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Install fmgr rewrite doc as README file.

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Need to update user docs still ...
REL7_1_STABLE
Tom Lane 26 years ago
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Proposal for function-manager redesign 24-May-2000
--------------------------------------
We know that the existing mechanism for calling Postgres functions needs
to be redesigned. It has portability problems because it makes
assumptions about parameter passing that violate ANSI C; it fails to
handle NULL arguments and results cleanly; and "function handlers" that
support a class of functions (such as fmgr_pl) can only be done via a
really ugly, non-reentrant kluge. (Global variable set during every
function call, forsooth.) Here is a proposal for fixing these problems.
In the past, the major objections to redoing the function-manager
interface have been (a) it'll be quite tedious to implement, since every
built-in function and everyplace that calls such functions will need to
be touched; (b) such wide-ranging changes will be difficult to make in
parallel with other development work; (c) it will break existing
user-written loadable modules that define "C language" functions. While
I have no solution to the "tedium" aspect, I believe I see an answer to
the other problems: by use of function handlers, we can support both old
and new interfaces in parallel for both callers and callees, at some
small efficiency cost for the old styles. That way, most of the changes
can be done on an incremental file-by-file basis --- we won't need a
"big bang" where everything changes at once. Support for callees
written in the old style can be left in place indefinitely, to provide
backward compatibility for user-written C functions.
Note that neither the old function manager nor the redesign are intended
to handle functions that accept or return sets. Those sorts of functions
need to be handled by special querytree structures.
Changes in pg_proc (system data about a function)
-------------------------------------------------
A new column "proisstrict" will be added to the system pg_proc table.
This is a boolean value which will be TRUE if the function is "strict",
that is it always returns NULL when any of its inputs are NULL. The
function manager will check this field and skip calling the function when
it's TRUE and there are NULL inputs. This allows us to remove explicit
NULL-value tests from many functions that currently need them. A function
that is not marked "strict" is responsible for checking whether its inputs
are NULL or not. Most builtin functions will be marked "strict".
An optional WITH parameter will be added to CREATE FUNCTION to allow
specification of whether user-defined functions are strict or not. I am
inclined to make the default be "not strict", since that seems to be the
more useful case for functions expressed in SQL or a PL language, but
am open to arguments for the other choice.
The new function-manager interface
----------------------------------
The core of the new design is revised data structures for representing
the result of a function lookup and for representing the parameters
passed to a specific function invocation. (We want to keep function
lookup separate from function call, since many parts of the system apply
the same function over and over; the lookup overhead should be paid once
per query, not once per tuple.)
When a function is looked up in pg_proc, the result is represented as
typedef struct
{
PGFunction fn_addr; /* pointer to function or handler to be called */
Oid fn_oid; /* OID of function (NOT of handler, if any) */
short fn_nargs; /* 0..FUNC_MAX_ARGS, or -1 if variable arg count */
bool fn_strict; /* function is "strict" (NULL in => NULL out) */
void *fn_extra; /* extra space for use by handler */
} FmgrInfo;
For an ordinary built-in function, fn_addr is just the address of the C
routine that implements the function. Otherwise it is the address of a
handler for the class of functions that includes the target function.
The handler can use the function OID and perhaps also the fn_extra slot
to find the specific code to execute. (fn_oid = InvalidOid can be used
to denote a not-yet-initialized FmgrInfo struct. fn_extra will always
be NULL when an FmgrInfo is first filled by the function lookup code, but
a function handler could set it to avoid making repeated lookups of its
own when the same FmgrInfo is used repeatedly during a query.) fn_nargs
is the number of arguments expected by the function, and fn_strict is
its strictness flag.
FmgrInfo already exists in the current code, but has fewer fields. This
change should be transparent at the source-code level.
