE_TYPE_BLOCK)) {
+                int result = virFileRelLinkPointsTo(parentDir, name,
+                                                    chain->canonPath);
+                if (result < 0)
+                    goto error;
+                if (result > 0)
+                    break;
+            }
         }
-        *parent = owner->backingStore;
-        owner = owner->backingMeta;
+        *parent = chain->canonPath;
+        parentDir = chain->relDir;
+        chain = chain->backingMeta;
     }
-    if (!owner)
+    if (!chain)
         goto error;
     if (meta)
-        *meta = owner->backingMeta;
-    return owner->backingStore;
+        *meta = chain;
+    return chain->canonPath;
 
  error:
     if (name)

tests/virstoragetest.c

@@ -935,8 +935,7 @@ mymain(void)
     TEST_LOOKUP(19, abswrap, chain->canonPath, chain, NULL);
     TEST_LOOKUP(20, "../qcow2", chain->backingStore, chain->backingMeta,
                 chain->canonPath);
-    TEST_LOOKUP(21, "qcow2", chain->backingStore, chain->backingMeta,
-                chain->canonPath);
+    TEST_LOOKUP(21, "qcow2", NULL, NULL, NULL);
     TEST_LOOKUP(22, absqcow2, chain->backingStore, chain->backingMeta,
                 chain->canonPath);
     TEST_LOOKUP(23, "raw", chain->backingMeta->backingStore,