44.4. Global Data in PL/Tcl

44.4. Global Data in PL/Tcl
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+ Sometimes it + is useful to have some global data that is held between two + calls to a function or is shared between different functions. + This is easily done in PL/Tcl, but there are some restrictions that + must be understood. +

+ For security reasons, PL/Tcl executes functions called by any one SQL + role in a separate Tcl interpreter for that role. This prevents + accidental or malicious interference by one user with the behavior of + another user's PL/Tcl functions. Each such interpreter will have its own + values for any “global” Tcl variables. Thus, two PL/Tcl + functions will share the same global variables if and only if they are + executed by the same SQL role. In an application wherein a single + session executes code under multiple SQL roles (via SECURITY + DEFINER functions, use of SET ROLE, etc.) you may need to + take explicit steps to ensure that PL/Tcl functions can share data. To + do that, make sure that functions that should communicate are owned by + the same user, and mark them SECURITY DEFINER. You must of + course take care that such functions can't be used to do anything + unintended. +

+ All PL/TclU functions used in a session execute in the same Tcl + interpreter, which of course is distinct from the interpreter(s) + used for PL/Tcl functions. So global data is automatically shared + between PL/TclU functions. This is not considered a security risk + because all PL/TclU functions execute at the same trust level, + namely that of a database superuser. +

+ To help protect PL/Tcl functions from unintentionally interfering + with each other, a global + array is made available to each function via the upvar + command. The global name of this variable is the function's internal + name, and the local name is GD. It is recommended that + GD be used + for persistent private data of a function. Use regular Tcl global + variables only for values that you specifically intend to be shared among + multiple functions. (Note that the GD arrays are only + global within a particular interpreter, so they do not bypass the + security restrictions mentioned above.) +

+ An example of using GD appears in the + spi_execp example below. +

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44.3. Data Values in PL/Tcl	Home	44.5. Database Access from PL/Tcl