During a call of a function, the following data structure is created
and passed to the function:
typedef struct
{
FmgrInfo *flinfo; /* ptr to lookup info used for this call */
Node *context; /* pass info about context of call */
Node *resultinfo; /* pass or return extra info about result */
bool isnull; /* function must set true if result is NULL */
short nargs; /* # arguments actually passed */
Datum arg[FUNC_MAX_ARGS]; /* Arguments passed to function */
bool argnull[FUNC_MAX_ARGS]; /* T if arg[i] is actually NULL */
} FunctionCallInfoData;
typedef FunctionCallInfoData* FunctionCallInfo;
flinfo points to the lookup info used to make the call. Ordinary functions
will probably ignore this field, but function class handlers will need it
to find out the OID of the specific function being called.
context is NULL for an "ordinary" function call, but may point to additional
info when the function is called in certain contexts. (For example, the
trigger manager will pass information about the current trigger event here.)
If context is used, it should point to some subtype of Node; the particular
kind of context can then be indicated by the node type field. (A callee
should always check the node type before assuming it knows what kind of
context is being passed.) fmgr itself puts no other restrictions on the use
of this field.
resultinfo is NULL when calling any function from which a simple Datum
result is expected. It may point to some subtype of Node if the function
returns more than a Datum. Like the context field, resultinfo is a hook
for expansion; fmgr itself doesn't constrain the use of the field.
nargs, arg[], and argnull[] hold the arguments being passed to the function.
Notice that all the arguments passed to a function (as well as its result
value) will now uniformly be of type Datum. As discussed below, callers
and callees should apply the standard Datum-to-and-from-whatever macros
to convert to the actual argument types of a particular function. The
value in arg[i] is unspecified when argnull[i] is true.
It is generally the responsibility of the caller to ensure that the
number of arguments passed matches what the callee is expecting; except
for callees that take a variable number of arguments, the callee will
typically ignore the nargs field and just grab values from arg[].
The isnull field will be initialized to "false" before the call. On
return from the function, isnull is the null flag for the function result:
if it is true the function's result is NULL, regardless of the actual
function return value. Note that simple "strict" functions can ignore
both isnull and argnull[], since they won't even get called when there
are any TRUE values in argnull[].
FunctionCallInfo replaces FmgrValues plus a bunch of ad-hoc parameter
conventions, global variables (fmgr_pl_finfo and CurrentTriggerData at
least), and other uglinesses.
Callees, whether they be individual functions or function handlers,
shall always have this signature:
Datum function (FunctionCallInfo fcinfo);
which is represented by the typedef
typedef Datum (*PGFunction) (FunctionCallInfo fcinfo);
The function is responsible for setting fcinfo->isnull appropriately
as well as returning a result represented as a Datum. Note that since
all callees will now have exactly the same signature, and will be called
through a function pointer declared with exactly that signature, we
should have no portability or optimization problems.
Function coding conventions
---------------------------
As an example, int4 addition goes from old-style
int32
int4pl(int32 arg1, int32 arg2)
{
return arg1 + arg2;
}
to new-style
Datum
int4pl(FunctionCallInfo fcinfo)
{
/* we assume the function is marked "strict", so we can ignore
* NULL-value handling */
return Int32GetDatum(DatumGetInt32(fcinfo->arg[0]) +
DatumGetInt32(fcinfo->arg[1]));
}
This is, of course, much uglier than the old-style code, but we can
improve matters with some well-chosen macros for the boilerplate parts.
I propose below macros that would make the code look like
Datum
int4pl(PG_FUNCTION_ARGS)
{
int32 arg1 = PG_GETARG_INT32(0);
int32 arg2 = PG_GETARG_INT32(1);
PG_RETURN_INT32( arg1 + arg2 );
}
This is still more code than before, but it's fairly readable, and it's
also amenable to machine processing --- for example, we could probably
write a script that scans code like this and extracts argument and result
type info for comparison to the pg_proc table.
For the standard data types float4, float8, and int8, these macros should
hide the indirection and space allocation involved, so that the function's
code is not explicitly aware that these types are pass-by-reference. This
will offer a considerable gain in readability, and it also opens up the
opportunity to make these types be pass-by-value on machines where it's
feasible to do so. (For example, on an Alpha it's pretty silly to make int8
be pass-by-ref, since Datum is going to be 64 bits anyway. float4 could
become pass-by-value on all machines...)
