nix repl: make symlinks with the :bl command

Requested by ppepino on the Matrix:
https://matrix.to/#/!KqkRjyTEzAGRiZFBYT:nixos.org/$Tb32BS3rVE2BSULAX4sPm0h6CDewX2hClOTGzTC7gwM?via=nixos.org&via=matrix.org&via=nixos.dev

This adds a new command, :bl, which works like :b but also creates
a GC root symlink to the various derivation outputs.

ckie@cookiemonster ~/git/nix -> ./outputs/out/bin/nix repl
Welcome to Nix 2.6.0. Type :? for help.

nix-repl> :l <nixpkgs>
Added 16118 variables.

nix-repl> :b runCommand "hello" {} "echo hi > $out"

This derivation produced the following outputs:
  ./repl-result-out -> /nix/store/kidqq2acdpi05c4a9mlbg2baikmzik44-hello
[1 built, 0.0 MiB DL]
ckie@cookiemonster ~/git/nix -> cat ./repl-result-out
hi

4 files changed, 31 insertions, 9 deletions

diff --git a/.gitignore b/.gitignore
index 58e7377fb..db363aefd 100644
--- a/.gitignore
+++ b/.gitignore
@@ -79,6 +79,7 @@ perl/Makefile.config
 /tests/shell.drv
 /tests/config.nix
 /tests/ca/config.nix
+/tests/repl-result-out
 
 # /tests/lang/
 /tests/lang/*.out
diff --git a/doc/manual/src/release-notes/rl-next.md b/doc/manual/src/release-notes/rl-next.md
index c869b5e2f..3e2998c6c 100644
--- a/doc/manual/src/release-notes/rl-next.md
+++ b/doc/manual/src/release-notes/rl-next.md
@@ -1 +1,4 @@
 # Release X.Y (202?-??-??)
+
+* `nix repl` has a new build-'n-link (`:bl`) command that builds a derivation
+  while creating GC root symlinks.
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index 1f9d4fb4e..b055698b3 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -33,6 +33,7 @@ extern "C" {
 #include "command.hh"
 #include "finally.hh"
 #include "markdown.hh"
+#include "local-fs-store.hh"
 
 #if HAVE_BOEHMGC
 #define GC_INCLUDE_NEW
@@ -419,7 +420,8 @@ bool NixRepl::processLine(std::string line)
              << "  <expr>        Evaluate and print expression\n"
              << "  <x> = <expr>  Bind expression to variable\n"
              << "  :a <expr>     Add attributes from resulting set to scope\n"
-             << "  :b <expr>     Build derivation\n"
+             << "  :b <expr>     Build a derivation\n"
+             << "  :bl <expr>    Build a derivation, creating GC roots in the working directory\n"
              << "  :e <expr>     Open package or function in $EDITOR\n"
              << "  :i <expr>     Build derivation, then install result into current profile\n"
              << "  :l <path>     Load Nix expression and add it to scope\n"
@@ -502,18 +504,26 @@ bool NixRepl::processLine(std::string line)
         runNix("nix-shell", {state->store->printStorePath(drvPath)});
     }
 
-    else if (command == ":b" || command == ":i" || command == ":s" || command == ":log") {
+    else if (command == ":b" || command == ":bl" || command == ":i" || command == ":s" || command == ":log") {
         Value v;
         evalString(arg, v);
         StorePath drvPath = getDerivationPath(v);
         Path drvPathRaw = state->store->printStorePath(drvPath);
 
-        if (command == ":b") {
+        if (command == ":b" || command == ":bl") {
             state->store->buildPaths({DerivedPath::Built{drvPath}});
             auto drv = state->store->readDerivation(drvPath);
             logger->cout("\nThis derivation produced the following outputs:");
-            for (auto & [outputName, outputPath] : state->store->queryDerivationOutputMap(drvPath))
-                logger->cout("  %s -> %s", outputName, state->store->printStorePath(outputPath));
+            for (auto & [outputName, outputPath] : state->store->queryDerivationOutputMap(drvPath)) {
+                auto localStore = state->store.dynamic_pointer_cast<LocalFSStore>();
+                if (localStore && command == ":bl") {
+                    std::string symlink = "repl-result-" + outputName;
+                    localStore->addPermRoot(outputPath, absPath(symlink));
+                    logger->cout("  ./%s -> %s", symlink, state->store->printStorePath(outputPath));
+                } else {
+                    logger->cout("  %s -> %s", outputName, state->store->printStorePath(outputPath));
+                }
+            }
         } else if (command == ":i") {
             runNix("nix-env", {"-i", drvPathRaw});
         } else if (command == ":log") {
diff --git a/tests/repl.sh b/tests/repl.sh
index 6505f1741..b6937b9e9 100644
--- a/tests/repl.sh
+++ b/tests/repl.sh
@@ -1,29 +1,37 @@
 source common.sh
 
+testDir="$PWD"
+cd "$TEST_ROOT"
+
 replCmds="
 simple = 1
-simple = import ./simple.nix
-:b simple
+simple = import $testDir/simple.nix
+:bl simple
 :log simple
 "
 
 replFailingCmds="
-failing = import ./simple-failing.nix
+failing = import $testDir/simple-failing.nix
 :b failing
 :log failing
 "
 
 replUndefinedVariable="
-import ./undefined-variable.nix
+import $testDir/undefined-variable.nix
 "
 
 testRepl () {
     local nixArgs=("$@")
+    rm -rf repl-result-out || true # cleanup from other runs backed by a foreign nix store
     local replOutput="$(nix repl "${nixArgs[@]}" <<< "$replCmds")"
     echo "$replOutput"
     local outPath=$(echo "$replOutput" |&
         grep -o -E "$NIX_STORE_DIR/\w*-simple")
     nix path-info "${nixArgs[@]}" "$outPath"
+    [ "$(realpath ./repl-result-out)" == "$outPath" ] || fail "nix repl :bl doesn't make a symlink"
+    # run it again without checking the output to ensure the previously created symlink gets overwritten
+    nix repl "${nixArgs[@]}" <<< "$replCmds" || fail "nix repl does not work twice with the same inputs"
+
     # simple.nix prints a PATH during build
     echo "$replOutput" | grep -qs 'PATH=' || fail "nix repl :log doesn't output logs"
     local replOutput="$(nix repl "${nixArgs[@]}" <<< "$replFailingCmds" 2>&1)"