Date: Thu, 7 Apr 2022 09:15:18 -0700
From: Mark Wong <markwkm@gmail.com>
To: chap@anastigmatix.net
Cc: pgsql-hackers@lists.postgresql.org,
	Konstantina Skovola <konskov@gmail.com>
Subject: Re: trigger example for plsample
Message-ID: <Yk8OFtSyKDLLhjPW@workstation-mark-wong>
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 <Yh/PkdpVPbeydjC/@workstation-mark-wong>
 <622A8B8C.1060306@anastigmatix.net>
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On Thu, Apr 07, 2022 at 10:30:13AM -0400, chap@anastigmatix.net wrote:
> On 2022-04-06 16:44, Mark Wong wrote:
> > I think I've applied all of these suggestions and attached a new patch.
> 
> That looks good to me, though I wonder about the pfree(source).
> In the simplest case of a PL that uses no advance compilation or
> augmentation step, the Code Execution block might naturally refer
> to source, so perhaps the example boilerplate shouldn't include
> a pfree that needs to be removed in that case.
> 
> In fact, I wonder about the need for any retail pfree()s here. Those
> added in this patch are the only ones in plsample.c. They are small
> allocations, and maybe it would both streamline the example to leave
> out the pfree calls, and be an illustration of best practice in letting
> the memory context machinery handle all the deallocation at once, where
> there isn't a special need to free something, like an especially large
> allocation, at retail.

Thanks, I've attached v4.

I've removed all of the pfree()'s and added an elog(DEBUG1) for source
to quiet a compiler warning about source's lack of use. :)  (Was that a
good way?)

Regards,
Mark

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diff --git a/src/test/modules/plsample/expected/plsample.out b/src/test/modules/plsample/expected/plsample.out
index a0c318b6df..a7912c7897 100644
--- a/src/test/modules/plsample/expected/plsample.out
+++ b/src/test/modules/plsample/expected/plsample.out
@@ -6,9 +6,6 @@ AS $$
   Example of source with text result.
 $$ LANGUAGE plsample;
 SELECT plsample_result_text(1.23, 'abc', '{4, 5, 6}');
-NOTICE:  source text of function "plsample_result_text": 
-  Example of source with text result.
-
 NOTICE:  argument: 0; name: a1; value: 1.23
 NOTICE:  argument: 1; name: a2; value: abc
 NOTICE:  argument: 2; name: a3; value: {4,5,6}
@@ -25,12 +22,57 @@ AS $$
   Example of source with void result.
 $$ LANGUAGE plsample;
 SELECT plsample_result_void('{foo, bar, hoge}');
-NOTICE:  source text of function "plsample_result_void": 
-  Example of source with void result.
-
 NOTICE:  argument: 0; name: a1; value: {foo,bar,hoge}
  plsample_result_void 
 ----------------------
  
 (1 row)
 
+CREATE FUNCTION my_trigger_func() RETURNS trigger AS $$
+if TD_event == "INSERT"
+    return TD_NEW
+elseif TD_event == "UPDATE"
+    return TD_NEW
+else
+    return "OK"
+end
+$$ language plsample;
+CREATE TABLE my_table (num integer, description text);
+CREATE TRIGGER my_trigger_func BEFORE INSERT OR UPDATE ON my_table
+       FOR EACH ROW EXECUTE FUNCTION my_trigger_func();
+CREATE TRIGGER my_trigger_func2 AFTER INSERT OR UPDATE ON my_table
+       FOR EACH ROW EXECUTE FUNCTION my_trigger_func(8);
+INSERT INTO my_table (num, description)
+VALUES (1, 'first');
+NOTICE:  trigger name: my_trigger_func
+NOTICE:  trigger relation: my_table
+NOTICE:  trigger relation schema: public
+NOTICE:  triggered by INSERT
+NOTICE:  triggered BEFORE
+NOTICE:  triggered per row
+NOTICE:  trigger name: my_trigger_func2
+NOTICE:  trigger relation: my_table
+NOTICE:  trigger relation schema: public
+NOTICE:  triggered by INSERT
+NOTICE:  triggered AFTER
+NOTICE:  triggered per row
+NOTICE:  trigger arg[0]: 8
+UPDATE my_table
+SET description = 'first, modified once'
+WHERE num = 1;
+NOTICE:  trigger name: my_trigger_func
+NOTICE:  trigger relation: my_table
+NOTICE:  trigger relation schema: public
+NOTICE:  triggered by UPDATE
+NOTICE:  triggered BEFORE
+NOTICE:  triggered per row
+NOTICE:  trigger name: my_trigger_func2
+NOTICE:  trigger relation: my_table
+NOTICE:  trigger relation schema: public
+NOTICE:  triggered by UPDATE
+NOTICE:  triggered AFTER
+NOTICE:  triggered per row
+NOTICE:  trigger arg[0]: 8
+DROP TRIGGER my_trigger_func ON my_table;
+DROP TRIGGER my_trigger_func2 ON my_table;
+DROP FUNCTION my_trigger_func;
diff --git a/src/test/modules/plsample/plsample.c b/src/test/modules/plsample/plsample.c
index 6fc33c728c..4e03dd5093 100644
--- a/src/test/modules/plsample/plsample.c
+++ b/src/test/modules/plsample/plsample.c
@@ -19,6 +19,7 @@
 #include "catalog/pg_type.h"
 #include "commands/event_trigger.h"
 #include "commands/trigger.h"
+#include "executor/spi.h"
 #include "funcapi.h"
 #include "utils/builtins.h"
 #include "utils/lsyscache.h"
@@ -29,6 +30,7 @@ PG_MODULE_MAGIC;
 PG_FUNCTION_INFO_V1(plsample_call_handler);
 