Here are the proposed macros and coding conventions:
The definition of an fmgr-callable function will always look like
Datum
function_name(PG_FUNCTION_ARGS)
{
...
}
"PG_FUNCTION_ARGS" just expands to "FunctionCallInfo fcinfo". The main
reason for using this macro is to make it easy for scripts to spot function
definitions. However, if we ever decide to change the calling convention
again, it might come in handy to have this macro in place.
A nonstrict function is responsible for checking whether each individual
argument is null or not, which it can do with PG_ARGISNULL(n) (which is
just "fcinfo->argnull[n]"). It should avoid trying to fetch the value
of any argument that is null.
Both strict and nonstrict functions can return NULL, if needed, with
PG_RETURN_NULL();
which expands to
{ fcinfo->isnull = true; return (Datum) 0; }
Argument values are ordinarily fetched using code like
int32 name = PG_GETARG_INT32(number);
For float4, float8, and int8, the PG_GETARG macros will hide the pass-by-
reference nature of the data types; for example PG_GETARG_FLOAT4 expands to
(* (float4 *) DatumGetPointer(fcinfo->arg[number]))
and would typically be called like this:
float4 arg = PG_GETARG_FLOAT4(0);
Note that "float4" and "float8" are the recommended typedefs to use, not
"float32data" and "float64data", and the macros are named accordingly.
But 64-bit ints should be declared as "int64".
Non-null values are returned with a PG_RETURN_XXX macro of the appropriate
type. For example, PG_RETURN_INT32 expands to
return Int32GetDatum(x)
PG_RETURN_FLOAT4, PG_RETURN_FLOAT8, and PG_RETURN_INT64 hide the pass-by-
reference nature of their datatypes.
fmgr.h will provide PG_GETARG and PG_RETURN macros for all the basic data
types. Modules or header files that define specialized SQL datatypes
(eg, timestamp) should define appropriate macros for those types, so that
functions manipulating the types can be coded in the standard style.
For non-primitive data types (particularly variable-length types) it
probably won't be very practical to hide the pass-by-reference nature of
the data type, so the PG_GETARG and PG_RETURN macros for those types
probably won't do more than DatumGetPointer/PointerGetDatum plus the
appropriate typecast. Functions returning such types will need to
palloc() their result space explicitly. I recommend naming the GETARG
and RETURN macros for such types to end in "_P", as a reminder that they
produce or take a pointer. For example, PG_GETARG_TEXT_P yields "text *".
For TOAST-able data types, the PG_GETARG macro will deliver a de-TOASTed
data value. There might be a few cases where the still-toasted value is
wanted, but I am having a hard time coming up with examples. For the
moment I'd say that any such code could use a lower-level macro that is
just ((struct varlena *) DatumGetPointer(fcinfo->arg[n])).
Note: the above examples assume that arguments will be counted starting at
zero. We could have the ARG macros subtract one from the argument number,
so that arguments are counted starting at one. I'm not sure if that would be
more or less confusing. Does anyone have a strong feeling either way about
it?
When a function needs to access fcinfo->flinfo or one of the other auxiliary
fields of FunctionCallInfo, it should just do it. I doubt that providing
syntactic-sugar macros for these cases is useful.
Call-site coding conventions
----------------------------
There are many places in the system that call either a specific function
(for example, the parser invokes "textin" by name in places) or a
particular group of functions that have a common argument list (for
example, the optimizer invokes selectivity estimation functions with
a fixed argument list). These places will need to change, but we should
try to avoid making them significantly uglier than before.
Places that invoke an arbitrary function with an arbitrary argument list
can simply be changed to fill a FunctionCallInfoData structure directly;
that'll be no worse and possibly cleaner than what they do now.
When invoking a specific built-in function by name, we have generally
just written something like
result = textin ( ... args ... )
which will not work after textin() is converted to the new call style.
