doc/src/sgml/plhandler.sgml

   1 <!-- doc/src/sgml/plhandler.sgml -->
   2
   3  <chapter id="plhandler">
   4    <title>Writing a Procedural Language Handler</title>
   5
   6    <indexterm zone="plhandler">
   7     <primary>procedural language</primary>
   8     <secondary>handler for</secondary>
   9    </indexterm>
  10
  11    <para>
  12     All calls to functions that are written in a language other than
  13     the current <quote>version 1</quote> interface for compiled
  14     languages (this includes functions in user-defined procedural languages
  15     and functions written in SQL) go through a <firstterm>call handler</firstterm>
  16     function for the specific language.  It is the responsibility of
  17     the call handler to execute the function in a meaningful way, such
  18     as by interpreting the supplied source text.  This chapter outlines
  19     how a new procedural language's call handler can be written.
  20    </para>
  21
  22    <para>
  23     The call handler for a procedural language is a
  24     <quote>normal</quote> function that must be written in a compiled
  25     language such as C, using the version-1 interface, and registered
  26     with <productname>PostgreSQL</productname> as taking no arguments
  27     and returning the type <type>language_handler</type>.  This
  28     special pseudo-type identifies the function as a call handler and
  29     prevents it from being called directly in SQL commands.
  30     For more details on C language calling conventions and dynamic loading,
  31     see <xref linkend="xfunc-c"/>.
  32    </para>
  33
  34    <para>
  35     The call handler is called in the same way as any other function:
  36     It receives a pointer to a
  37     <structname>FunctionCallInfoBaseData</structname> <type>struct</type> containing
  38     argument values and information about the called function, and it
  39     is expected to return a <type>Datum</type> result (and possibly
  40     set the <structfield>isnull</structfield> field of the
  41     <structname>FunctionCallInfoBaseData</structname> structure, if it wishes
  42     to return an SQL null result).  The difference between a call
  43     handler and an ordinary callee function is that the
  44     <structfield>flinfo-&gt;fn_oid</structfield> field of the
  45     <structname>FunctionCallInfoBaseData</structname> structure will contain
  46     the OID of the actual function to be called, not of the call
  47     handler itself.  The call handler must use this field to determine
  48     which function to execute.  Also, the passed argument list has
  49     been set up according to the declaration of the target function,
  50     not of the call handler.
  51    </para>
  52
  53    <para>
  54     It's up to the call handler to fetch the entry of the function from the
  55     <classname>pg_proc</classname> system catalog and to analyze the argument
  56     and return types of the called function. The <literal>AS</literal> clause from the
  57     <command>CREATE FUNCTION</command> command for the function will be found
  58     in the <literal>prosrc</literal> column of the
  59     <classname>pg_proc</classname> row. This is commonly source
  60     text in the procedural language, but in theory it could be something else,
  61     such as a path name to a file, or anything else that tells the call handler
  62     what to do in detail.
  63    </para>
  64
  65    <para>
  66     Often, the same function is called many times per SQL statement.
  67     A call handler can avoid repeated lookups of information about the
  68     called function by using the
  69     <structfield>flinfo-&gt;fn_extra</structfield> field.  This will
  70     initially be <symbol>NULL</symbol>, but can be set by the call handler to point at
  71     information about the called function.  On subsequent calls, if
  72     <structfield>flinfo-&gt;fn_extra</structfield> is already non-<symbol>NULL</symbol>
  73     then it can be used and the information lookup step skipped.  The
  74     call handler must make sure that
  75     <structfield>flinfo-&gt;fn_extra</structfield> is made to point at
  76     memory that will live at least until the end of the current query,
  77     since an <structname>FmgrInfo</structname> data structure could be
  78     kept that long.  One way to do this is to allocate the extra data
  79     in the memory context specified by
  80     <structfield>flinfo-&gt;fn_mcxt</structfield>; such data will
  81     normally have the same lifespan as the
  82     <structname>FmgrInfo</structname> itself.  But the handler could
  83     also choose to use a longer-lived memory context so that it can cache
  84     function definition information across queries.
  85    </para>
  86
  87    <para>
  88     When a procedural-language function is invoked as a trigger, no arguments
  89     are passed in the usual way, but the
  90     <structname>FunctionCallInfoBaseData</structname>'s
  91     <structfield>context</structfield> field points at a
  92     <structname>TriggerData</structname> structure, rather than being <symbol>NULL</symbol>
  93     as it is in a plain function call.  A language handler should
  94     provide mechanisms for procedural-language functions to get at the trigger
  95     information.
