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diff --git a/doc/src/sgml/plhandler.sgml b/doc/src/sgml/plhandler.sgml new file mode 100644 index 0000000..980c95e --- /dev/null +++ b/doc/src/sgml/plhandler.sgml @@ -0,0 +1,192 @@ +<!-- doc/src/sgml/plhandler.sgml --> + + <chapter id="plhandler"> + <title>Writing a Procedural Language Handler</title> + + <indexterm zone="plhandler"> + <primary>procedural language</primary> + <secondary>handler for</secondary> + </indexterm> + + <para> + All calls to functions that are written in a language other than + the current <quote>version 1</quote> interface for compiled + languages (this includes functions in user-defined procedural languages + and functions written in SQL) go through a <firstterm>call handler</firstterm> + function for the specific language. It is the responsibility of + the call handler to execute the function in a meaningful way, such + as by interpreting the supplied source text. This chapter outlines + how a new procedural language's call handler can be written. + </para> + + <para> + The call handler for a procedural language is a + <quote>normal</quote> function that must be written in a compiled + language such as C, using the version-1 interface, and registered + with <productname>PostgreSQL</productname> as taking no arguments + and returning the type <type>language_handler</type>. This + special pseudo-type identifies the function as a call handler and + prevents it from being called directly in SQL commands. + For more details on C language calling conventions and dynamic loading, + see <xref linkend="xfunc-c"/>. + </para> + + <para> + The call handler is called in the same way as any other function: + It receives a pointer to a + <structname>FunctionCallInfoBaseData</structname> <type>struct</type> containing + argument values and information about the called function, and it + is expected to return a <type>Datum</type> result (and possibly + set the <structfield>isnull</structfield> field of the + <structname>FunctionCallInfoBaseData</structname> structure, if it wishes + to return an SQL null result). The difference between a call + handler and an ordinary callee function is that the + <structfield>flinfo->fn_oid</structfield> field of the + <structname>FunctionCallInfoBaseData</structname> structure will contain + the OID of the actual function to be called, not of the call + handler itself. The call handler must use this field to determine + which function to execute. Also, the passed argument list has + been set up according to the declaration of the target function, + not of the call handler. + </para> + + <para> + It's up to the call handler to fetch the entry of the function from the + <classname>pg_proc</classname> system catalog and to analyze the argument + and return types of the called function. The <literal>AS</literal> clause from the + <command>CREATE FUNCTION</command> command for the function will be found + in the <literal>prosrc</literal> column of the + <classname>pg_proc</classname> row. This is commonly source + text in the procedural language, but in theory it could be something else, + such as a path name to a file, or anything else that tells the call handler + what to do in detail. + </para> + + <para> + Often, the same function is called many times per SQL statement. + A call handler can avoid repeated lookups of information about the + called function by using the + <structfield>flinfo->fn_extra</structfield> field. This will + initially be <symbol>NULL</symbol>, but can be set by the call handler to point at + information about the called function. On subsequent calls, if + <structfield>flinfo->fn_extra</structfield> is already non-<symbol>NULL</symbol> + then it can be used and the information lookup step skipped. The + call handler must make sure that + <structfield>flinfo->fn_extra</structfield> is made to point at + memory that will live at least until the end of the current query, + since an <structname>FmgrInfo</structname> data structure could be + kept that long. One way to do this is to allocate the extra data + in the memory context specified by + <structfield>flinfo->fn_mcxt</structfield>; such data will + normally have the same lifespan as the + <structname>FmgrInfo</structname> itself. But the handler could + also choose to use a longer-lived memory context so that it can cache + function definition information across queries. + </para> + + <para> + When a procedural-language function is invoked as a trigger, no arguments + are passed in the usual way, but the + <structname>FunctionCallInfoBaseData</structname>'s + <structfield>context</structfield> field points at a + <structname>TriggerData</structname> structure, rather than being <symbol>NULL</symbol> + as it is in a plain function call. A language handler should + provide mechanisms for procedural-language functions to get at the trigger + information. + </para> + + <para> + A template for a procedural-language handler