buflib: add pin/unpin operation

An allocation is pinned by calling buflib_pin() to up its pin count.
The pin count is like a reference count: when above 0, buflib won't
move the allocation and won't call its move callbacks. This makes it
safe to hold the pointer returned by buflib_get_data() across yields
or allocations.

Note that pinned allocations can still shrink because there are some
use cases where this would be valid, if buffer users coordinate with
the shrink callback.

Change-Id: I0d0c2a8ac7d891d3ad6b3d0eb80c5b5a1b4b9a9d

firmware/include/buflib.h

@@ -38,6 +38,7 @@ union buflib_data
     intptr_t val;                 /* length of the block in n*sizeof(union buflib_data).
                                      Includes buflib metadata overhead. A negative value
                                      indicates block is unallocated */
+    volatile unsigned pincount;   /* number of pins */
     struct buflib_callbacks* ops; /* callback functions for move and shrink. Can be NULL */
     char* alloc;                  /* start of allocated memory area */
     union buflib_data *handle;    /* pointer to entry in the handle table.
@@ -291,6 +292,25 @@ static inline void* buflib_get_data(struct buflib_context *ctx, int handle)
  */
 bool buflib_shrink(struct buflib_context *ctx, int handle, void* newstart, size_t new_size);
 
+/**
+ * Increment the pin count for a handle. When pinned the handle will not
+ * be moved and move callbacks will not be triggered, allowing a pointer
+ * to the buffer to be kept across yields or used for I/O.
+ *
+ * Note that shrink callbacks can still be invoked for pinned handles.
+ */
+void buflib_pin(struct buflib_context *ctx, int handle);
+
+/**
+ * Decrement the pin count for a handle.
+ */
+void buflib_unpin(struct buflib_context *ctx, int handle);
+
+/**
+ * Get the current pin count of a handle. Zero means the handle is not pinned.
+ */
+unsigned buflib_pin_count(struct buflib_context *ctx, int handle);
+
 /**
  * Frees memory associated with the given handle
  *