Minor cleanup/code review for "indirect toast" stuff.

tglsfdc · tglsfdc · commit ccf655c446d9 · 2015-02-09T12:30:55.000-05:00
Fix some issues I noticed while fooling with an extension to allow an
additional kind of toast pointer.  Much of this is just comment
improvement, but there are a couple of actual bugs, which might or might
not be reachable today depending on what can happen during logical
decoding.  An example is that toast_flatten_tuple() failed to cover the
possibility of an indirection pointer in its input.  Back-patch to 9.4
just in case that is reachable now.

In HEAD, also correct some really minor issues with recent compression
reorganization, such as dangerously underparenthesized macros.
diff --git a/src/backend/access/heap/tuptoaster.c b/src/backend/access/heap/tuptoaster.c
@@ -68,8 +68,9 @@ static void toast_close_indexes(Relation *toastidxs, int num_indexes,
  *
  * This will return a datum that contains all the data internally, ie, not
  * relying on external storage or memory, but it can still be compressed or
- * have a short header.
- ----------
+ * have a short header.  Note some callers assume that if the input is an
+ * EXTERNAL datum, the result will be a pfree'able chunk.
+ * ----------
  */
 struct varlena *
 heap_tuple_fetch_attr(struct varlena * attr)
@@ -86,9 +87,7 @@ heap_tuple_fetch_attr(struct varlena * attr)
 	else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
 	{
 		/*
-		 * copy into the caller's memory context. That's not required in all
-		 * cases but sufficient for now since this is mainly used when we need
-		 * to persist a Datum for unusually long time, like in a HOLD cursor.
+		 * This is an indirect pointer --- dereference it
 		 */
 		struct varatt_indirect redirect;
 
@@ -98,11 +97,14 @@ heap_tuple_fetch_attr(struct varlena * attr)
 		/* nested indirect Datums aren't allowed */
 		Assert(!VARATT_IS_EXTERNAL_INDIRECT(attr));
 
-		/* doesn't make much sense, but better handle it */
-		if (VARATT_IS_EXTERNAL_ONDISK(attr))
+		/* recurse if value is still external in some other way */
+		if (VARATT_IS_EXTERNAL(attr))
 			return heap_tuple_fetch_attr(attr);
 
-		/* copy datum verbatim */
+		/*
+		 * Copy into the caller's memory context, in case caller tries to
+		 * pfree the result.
+		 */
 		result = (struct varlena *) palloc(VARSIZE_ANY(attr));
 		memcpy(result, attr, VARSIZE_ANY(attr));
 	}
@@ -122,7 +124,10 @@ heap_tuple_fetch_attr(struct varlena * attr)
  * heap_tuple_untoast_attr -
  *
  *	Public entry point to get back a toasted value from compression
- *	or external storage.
+ *	or external storage.  The result is always non-extended varlena form.
+ *
+ * Note some callers assume that if the input is an EXTERNAL or COMPRESSED
+ * datum, the result will be a pfree'able chunk.
  * ----------
  */
 struct varlena *
@@ -147,6 +152,9 @@ heap_tuple_untoast_attr(struct varlena * attr)
 	}
 	else if (VARATT_IS_EXTERNAL_INDIRECT(attr))
 	{
+		/*
+		 * This is an indirect pointer --- dereference it
+		 */
 		struct varatt_indirect redirect;
 
 		VARATT_EXTERNAL_GET_POINTER(redirect, attr);
@@ -155,7 +163,18 @@ heap_tuple_untoast_attr(struct varlena * attr)
 		/* nested indirect Datums aren't allowed */
 		Assert(!VARATT_IS_EXTERNAL_INDIRECT(attr));
 
+		/* recurse in case value is still extended in some other way */
 		attr = heap_tuple_untoast_attr(attr);
+
+		/* if it isn't, we'd better copy it */
+		if (attr == (struct varlena *) redirect.pointer)
+		{
+			struct varlena *result;
+
+			result = (struct varlena *) palloc(VARSIZE_ANY(attr));
+			memcpy(result, attr, VARSIZE_ANY(attr));
+			attr = result;
+		}
 	}
 	else if (VARATT_IS_COMPRESSED(attr))
 	{
@@ -231,6 +250,8 @@ heap_tuple_untoast_attr_slice(struct varlena * attr,
 	else
 		preslice = attr;
 
+	Assert(!VARATT_IS_EXTERNAL(preslice));
+
 	if (VARATT_IS_COMPRESSED(preslice))
 	{
 		PGLZ_Header *tmp = (PGLZ_Header *) preslice;
@@ -437,8 +458,6 @@ toast_delete(Relation rel, HeapTuple oldtup)
 				continue;
 			else if (VARATT_IS_EXTERNAL_ONDISK(PointerGetDatum(value)))
 				toast_delete_datum(rel, value);
-			else if (VARATT_IS_EXTERNAL_INDIRECT(PointerGetDatum(value)))
-				elog(ERROR, "attempt to delete tuple containing indirect datums");
 		}
 	}
 }
@@ -600,7 +619,8 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
 
 			/*
 			 * We took care of UPDATE above, so any external value we find
-			 * still in the tuple must be someone else's we cannot reuse.
+			 * still in the tuple must be someone else's that we cannot reuse
+			 * (this includes the case of an out-of-line in-memory datum).
 			 * Fetch it back (without decompression, unless we are forcing
 			 * PLAIN storage).  If necessary, we'll push it out as a new
 			 * external value below.
@@ -1032,7 +1052,7 @@ toast_flatten_tuple(HeapTuple tup, TupleDesc tupleDesc)
 			new_value = (struct varlena *) DatumGetPointer(toast_values[i]);
 			if (VARATT_IS_EXTERNAL(new_value))
 			{
-				new_value = toast_fetch_datum(new_value);
+				new_value = heap_tuple_fetch_attr(new_value);
 				toast_values[i] = PointerGetDatum(new_value);
 				toast_free[i] = true;
 			}
@@ -1726,8 +1746,8 @@ toast_fetch_datum(struct varlena * attr)
 	int			num_indexes;
 	int			validIndex;
 
