postgresql: provide pltcl extension as package

This allows us to always build the extension, but still have the user
explicitly enable it without causing rebuilds.

doc/release-notes/rl-2505.section.md

···
   The `nixLog` function, which logs unconditionally, was also re-introduced and modified to prefix messages with the function name of the caller.
   For more information, [see this PR](https://github.com/NixOS/nixpkgs/pull/370742).
 
-- `postgresql`'s `pythonSupport` argument has been changed. It is now enabled by default, but to use PL/Python the extension needs to be added explicitly with `postgresql.withPackages`. If you were using `postgresql.override { pythonSupport = true; }` before, change it to `postgresql.withPackages (ps: [ ps.plpython3 ])`. The same applies to `perlSupport`/`plperl` respectively.
+- `postgresql`'s `pythonSupport` argument has been changed. It is now enabled by default, but to use PL/Python the extension needs to be added explicitly with `postgresql.withPackages`. If you were using `postgresql.override { pythonSupport = true; }` before, change it to `postgresql.withPackages (ps: [ ps.plpython3 ])`. The same applies to `perlSupport`/`plperl` and `tclSupport`/`pltcl` respectively.
 
 - The `rustPlatform.fetchCargoTarball` function is deprecated, because it relied on `cargo vendor` not changing its output format to keep fixed-output derivation hashes the same, which is a Nix invariant, and Cargo 1.84.0 changed `cargo vendor`'s output format.
   It should generally be replaced with `rustPlatform.fetchCargoVendor`, but `rustPlatform.importCargoLock` may also be appropriate in some circumstances.

pkgs/servers/sql/postgresql/ext.nix

···
 // lib.optionalAttrs (!self.pythonSupport) {
   plpython3 = throw "PostgreSQL extension `plpython3` is not available, because `postgresql` was built without Python support. Override with `pythonSupport = true` to enable the extension.";
 }
+// lib.optionalAttrs (!self.tclSupport) {
+  pltcl = throw "PostgreSQL extension `pltcl` is not available, because `postgresql` was built without Tcl support. Override with `tclSupport = true` to enable the extension.";
+}
 // lib.optionalAttrs config.allowAliases {
   pg_embedding = throw "PostgreSQL extension `pg_embedding` has been removed since the project has been abandoned. Upstream's recommendation is to use pgvector instead (https://neon.tech/docs/extensions/pg_embedding#migrate-from-pg_embedding-to-pgvector)";
 }

pkgs/servers/sql/postgresql/ext/pltcl.nix

···
+{
+  buildEnv,
+  lib,
+  postgresql,
+  postgresqlTestExtension,
+  tcl,
+  tclPackages,
+}:
+
+let
+  withPackages =
+    f:
+    let
+      pkgs = f tclPackages;
+      paths = lib.concatMapStringsSep " " (pkg: "${pkg}/lib") pkgs;
+      finalPackage = buildEnv {
+        name = "${postgresql.pname}-pltcl-${postgresql.version}";
+        paths = [ postgresql.pltcl ];
+        passthru = {
+          inherit withPackages;
+          wrapperArgs = [
+            ''--set TCLLIBPATH "${paths}"''
+          ];
+          tests.extension = postgresqlTestExtension {
+            finalPackage = finalPackage.withPackages (ps: [
+              ps.mustache-tcl
+              ps.tcllib
+            ]);
+            sql = ''
+              CREATE EXTENSION pltclu;
+              CREATE FUNCTION test() RETURNS VOID
+              LANGUAGE pltclu AS $$
+                package require mustache
+              $$;
+              SELECT test();
+            '';
+          };
+        };
+        meta = {
+          inherit (postgresql.meta)
+            homepage
+            license
+            changelog
+            maintainers
+            platforms
+            ;
+          description = "PL/Tcl - Tcl Procedural Language";
+        };
+      };
+    in
+    finalPackage;
+in
+withPackages (_: [ ])

pkgs/servers/sql/postgresql/generic.nix

···
       python3,
 
       # PL/Tcl
-      tclSupport ? false,
+      tclSupport ?
+        lib.meta.availableOn stdenv.hostPlatform tcl
+        # tcl is broken in pkgsStatic
+        && !stdenv.hostPlatform.isStatic
+        # configure fails with:
+        #   configure: error: file 'tclConfig.sh' is required for Tcl
+        && stdenv.buildPlatform.canExecute stdenv.hostPlatform,
       tcl,
 
       # SELinux
···
           "man"
         ]
         ++ lib.optionals perlSupport [ "plperl" ]
-        ++ lib.optionals pythonSupport [ "plpython3" ];
+        ++ lib.optionals pythonSupport [ "plpython3" ]
+        ++ lib.optionals tclSupport [ "pltcl" ];
       outputChecks = {
         out = {
           disallowedReferences = [
···
         + lib.optionalString pythonSupport ''
           moveToOutput "lib/*plpython3*" "$plpython3"
           moveToOutput "share/postgresql/extension/*plpython3*" "$plpython3"
+        ''
+        + lib.optionalString tclSupport ''
+          moveToOutput "lib/*pltcl*" "$pltcl"
+          moveToOutput "share/postgresql/extension/*pltcl*" "$pltcl"
         '';
 
       postFixup = lib.optionalString stdenv'.hostPlatform.isGnu ''
···
           pkgs =
             let
               scope = {
-                inherit jitSupport pythonSupport perlSupport;
+                inherit
+                  jitSupport
+                  pythonSupport
+                  perlSupport
+                  tclSupport
+                  ;
                 inherit (llvmPackages) llvm;
                 postgresql = this;
                 stdenv = stdenv';