written as a C extension is + provided in <literal>src/test/modules/plsample</literal>. This is a + working sample demonstrating one way to create a procedural-language + handler, process parameters, and return a value. + </para> + + <para> + Although providing a call handler is sufficient to create a minimal + procedural language, there are two other functions that can optionally + be provided to make the language more convenient to use. These + are a <firstterm>validator</firstterm> and an + <firstterm>inline handler</firstterm>. A validator can be provided + to allow language-specific checking to be done during + <xref linkend="sql-createfunction"/>. + An inline handler can be provided to allow the language to support + anonymous code blocks executed via the <xref linkend="sql-do"/> command. + </para> + + <para> + If a validator is provided by a procedural language, it + must be declared as a function taking a single parameter of type + <type>oid</type>. The validator's result is ignored, so it is customarily + declared to return <type>void</type>. The validator will be called at + the end of a <command>CREATE FUNCTION</command> command that has created + or updated a function written in the procedural language. + The passed-in OID is the OID of the function's <classname>pg_proc</classname> + row. The validator must fetch this row in the usual way, and do + whatever checking is appropriate. + First, call <function>CheckFunctionValidatorAccess()</function> to diagnose + explicit calls to the validator that the user could not achieve through + <command>CREATE FUNCTION</command>. Typical checks then include verifying + that the function's argument and result types are supported by the + language, and that the function's body is syntactically correct + in the language. If the validator finds the function to be okay, + it should just return. If it finds an error, it should report that + via the normal <function>ereport()</function> error reporting mechanism. + Throwing an error will force a transaction rollback and thus prevent + the incorrect function definition from being committed. + </para> + + <para> + Validator functions should typically honor the <xref + linkend="guc-check-function-bodies"/> parameter: if it is turned off then + any expensive or context-sensitive checking should be skipped. If the + language provides for code execution at compilation time, the validator + must suppress checks that would induce such execution. In particular, + this parameter is turned off by <application>pg_dump</application> so that it can + load procedural language functions without worrying about side effects or + dependencies of the function bodies on other database objects. + (Because of this requirement, the call handler should avoid + assuming that the validator has fully checked the function. The point + of having a validator is not to let the call handler omit checks, but + to notify the user immediately if there are obvious errors in a + <command>CREATE FUNCTION</command> command.) + While the choice of exactly what to check is mostly left to the + discretion of the validator function, note that the core + <command>CREATE FUNCTION</command> code only executes <literal>SET</literal> clauses + attached to a function when <varname>check_function_bodies</varname> is on. + Therefore, checks whose results might be affected by GUC parameters + definitely should be skipped when <varname>check_function_bodies</varname> is + off, to avoid false failures when restoring a dump. + </para> + + <para> + If an inline handler is provided by a procedural language, it + must be declared as a function taking a single parameter of type + <type>internal</type>. The inline handler's result is ignored, so it is + customarily declared to return <type>void</type>. The inline handler + will be called when a <command>DO</command> statement is executed specifying + the procedural language. The parameter actually passed is a pointer + to an <structname>InlineCodeBlock</structname> struct, which contains information + about the <command>DO</command> statement's parameters, in particular the + text of the anonymous code block to be executed. The inline handler + should execute this code and return. + </para> + + <para> + It's recommended that you wrap all these function declarations, + as well as the <command>CREATE LANGUAGE</command> command itself, into + an <firstterm>extension</firstterm> so that a simple <command>CREATE EXTENSION</command> + command is sufficient to install the language. See + <xref linkend="extend-extensions"/> for information about writing + extensions. + </para> + + <para> + The procedural languages included in the standard distribution + are good references when trying to write your own language handler. + Look into the <filename>src/pl</filename> subdirectory of the source tree. + The <xref linkend="sql-createlanguage"/> + reference page also has some useful details. + </para> + + </chapter> |