 static Datum plsample_func_handler(PG_FUNCTION_ARGS);
+static HeapTuple plsample_trigger_handler(PG_FUNCTION_ARGS);
 
 /*
  * Handle function, procedure, and trigger calls.
@@ -51,6 +53,7 @@ plsample_call_handler(PG_FUNCTION_ARGS)
 			 * (TriggerData *) fcinfo->context includes the information of the
 			 * context.
 			 */
+			retval = PointerGetDatum(plsample_trigger_handler(fcinfo));
 		}
 		else if (CALLED_AS_EVENT_TRIGGER(fcinfo))
 		{
@@ -119,9 +122,6 @@ plsample_func_handler(PG_FUNCTION_ARGS)
 		elog(ERROR, "could not find source text of function \"%s\"",
 			 proname);
 	source = DatumGetCString(DirectFunctionCall1(textout, ret));
-	ereport(NOTICE,
-			(errmsg("source text of function \"%s\": %s",
-					proname, source)));
 
 	/*
 	 * Allocate a context that will hold all the Postgres data for the
@@ -185,3 +185,156 @@ plsample_func_handler(PG_FUNCTION_ARGS)
 	ret = InputFunctionCall(&result_in_func, source, result_typioparam, -1);
 	PG_RETURN_DATUM(ret);
 }
+
+/*
+ * plsample_trigger_handler
+ *
+ * Function called by the call handler for trigger execution.
+ */
+static HeapTuple
+plsample_trigger_handler(PG_FUNCTION_ARGS)
+{
+	TriggerData *trigdata = (TriggerData *) fcinfo->context;
+	char	   *string;
+	volatile HeapTuple rettup;
+	HeapTuple	pl_tuple;
+	Datum		ret;
+	char	   *source;
+	bool		isnull;
+	Form_pg_proc pl_struct;
+	char	   *proname;
+	int			rc PG_USED_FOR_ASSERTS_ONLY;
+
+	/* Make sure this is being called from a trigger. */
+	if (!CALLED_AS_TRIGGER(fcinfo))
+		elog(ERROR, "not called by trigger manager");
+
+	/* Connect to the SPI manager */
+	if (SPI_connect() != SPI_OK_CONNECT)
+		elog(ERROR, "could not connect to SPI manager");
+
+	rc = SPI_register_trigger_data(trigdata);
+	Assert(rc >= 0);
+
+	/* Fetch the source text of the function. */
+	pl_tuple = SearchSysCache(PROCOID,
+							  ObjectIdGetDatum(fcinfo->flinfo->fn_oid), 0, 0, 0);
+	if (!HeapTupleIsValid(pl_tuple))
+		elog(ERROR, "cache lookup failed for function %u",
+			 fcinfo->flinfo->fn_oid);
+
+	/*
+	 * Code Retrieval
+	 *
+	 * Extract and print the source text of the function.  This can be used as
+	 * a base for the function validation and execution.
+	 */
+	pl_struct = (Form_pg_proc) GETSTRUCT(pl_tuple);
+	proname = pstrdup(NameStr(pl_struct->proname));
+	ret = SysCacheGetAttr(PROCOID, pl_tuple, Anum_pg_proc_prosrc, &isnull);
+	if (isnull)
+		elog(ERROR, "could not find source text of function \"%s\"",
+			 proname);
+	source = DatumGetCString(DirectFunctionCall1(textout, ret));
+	ereport(DEBUG1,
+			(errmsg("source text of function \"%s\": %s", proname, source)));
+
+
+
+	/*
+	 * Code Augmentation
+	 *
+	 * The source text may be augmented here, such as by wrapping it as the
+	 * body of a function in the target language, prefixing a parameter list
+	 * with names like TD_name, TD_relid, TD_table_name, TD_table_schema,
+	 * TD_event, TD_when, TD_level, TD_NEW, TD_OLD, and args, using whatever
+	 * types in the target language are convenient. The augmented text can be
+	 * cached in a longer-lived memory context, or, if the target language uses
+	 * a compilation step, that can be done here, caching the result of the
+	 * compilation.
+	 */
+
+	ReleaseSysCache(pl_tuple);
+
+	/*
+	 * Code Execution
+	 *
+	 * Here the function (the possibly-augmented source text, or the result of
+	 * compilation if the target language uses such a step) should be executed,
+	 * after binding these values from the TriggerData struct to the expected
