Move handles.h to src/lib/container

There might be a better place for it in the long run, but this is
the best we can think of for now.

1 files changed, 153 insertions, 0 deletions

diff --git a/src/lib/container/handles.h b/src/lib/container/handles.h
new file mode 100644
index 0000000000..21ec0dfeec
--- /dev/null
+++ b/src/lib/container/handles.h
@@ -0,0 +1,153 @@
+/* Copyright (c) 2016-2018, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file handles.h
+ * \brief Macros for C weak-handle implementation.
+ *
+ * A 'handle' is a pointer to an object that is allowed to go away while
+ * the handle stays alive.  When you dereference the handle, you might get
+ * the object, or you might get "NULL".
+ *
+ * Use this pattern when an object has a single obvious lifespan, so you don't
+ * want to use reference counting, but when other objects might need to refer
+ * to the first object without caring about its lifetime.
+ *
+ * To enable a type to have handles, add a HANDLE_ENTRY() field in its
+ * definition, as in:
+ *
+ *     struct walrus {
+ *         HANDLE_ENTRY(wlr, walrus);
+ *         // ...
+ *     };
+ *
+ * And invoke HANDLE_DECL(wlr, walrus, [static]) to declare the handle
+ * manipulation functions (typically in a header):
+ *
+ *     // opaque handle to walrus.
+ *     typedef struct wlr_handle_t wlr_handle_t;
+ *
+ *     // make a new handle
+ *     struct wlr_handle_t *wlr_handle_new(struct walrus *);
+ *
+ *     // release a handle
+ *     void wlr_handle_free(wlr_handle_t *);
+ *
+ *     // return the pointed-to walrus, or NULL.
+ *     struct walrus *wlr_handle_get(wlr_handle_t *).
+ *
+ *     // call this function when you're about to free the walrus;
+ *     // it invalidates all handles. (IF YOU DON'T, YOU WILL HAVE
+ *     // DANGLING REFERENCES)
+ *     void wlr_handles_clear(struct walrus *);
+ *
+ * Finally, use HANDLE_IMPL() to define the above functions in some
+ * appropriate C file: HANDLE_IMPL(wlr, walrus, [static])
+ *
+ **/
+
+#ifndef TOR_HANDLE_H
+#define TOR_HANDLE_H
+
+#include "orconfig.h"
+
+#include "lib/log/util_bug.h"
+#include "lib/malloc/util_malloc.h"
+
+#define HANDLE_ENTRY(name, structname)         \
+  struct name ## _handle_head_t *handle_head
+
+#define HANDLE_DECL(name, structname, linkage)                          \
+  typedef struct name ## _handle_t name ## _handle_t;                   \
+  linkage  name ## _handle_t *name ## _handle_new(struct structname *object); \
+  linkage void name ## _handle_free_(name ## _handle_t *);              \
+  linkage struct structname *name ## _handle_get(name ## _handle_t *);  \
+  linkage void name ## _handles_clear(struct structname *object);
+
+/*
+ * Implementation notes: there are lots of possible implementations here.  We
+ * could keep a linked list of handles, each with a backpointer to the object,
+ * and set all of their backpointers to NULL when the object is freed.  Or we
+ * could have the clear function invalidate the object, but not actually let
+ * the object get freed until the all the handles went away.  We could even
+ * have a hash-table mapping unique identifiers to objects, and have each
+ * handle be a copy of the unique identifier.  (We'll want to build that last
+ * one eventually if we want cross-process handles.)
+ *
+ * But instead we're opting for a single independent 'head' that knows how
+ * many handles there are, and where the object is (or isn't).  This makes
+ * all of our functions O(1), and most as fast as a single pointer access.
+ *
+ * The handles themselves are opaque structures holding a pointer to the head.
+ * We could instead have each foo_handle_t* be identical to foo_handle_head_t
+ * *, and save some allocations ... but doing so would make handle leaks
+ * harder to debug.  As it stands, every handle leak is a memory leak, and
+ * existing memory debugging tools should help with those.  We can revisit
+ * this decision if handles are too slow.
+ */
+
+#define HANDLE_IMPL(name, structname, linkage)                          \
+  /* The 'head' object for a handle-accessible type. This object */     \
+  /* persists for as long as the object, or any handles, exist. */      \
+  typedef struct name ## _handle_head_t {                               \
+    struct structname *object; /* pointed-to object, or NULL */         \
+    unsigned int references; /* number of existing handles */           \
+  } name ## _handle_head_t;                                             \
+                                                                        \
+  struct name ## _handle_t {                                            \
+    struct name ## _handle_head_t *head; /* reference to the 'head'. */ \
+  };                                                                    \
+                                                                        \
+  linkage struct name ## _handle_t *                                    \
+  name ## _handle_new(struct structname *object)                        \
+  {                                                                     \
+    tor_assert(object);                                                 \
+    name ## _handle_head_t *head = object->handle_head;                 \
+    if (PREDICT_UNLIKELY(head == NULL)) {                               \
+      head = object->handle_head = tor_malloc_zero(sizeof(*head));      \
+      head->object = object;                                            \
+    }                                                                   \
+    name ## _handle_t *new_ref = tor_malloc_zero(sizeof(*new_ref));     \
+    new_ref->head = head;                                               \
+    ++head->references;                                                 \
+    return new_ref;                                                     \
+  }                                                                     \
+                                                                        \
+  linkage void                                                          \
+  name ## _handle_free_(struct name ## _handle_t *ref)                   \
+  {                                                                     \
+    if (! ref) return;                                                  \
+    name ## _handle_head_t *head = ref->head;                           \
+    tor_assert(head);                                                   \
+    --head->references;                                                 \
+    tor_free(ref);                                                      \
+    if (head->object == NULL && head->references == 0) {                \
+      tor_free(head);                                                   \
+      return;                                                           \
+    }                                                                   \
+  }                                                                     \
+                                                                        \
+  linkage struct structname *                                           \
+  name ## _handle_get(struct name ## _handle_t *ref)                    \
+  {                                                                     \
+    tor_assert(ref);                                                    \
+    name ## _handle_head_t *head = ref->head;                           \
+    tor_assert(head);                                                   \
+    return head->object;                                                \
+  }                                                                     \
+                                                                        \
+  linkage void                                                          \
+  name ## _handles_clear(struct structname *object)                     \
+  {                                                                     \
+    tor_assert(object);                                                 \
+    name ## _handle_head_t *head = object->handle_head;                 \
+    if (! head)                                                         \
+      return;                                                           \
+    object->handle_head = NULL;                                         \
+    head->object = NULL;                                                \
+    if (head->references == 0) {                                        \
+      tor_free(head);                                                   \
+    }                                                                   \
+  }
+
+#endif /* !defined(TOR_HANDLE_H) */