  96    </para>
  97
  98    <para>
  99     A template for a procedural-language handler written as a C extension is
 100     provided in <literal>src/test/modules/plsample</literal>.  This is a
 101     working sample demonstrating one way to create a procedural-language
 102     handler, process parameters, and return a value.
 103    </para>
 104
 105    <para>
 106     Although providing a call handler is sufficient to create a minimal
 107     procedural language, there are two other functions that can optionally
 108     be provided to make the language more convenient to use.  These
 109     are a <firstterm>validator</firstterm> and an
 110     <firstterm>inline handler</firstterm>.  A validator can be provided
 111     to allow language-specific checking to be done during
 112     <xref linkend="sql-createfunction"/>.
 113     An inline handler can be provided to allow the language to support
 114     anonymous code blocks executed via the <xref linkend="sql-do"/> command.
 115    </para>
 116
 117    <para>
 118     If a validator is provided by a procedural language, it
 119     must be declared as a function taking a single parameter of type
 120     <type>oid</type>.  The validator's result is ignored, so it is customarily
 121     declared to return <type>void</type>.  The validator will be called at
 122     the end of a <command>CREATE FUNCTION</command> command that has created
 123     or updated a function written in the procedural language.
 124     The passed-in OID is the OID of the function's <classname>pg_proc</classname>
 125     row.  The validator must fetch this row in the usual way, and do
 126     whatever checking is appropriate.
 127     First, call <function>CheckFunctionValidatorAccess()</function> to diagnose
 128     explicit calls to the validator that the user could not achieve through
 129     <command>CREATE FUNCTION</command>.  Typical checks then include verifying
 130     that the function's argument and result types are supported by the
 131     language, and that the function's body is syntactically correct
 132     in the language.  If the validator finds the function to be okay,
 133     it should just return.  If it finds an error, it should report that
 134     via the normal <function>ereport()</function> error reporting mechanism.
 135     Throwing an error will force a transaction rollback and thus prevent
 136     the incorrect function definition from being committed.
 137    </para>
 138
 139    <para>
 140     Validator functions should typically honor the <xref
 141     linkend="guc-check-function-bodies"/> parameter: if it is turned off then
 142     any expensive or context-sensitive checking should be skipped.  If the
 143     language provides for code execution at compilation time, the validator
 144     must suppress checks that would induce such execution.  In particular,
 145     this parameter is turned off by <application>pg_dump</application> so that it can
 146     load procedural language functions without worrying about side effects or
 147     dependencies of the function bodies on other database objects.
 148     (Because of this requirement, the call handler should avoid
 149     assuming that the validator has fully checked the function.  The point
 150     of having a validator is not to let the call handler omit checks, but
 151     to notify the user immediately if there are obvious errors in a
 152     <command>CREATE FUNCTION</command> command.)
 153     While the choice of exactly what to check is mostly left to the
 154     discretion of the validator function, note that the core
 155     <command>CREATE FUNCTION</command> code only executes <literal>SET</literal> clauses
 156     attached to a function when <varname>check_function_bodies</varname> is on.
 157     Therefore, checks whose results might be affected by GUC parameters
 158     definitely should be skipped when <varname>check_function_bodies</varname> is
 159     off, to avoid false failures when restoring a dump.
 160    </para>
 161
 162    <para>
 163     If an inline handler is provided by a procedural language, it
 164     must be declared as a function taking a single parameter of type
 165     <type>internal</type>.  The inline handler's result is ignored, so it is
 166     customarily declared to return <type>void</type>.  The inline handler
 167     will be called when a <command>DO</command> statement is executed specifying
 168     the procedural language.  The parameter actually passed is a pointer
 169     to an <structname>InlineCodeBlock</structname> struct, which contains information
 170     about the <command>DO</command> statement's parameters, in particular the
 171     text of the anonymous code block to be executed.  The inline handler
 172     should execute this code and return.
 173    </para>
 174
 175    <para>
 176     It's recommended that you wrap all these function declarations,
 177     as well as the <command>CREATE LANGUAGE</command> command itself, into
 178     an <firstterm>extension</firstterm> so that a simple <command>CREATE EXTENSION</command>
 179     command is sufficient to install the language.  See
 180     <xref linkend="extend-extensions"/> for information about writing
 181     extensions.
 182    </para>
 183
 184    <para>
 185     The procedural languages included in the standard distribution
 186     are good references when trying to write your own language handler.
 187     Look into the <filename>src/pl</filename> subdirectory of the source tree.
 188     The <xref linkend="sql-createlanguage"/>
 189     reference page also has some useful details.
 190    </para>
 191
 192  </chapter>