+	 * parameters.
+	 */
+
+	PG_TRY();
+	{
+		int			i;
+
+		ereport(NOTICE,
+				(errmsg("trigger name: %s", trigdata->tg_trigger->tgname)));
+		string = SPI_getrelname(trigdata->tg_relation);
+		ereport(NOTICE, (errmsg("trigger relation: %s", string)));
+
+		string = SPI_getnspname(trigdata->tg_relation);
+		ereport(NOTICE, (errmsg("trigger relation schema: %s", string)));
+
+		/* Example handling of different trigger aspects. */
+
+		if (TRIGGER_FIRED_BY_INSERT(trigdata->tg_event))
+		{
+			ereport(NOTICE, (errmsg("triggered by INSERT")));
+			rettup = trigdata->tg_trigtuple;
+		}
+		else if (TRIGGER_FIRED_BY_DELETE(trigdata->tg_event))
+		{
+			ereport(NOTICE, (errmsg("triggered by DELETE")));
+			rettup = trigdata->tg_trigtuple;
+		}
+		else if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
+		{
+			ereport(NOTICE, (errmsg("triggered by UPDATE")));
+			rettup = trigdata->tg_trigtuple;
+		}
+		else if (TRIGGER_FIRED_BY_TRUNCATE(trigdata->tg_event))
+		{
+			ereport(NOTICE, (errmsg("triggered by TRUNCATE")));
+			rettup = trigdata->tg_trigtuple;
+		}
+		else
+			elog(ERROR, "unrecognized event: %u", trigdata->tg_event);
+
+		if (TRIGGER_FIRED_BEFORE(trigdata->tg_event))
+			ereport(NOTICE, (errmsg("triggered BEFORE")));
+		else if (TRIGGER_FIRED_AFTER(trigdata->tg_event))
+			ereport(NOTICE, (errmsg("triggered AFTER")));
+		else if (TRIGGER_FIRED_INSTEAD(trigdata->tg_event))
+			ereport(NOTICE, (errmsg("triggered INSTEAD OF")));
+		else
+			elog(ERROR, "unrecognized when: %u", trigdata->tg_event);
+
+		if (TRIGGER_FIRED_FOR_ROW(trigdata->tg_event))
+			ereport(NOTICE, (errmsg("triggered per row")));
+		else if (TRIGGER_FIRED_FOR_STATEMENT(trigdata->tg_event))
+			ereport(NOTICE, (errmsg("triggered per statement")));
+		else
+			elog(ERROR, "unrecognized level: %u", trigdata->tg_event);
+
+		/*
+		 * Iterate through all of the trigger arguments, printing each input
+		 * value.
+		 */
+		for (i = 0; i < trigdata->tg_trigger->tgnargs; i++)
+			ereport(NOTICE,
+					(errmsg("trigger arg[%i]: %s", i,
+							trigdata->tg_trigger->tgargs[i])));
+	}
+	PG_CATCH();
+	{
+		PG_RE_THROW();
+	}
+	PG_END_TRY();
+
+	if (SPI_finish() != SPI_OK_FINISH)
+		elog(ERROR, "SPI_finish() failed");
+
+	return rettup;
+}
diff --git a/src/test/modules/plsample/sql/plsample.sql b/src/test/modules/plsample/sql/plsample.sql
index bf0fddac7f..ac06e99643 100644
--- a/src/test/modules/plsample/sql/plsample.sql
+++ b/src/test/modules/plsample/sql/plsample.sql
@@ -13,3 +13,31 @@ AS $$
   Example of source with void result.
 $$ LANGUAGE plsample;
 SELECT plsample_result_void('{foo, bar, hoge}');
+
+CREATE FUNCTION my_trigger_func() RETURNS trigger AS $$
+if TD_event == "INSERT"
+    return TD_NEW
+elseif TD_event == "UPDATE"
+    return TD_NEW
+else
+    return "OK"
+end
+$$ language plsample;
+
+CREATE TABLE my_table (num integer, description text);
+CREATE TRIGGER my_trigger_func BEFORE INSERT OR UPDATE ON my_table
+       FOR EACH ROW EXECUTE FUNCTION my_trigger_func();
+CREATE TRIGGER my_trigger_func2 AFTER INSERT OR UPDATE ON my_table
+       FOR EACH ROW EXECUTE FUNCTION my_trigger_func(8);
+
+INSERT INTO my_table (num, description)
+VALUES (1, 'first');
+
+UPDATE my_table
+SET description = 'first, modified once'
+WHERE num = 1;
+
+DROP TRIGGER my_trigger_func ON my_table;
+DROP TRIGGER my_trigger_func2 ON my_table;
+DROP FUNCTION my_trigger_func;
+

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