Split tuptoaster.c into three separate files.

detoast.c/h contain functions required to detoast a datum, partially
or completely, plus a few other utility functions for examining the
size of toasted datums.

toast_internals.c/h contain functions that are used internally to the
TOAST subsystem but which (mostly) do not need to be accessed from
outside.

heaptoast.c/h contains code that is intrinsically specific to the
heap AM, either because it operates on HeapTuples or is based on the
layout of a heap page.

detoast.c and toast_internals.c are placed in
src/backend/access/common rather than src/backend/access/heap.  At
present, both files still have dependencies on the heap, but that will
be improved in a future commit.

Patch by me, reviewed and tested by Prabhat Sabu, Thomas Munro,
Andres Freund, and Álvaro Herrera.

Discussion: http://postgr.es/m/CA+TgmoZv-=2iWM4jcw5ZhJeL18HF96+W1yJeYrnGMYdkFFnEpQ@mail.gmail.com

src/include/access/detoast.h

@@ -0,0 +1,92 @@
+/*-------------------------------------------------------------------------
+ *
+ * detoast.h
+ *	  Access to compressed and external varlena values.
+ *
+ * Copyright (c) 2000-2019, PostgreSQL Global Development Group
+ *
+ * src/include/access/detoast.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef DETOAST_H
+#define DETOAST_H
+
+/*
+ * Testing whether an externally-stored value is compressed now requires
+ * comparing extsize (the actual length of the external data) to rawsize
+ * (the original uncompressed datum's size).  The latter includes VARHDRSZ
+ * overhead, the former doesn't.  We never use compression unless it actually
+ * saves space, so we expect either equality or less-than.
+ */
+#define VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer) \
+	((toast_pointer).va_extsize < (toast_pointer).va_rawsize - VARHDRSZ)
+
+/*
+ * Macro to fetch the possibly-unaligned contents of an EXTERNAL datum
+ * into a local "struct varatt_external" toast pointer.  This should be
+ * just a memcpy, but some versions of gcc seem to produce broken code
+ * that assumes the datum contents are aligned.  Introducing an explicit
+ * intermediate "varattrib_1b_e *" variable seems to fix it.
+ */
+#define VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr) \
+do { \
+	varattrib_1b_e *attre = (varattrib_1b_e *) (attr); \
+	Assert(VARATT_IS_EXTERNAL(attre)); \
+	Assert(VARSIZE_EXTERNAL(attre) == sizeof(toast_pointer) + VARHDRSZ_EXTERNAL); \
+	memcpy(&(toast_pointer), VARDATA_EXTERNAL(attre), sizeof(toast_pointer)); \
+} while (0)
+
+/* Size of an EXTERNAL datum that contains a standard TOAST pointer */
+#define TOAST_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(varatt_external))
+
+/* Size of an EXTERNAL datum that contains an indirection pointer */
+#define INDIRECT_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(varatt_indirect))
+
+/* ----------
+ * heap_tuple_fetch_attr() -
+ *
+ *		Fetches an external stored attribute from the toast
+ *		relation. Does NOT decompress it, if stored external
+ *		in compressed format.
+ * ----------
+ */
+extern struct varlena *heap_tuple_fetch_attr(struct varlena *attr);
+
+/* ----------
+ * heap_tuple_untoast_attr() -
+ *
+ *		Fully detoasts one attribute, fetching and/or decompressing
+ *		it as needed.
+ * ----------
+ */
+extern struct varlena *heap_tuple_untoast_attr(struct varlena *attr);
+
+/* ----------
+ * heap_tuple_untoast_attr_slice() -
+ *
+ *		Fetches only the specified portion of an attribute.
+ *		(Handles all cases for attribute storage)
+ * ----------
+ */
+extern struct varlena *heap_tuple_untoast_attr_slice(struct varlena *attr,
+							  int32 sliceoffset,
+							  int32 slicelength);
+
+/* ----------
+ * toast_raw_datum_size -
+ *
+ *	Return the raw (detoasted) size of a varlena datum
+ * ----------
+ */
+extern Size toast_raw_datum_size(Datum value);
+
+/* ----------
+ * toast_datum_size -
+ *
+ *	Return the storage size of a varlena datum
+ * ----------
+ */
+extern Size toast_datum_size(Datum value);
+
+#endif							/* DETOAST_H */