util: Forbid calling hash APIs from iterator callback

Calling most hash APIs is not safe from inside of an iterator callback.
Exceptions are APIs that do not modify the hash table and removing
current hash entry from virHashFroEach callback.

This patch make all APIs which are not safe fail instead of just relying
on the callback being nice not calling any unsafe APIs.

src/util/hash.c

@@ -35,6 +35,12 @@
 
 /* #define DEBUG_GROW */
 
+#define virHashIterationError(ret)                                      \
+    do {                                                                \
+        VIR_ERROR0(_("Hash operation not allowed during iteration"));   \
+        return ret;                                                     \
+    } while (0)
+
 /*
  * A single entry in the hash table
  */
@@ -54,6 +60,10 @@ struct _virHashTable {
     struct _virHashEntry *table;
     int size;
     int nbElems;
+    /* True iff we are iterating over hash entries. */
+    bool iterating;
+    /* Pointer to the current entry during iteration. */
+    virHashEntryPtr current;
     virHashDataFree dataFree;
     virHashKeyCode keyCode;
     virHashKeyEqual keyEqual;
@@ -316,6 +326,9 @@ virHashAddOrUpdateEntry(virHashTablePtr table, const void *name,
     if ((table == NULL) || (name == NULL))
         return (-1);
 
+    if (table->iterating)
+        virHashIterationError(-1);
+
     /*
      * Check for duplicate and insertion location.
      */
@@ -513,6 +526,8 @@ virHashRemoveEntry(virHashTablePtr table, const void *name)
         for (entry = &(table->table[key]); entry != NULL;
              entry = entry->next) {
             if (table->keyEqual(entry->name, name)) {
+                if (table->iterating && table->current != entry)
+                    virHashIterationError(-1);
                 if (table->dataFree && (entry->payload != NULL))
                     table->dataFree(entry->payload, entry->name);
                 entry->payload = NULL;
@@ -548,28 +563,38 @@ virHashRemoveEntry(virHashTablePtr table, const void *name)
  * @data: opaque data to pass to the iterator
  *
  * Iterates over every element in the hash table, invoking the
- * 'iter' callback. The callback is allowed to remove the element using
+ * 'iter' callback. The callback is allowed to remove the current element
- * virHashRemoveEntry but calling other virHash* functions is prohibited.
+ * using virHashRemoveEntry but calling other virHash* functions is prohibited.
  *
  * Returns number of items iterated over upon completion, -1 on failure
  */
-int virHashForEach(virHashTablePtr table, virHashIterator iter, void *data) {
+int virHashForEach(virHashTablePtr table, virHashIterator iter, void *data)
+{
     int i, count = 0;
 
     if (table == NULL || iter == NULL)
         return (-1);
 
+    if (table->iterating)
+        virHashIterationError(-1);
+
+    table->iterating = true;
+    table->current = NULL;
     for (i = 0 ; i < table->size ; i++) {
         virHashEntryPtr entry = table->table + i;
         while (entry) {
             virHashEntryPtr next = entry->next;
             if (entry->valid) {
+                table->current = entry;
                 iter(entry->payload, entry->name, data);
+                table->current = NULL;
                 count++;
             }
             entry = next;
         }
     }
+    table->iterating = false;
+
     return (count);
 }
 
@@ -593,6 +618,11 @@ int virHashRemoveSet(virHashTablePtr table, virHashSearcher iter, const void *da
     if (table == NULL || iter == NULL)
         return (-1);
 
+    if (table->iterating)
+        virHashIterationError(-1);
+
+    table->iterating = true;
+    table->current = NULL;
     for (i = 0 ; i < table->size ; i++) {
         virHashEntryPtr prev = NULL;
         virHashEntryPtr entry = &(table->table[i]);
@@ -629,6 +659,8 @@ int virHashRemoveSet(virHashTablePtr table, virHashSearcher iter, const void *da
             }
         }
     }
+    table->iterating = false;
+
     return (count);
 }
 
@@ -649,15 +681,24 @@ void *virHashSearch(virHashTablePtr table, virHashSearcher iter, const void *dat
     if (table == NULL || iter == NULL)
         return (NULL);
 
+    if (table->iterating)
+        virHashIterationError(NULL);
+
+    table->iterating = true;
+    table->current = NULL;
     for (i = 0 ; i < table->size ; i++) {
         virHashEntryPtr entry = table->table + i;
         while (entry) {
             if (entry->valid) {
-                if (iter(entry->payload, entry->name, data))
+                if (iter(entry->payload, entry->name, data)) {
+                    table->iterating = false;
                     return entry->payload;
+                }
             }
             entry = entry->next;
         }
     }
+    table->iterating = false;
+
     return (NULL);
 }