-	if (VARATT_IS_EXTERNAL_INDIRECT(attr))
-		elog(ERROR, "shouldn't be called for indirect tuples");
+	if (!VARATT_IS_EXTERNAL_ONDISK(attr))
+		elog(ERROR, "toast_fetch_datum shouldn't be called for non-ondisk datums");
 
 	/* Must copy to access aligned fields */
 	VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
@@ -1903,7 +1923,8 @@ toast_fetch_datum_slice(struct varlena * attr, int32 sliceoffset, int32 length)
 	int			num_indexes;
 	int			validIndex;
 
-	Assert(VARATT_IS_EXTERNAL_ONDISK(attr));
+	if (!VARATT_IS_EXTERNAL_ONDISK(attr))
+		elog(ERROR, "toast_fetch_datum_slice shouldn't be called for non-ondisk datums");
 
 	/* Must copy to access aligned fields */
 	VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
diff --git a/src/include/access/tuptoaster.h b/src/include/access/tuptoaster.h
@@ -96,10 +96,10 @@
 	 VARHDRSZ)
 
 /* Size of an EXTERNAL datum that contains a standard TOAST pointer */
-#define TOAST_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(struct varatt_external))
+#define TOAST_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(varatt_external))
 
-/* Size of an indirect datum that contains a standard TOAST pointer */
-#define INDIRECT_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(struct varatt_indirect))
+/* Size of an EXTERNAL datum that contains an indirection pointer */
+#define INDIRECT_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(varatt_indirect))
 
 /*
  * Testing whether an externally-stored value is compressed now requires
diff --git a/src/include/postgres.h b/src/include/postgres.h
@@ -54,11 +54,11 @@
  */
 
 /*
- * struct varatt_external is a "TOAST pointer", that is, the information needed
- * to fetch a Datum stored in an out-of-line on-disk Datum. The data is
- * compressed if and only if va_extsize < va_rawsize - VARHDRSZ.  This struct
- * must not contain any padding, because we sometimes compare pointers using
- * memcmp.
+ * struct varatt_external is a traditional "TOAST pointer", that is, the
+ * information needed to fetch a Datum stored out-of-line in a TOAST table.
+ * The data is compressed if and only if va_extsize < va_rawsize - VARHDRSZ.
+ * This struct must not contain any padding, because we sometimes compare
+ * these pointers using memcmp.
  *
  * Note that this information is stored unaligned within actual tuples, so
  * you need to memcpy from the tuple into a local struct variable before
@@ -74,22 +74,23 @@ typedef struct varatt_external
 }	varatt_external;
 
 /*
- * Out-of-line Datum thats stored in memory in contrast to varatt_external
- * pointers which points to data in an external toast relation.
+ * struct varatt_indirect is a "TOAST pointer" representing an out-of-line
+ * Datum that's stored in memory, not in an external toast relation.
+ * The creator of such a Datum is entirely responsible that the referenced
+ * storage survives for as long as referencing pointer Datums can exist.
  *
- * Note that just as varatt_external's this is stored unaligned within the
- * tuple.
+ * Note that just as for struct varatt_external, this struct is stored
+ * unaligned within any containing tuple.
  */
 typedef struct varatt_indirect
 {
 	struct varlena *pointer;	/* Pointer to in-memory varlena */
 }	varatt_indirect;
 
-
 /*
- * Type of external toast datum stored. The peculiar value for VARTAG_ONDISK
- * comes from the requirement for on-disk compatibility with the older
- * definitions of varattrib_1b_e where v_tag was named va_len_1be...
+ * Type tag for the various sorts of "TOAST pointer" datums.  The peculiar
+ * value for VARTAG_ONDISK comes from a requirement for on-disk compatibility
+ * with a previous notion that the tag field was the pointer datum's length.
  */
 typedef enum vartag_external
 {
@@ -98,9 +99,9 @@ typedef enum vartag_external
 } vartag_external;
 
 #define VARTAG_SIZE(tag) \
-	((tag) == VARTAG_INDIRECT ? sizeof(varatt_indirect) :		\
+	((tag) == VARTAG_INDIRECT ? sizeof(varatt_indirect) : \
 	 (tag) == VARTAG_ONDISK ? sizeof(varatt_external) : \
-	 TrapMacro(true, "unknown vartag"))
+	 TrapMacro(true, "unrecognized TOAST vartag"))
 
 /*
  * These structs describe the header of a varlena object that may have been
@@ -132,7 +133,7 @@ typedef struct
 	char		va_data[1];		/* Data begins here */
 } varattrib_1b;
 
-/* inline portion of a short varlena pointing to an external resource */
+/* TOAST pointers are a subset of varattrib_1b with an identifying tag byte */
 typedef struct
 {
 	uint8		va_header;		/* Always 0x80 or 0x01 */
@@ -162,8 +163,8 @@ typedef struct
  * this lets us disambiguate alignment padding bytes from the start of an
  * unaligned datum.  (We now *require* pad bytes to be filled with zero!)
  *
- * In TOAST datums the tag field in varattrib_1b_e is used to discern whether
- * its an indirection pointer or more commonly an on-disk tuple.
+ * In TOAST pointers the va_tag field (see varattrib_1b_e) is used to discern
+ * the specific type and length of the pointer datum.
  */
 
 /*
