static-nix-shell: only generate preambles when theyre needed

the previous logic would append : to PATH when the binary did not require PATH additions, however that's not a no-op. i think it causes PATH lookups in PWD. guard against that by only outputting preambles when we would expect them to have a side effect

pkgs/by-name/static-nix-shell/package.nix

@@ -221,12 +221,17 @@ in rec {
       inherit pkgsEnv pkgExprs;
       interpreter = lib.getExe bash;
       postConfigure = ''
-        shellPreamble='
+      if [[ -n "$append_PATH" ]]; then
-        export PATH=''${PATH:+$PATH:}'"$append_PATH"'
+        shellPreamble="$shellPreamble"'
-        export XDG_DATA_DIRS=''${XDG_DATA_DIRS:+$XDG_DATA_DIRS:}'"$append_XDG_DATA_DIRS"'
+      export PATH=''${PATH:+$PATH:}'"$append_PATH"
-        '
         unset append_PATH
+      fi
+
+      if [[ -n "$append_XDG_DATA_DIRS" ]]; then
+        shellPreamble="$shellPreamble"'
+      export XDG_DATA_DIRS=''${XDG_DATA_DIRS:+$XDG_DATA_DIRS:}'"$append_XDG_DATA_DIRS"
         unset append_XDG_DATA_DIRS
+      fi
       '';
     } // (removeAttrs attrs [ "bash" "pkgs" ])
   );
@@ -241,6 +246,18 @@ in rec {
       inherit pkgsEnv pkgExprs;
       interpreter = lib.getExe' oils-for-unix "ysh";
       postConfigure = ''
+        shellPreambleBody=
+        if [[ -n "$append_PATH" ]]; then
+          shellPreambleBody="$shellPreambleBody"'
+          addToSearchPath "PATH" "'"$append_PATH"'"'
+          unset append_PATH
+        fi
+        if [[ -n "$append_XDG_DATA_DIRS" ]]; then
+          shellPreambleBody="$shellPreambleBody"'
+          addToSearchPath "XDG_DATA_DIRS" "'"$append_XDG_DATA_DIRS"'"'
+          unset append_XDG_DATA_DIRS
+        fi
+        if [[ -n "$shellPreambleBody" ]]; then
         shellPreamble='
         {
           proc addToSearchPath(envName, appendValue) {
@@ -251,15 +268,12 @@ in rec {
               setglobal ENV[envName] = "$value:$appendValue"
             }
           }
-
+        '"$shellPreambleBody"'
-          addToSearchPath "PATH" "'"$append_PATH"'"
-          addToSearchPath "XDG_DATA_DIRS" "'"$append_XDG_DATA_DIRS"'"
 
           unset addToSearchPath
         }
         '
-        unset append_PATH
+        fi
-        unset append_XDG_DATA_DIRS
       '';
     } // (removeAttrs attrs [ "oils-for-unix" "pkgs" ])
   );
@@ -274,12 +288,17 @@ in rec {
       inherit pkgsEnv pkgExprs;
       interpreter = lib.getExe zsh;
       postConfigure = ''
-        shellPreamble='
+        if [[ -n "$append_PATH" ]]; then
-        export PATH=''${PATH:+$PATH:}'"$append_PATH"'
+          shellPreamble="$shellPreamble"'
-        export XDG_DATA_DIRS=''${XDG_DATA_DIRS:+$XDG_DATA_DIRS:}'"$append_XDG_DATA_DIRS"'
+        export PATH=''${PATH:+$PATH:}'"$append_PATH"
-        '
+          unset append_PATH
-        unset append_PATH
+        fi
-        unset append_XDG_DATA_DIRS
+
+        if [[ -n "$append_XDG_DATA_DIRS" ]]; then
+          shellPreamble="$shellPreamble"'
+        export XDG_DATA_DIRS=''${XDG_DATA_DIRS:+$XDG_DATA_DIRS:}'"$append_XDG_DATA_DIRS"
+          unset append_XDG_DATA_DIRS
+        fi
       '';
     } // (removeAttrs attrs [ "pkgs" "zsh" ])
   );
@@ -302,15 +321,32 @@ in rec {
           addToSearchPath append_PYTHONPATH "$p/${python3.sitePackages}"
         done
 
-        shellPreamble='
+        shellPreambleBody=
+        if [[ -n "$append_PATH" ]]; then
+          shellPreambleBody="$shellPreambleBody"'
+        addToSearchPath("PATH", "'"$append_PATH"'")'
+          unset append_PATH
+        fi
+        if [[ -n "$append_XDG_DATA_DIRS" ]]; then
+          shellPreambleBody="$shellPreambleBody"'
+        addToSearchPath("XDG_DATA_DIRS", "'"$append_XDG_DATA_DIRS"'")'
+          unset append_XDG_DATA_DIRS
+        fi
+
+        if [[ -n "$append_PYTHONPATH" ]]; then
+          shellPreambleBody="$shellPreambleBody"'
+        addSiteDirs("'"$append_PYTHONPATH"'")'
+          unset append_PYTHONPATH
+        fi
+
+        if [[ -n "$shellPreambleBody" ]]; then
+          shellPreamble='
         # this preamble is unaware of the tab-style used in the rest of the file,
         # so wrap it in a big `exec` to avoid conflicting tab styles
         # (and to avoid polluting the globals)
         exec("""
-        import os
-        import site
-
         def addToSearchPath(envName, appendValue):
+          import os
           value = os.environ.get(envName)
           if value is None:
             os.environ[envName] = appendValue
@@ -318,18 +354,15 @@ in rec {
             os.environ[envName] = value + ":" + appendValue
 
         def addSiteDirs(joinedDirs):
+          import site
           known = site._init_pathinfo()
           for p in joinedDirs.split(":"):
             known = site.addsitedir(p, known)
 
-        addToSearchPath("PATH", "'"$append_PATH"'")
+        '"$shellPreambleBody"'
-        addToSearchPath("XDG_DATA_DIRS", "'"$append_XDG_DATA_DIRS"'")
-        addSiteDirs("'"$append_PYTHONPATH"'")
         """, globals={})
         '
-        unset append_PATH
+        fi
-        unset append_PYTHONPATH
-        unset append_XDG_DATA_DIRS
       '';
     } // (removeAttrs attrs [ "pkgs" "python3" ])
   );