I suggest that code like this be converted to use "helper" functions
that will create and fill in a FunctionCallInfoData struct. For
example, if textin is being called with one argument, it'd look
something like
result = DirectFunctionCall1(textin, PointerGetDatum(argument));
These helper routines will have declarations like
Datum DirectFunctionCall2(PGFunction func, Datum arg1, Datum arg2);
Note it will be the caller's responsibility to convert to and from
Datum; appropriate conversion macros should be used.
The DirectFunctionCallN routines will not bother to fill in
fcinfo->flinfo (indeed cannot, since they have no idea about an OID for
the target function); they will just set it NULL. This is unlikely to
bother any built-in function that could be called this way. Note also
that this style of coding cannot pass a NULL input value nor cope with
a NULL result (it couldn't before, either!). We can make the helper
routines elog an error if they see that the function returns a NULL.
(Note: direct calls like this will have to be changed at the same time
that their called routines are changed to the new style. But that will
still be a lot less of a constraint than a "big bang" conversion.)
When invoking a function that has a known argument signature, we have
usually written either
result = fmgr(targetfuncOid, ... args ... );
or
result = fmgr_ptr(FmgrInfo *finfo, ... args ... );
depending on whether an FmgrInfo lookup has been done yet or not.
This kind of code can be recast using helper routines, in the same
style as above:
result = OidFunctionCall1(funcOid, PointerGetDatum(argument));
result = FunctionCall2(funcCallInfo,
PointerGetDatum(argument),
Int32GetDatum(argument));
Again, this style of coding does not allow for expressing NULL inputs
or receiving a NULL result.
As with the callee-side situation, I propose adding argument conversion
macros that hide the pass-by-reference nature of int8, float4, and
float8, with an eye to making those types relatively painless to convert
to pass-by-value.
The existing helper functions fmgr(), fmgr_c(), etc will be left in
place until all uses of them are gone. Of course their internals will
have to change in the first step of implementation, but they can
continue to support the same external appearance.
Notes about function handlers
-----------------------------
Handlers for classes of functions should find life much easier and
cleaner in this design. The OID of the called function is directly
reachable from the passed parameters; we don't need the global variable
fmgr_pl_finfo anymore. Also, by modifying fcinfo->flinfo->fn_extra,
the handler can cache lookup info to avoid repeat lookups when the same
function is invoked many times. (fn_extra can only be used as a hint,
since callers are not required to re-use an FmgrInfo struct.
But in performance-critical paths they normally will do so.)
Issue: in what context should a handler allocate memory that it intends
to use for fn_extra data? The current palloc context when the handler
is actually called might be considerably shorter-lived than the FmgrInfo
struct, which would lead to dangling-pointer problems at the next use
of the FmgrInfo. Perhaps FmgrInfo should also store a memory context
identifier that the handler could use to allocate space of the right
lifespan. (Having fmgr_info initialize this to CurrentMemoryContext
should work in nearly all cases, though a few places might have to
set it differently.) At the moment I have not done this, since the
existing PL handlers only need to set fn_extra to point at long-lived
structures (data in their own caches) and don't really care which
context the FmgrInfo is in anyway.
Are there any other things needed by the call handlers for PL/pgsql and
other languages?
During the conversion process, support for old-style builtin functions
and old-style user-written C functions will be provided by appropriate
function handlers. For example, the handler for old-style builtins
looks roughly like fmgr_c() used to.
System table updates
--------------------
In the initial phase, two new entries will be added to pg_language
for language types "newinternal" and "newC", corresponding to
builtin and dynamically-loaded functions having the new calling
convention.
There will also be a change to pg_proc to add the new "proisstrict"
column.
Then pg_proc entries will be changed from language code "internal" to
"newinternal" piecemeal, as the associated routines are rewritten.
(This will imply several rounds of forced initdbs as the contents of
pg_proc change, but I think we can live with that.)
The old language names "internal" and "C" will continue to refer to
functions with the old calling convention. We should deprecate
old-style functions because of their portability problems, but the
support for them will only be one small function handler routine,
so we can leave them in place for as long as necessary.
The expected calling convention for PL call handlers will need to change
all-at-once, but fortunately there are not very many of them to fix.
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