Invent a new memory context primitive, MemoryContextSetParent.

This function will be useful for altering the lifespan of a context after
creation (for example, by creating it under a transient context and later
reparenting it to belong to a long-lived context).  It costs almost no new
code, since we can refactor what was there.  Per my proposal of yesterday.

diff --git a/src/backend/utils/mmgr/mcxt.c b/src/backend/utils/mmgr/mcxt.c
index 7ed35d5bdf123ed6b1725edf27d32e94c20aebb4..4eab24c46c7ab0f0b2a09770ac44ce2c6f4c5554 100644 (file)
--- a/src/backend/utils/mmgr/mcxt.c
+++ b/src/backend/utils/mmgr/mcxt.c
@@ -178,26 +178,8 @@ MemoryContextDelete(MemoryContext context)
     * there's an error we won't have deleted/busted contexts still attached
     * to the context tree.  Better a leak than a crash.
     */
-   if (context->parent)
-   {
-       MemoryContext parent = context->parent;
-
-       if (context == parent->firstchild)
-           parent->firstchild = context->nextchild;
-       else
-       {
-           MemoryContext child;
+   MemoryContextSetParent(context, NULL);
 
-           for (child = parent->firstchild; child; child = child->nextchild)
-           {
-               if (context == child->nextchild)
-               {
-                   child->nextchild = context->nextchild;
-                   break;
-               }
-           }
-       }
-   }
    (*context->methods->delete_context) (context);
    pfree(context);
 }
@@ -237,6 +219,67 @@ MemoryContextResetAndDeleteChildren(MemoryContext context)
    MemoryContextReset(context);
 }
 
+/*
+ * MemoryContextSetParent
+ *     Change a context to belong to a new parent (or no parent).
+ *
+ * We provide this as an API function because it is sometimes useful to
+ * change a context's lifespan after creation.  For example, a context
+ * might be created underneath a transient context, filled with data,
+ * and then reparented underneath CacheMemoryContext to make it long-lived.
+ * In this way no special effort is needed to get rid of the context in case
+ * a failure occurs before its contents are completely set up.
+ *
+ * Callers often assume that this function cannot fail, so don't put any
+ * elog(ERROR) calls in it.
+ *
+ * A possible caller error is to reparent a context under itself, creating
+ * a loop in the context graph.  We assert here that context != new_parent,
+ * but checking for multi-level loops seems more trouble than it's worth.
+ */
+void
+MemoryContextSetParent(MemoryContext context, MemoryContext new_parent)
+{
+   AssertArg(MemoryContextIsValid(context));
+   AssertArg(context != new_parent);
+
+   /* Delink from existing parent, if any */
+   if (context->parent)
+   {
+       MemoryContext parent = context->parent;
+
+       if (context == parent->firstchild)
+           parent->firstchild = context->nextchild;
+       else
+       {
+           MemoryContext child;
+
+           for (child = parent->firstchild; child; child = child->nextchild)
+           {
+               if (context == child->nextchild)
+               {
+                   child->nextchild = context->nextchild;
+                   break;
+               }
+           }
+       }
+   }
+
+   /* And relink */
+   if (new_parent)
+   {
+       AssertArg(MemoryContextIsValid(new_parent));
+       context->parent = new_parent;
+       context->nextchild = new_parent->firstchild;
+       new_parent->firstchild = context;
+   }
+   else
+   {
+       context->parent = NULL;
+       context->nextchild = NULL;
+   }
+}
+
 /*
  * GetMemoryChunkSpace
  *     Given a currently-allocated chunk, determine the total space
@@ -489,6 +532,7 @@ MemoryContextCreate(NodeTag tag, Size size,
    (*node->methods->init) (node);
 
    /* OK to link node to parent (if any) */
+   /* Could use MemoryContextSetParent here, but doesn't seem worthwhile */
    if (parent)
    {
        node->parent = parent;

diff --git a/src/include/utils/memutils.h b/src/include/utils/memutils.h
index 7c1202478e5a0ae38c71bcda7da8e5ab4e036a2d..94c78289b6e818c0a097f6492b6e19bd98c04430 100644 (file)
--- a/src/include/utils/memutils.h
+++ b/src/include/utils/memutils.h
@@ -90,6 +90,8 @@ extern void MemoryContextDelete(MemoryContext context);
 extern void MemoryContextResetChildren(MemoryContext context);
 extern void MemoryContextDeleteChildren(MemoryContext context);
 extern void MemoryContextResetAndDeleteChildren(MemoryContext context);
+extern void MemoryContextSetParent(MemoryContext context,
+                                  MemoryContext new_parent);
 extern Size GetMemoryChunkSpace(void *pointer);
 extern MemoryContext GetMemoryChunkContext(void *pointer);
 extern bool MemoryContextIsEmpty(MemoryContext context);