nix-shell: Support --out-link

doc/manual/src/command-ref/nix-shell.md

@@ -13,6 +13,8 @@
   [`--exclude` *regexp*]
   [`--pure`]
   [`--keep` *name*]
+  [`--no-out-link`]
+  [{`--out-link` | `-o`} *outlink*]
   {{`--packages` | `-p`} {*packages* | *expressions*} … | [*path*]}
 
 # Disambiguation
@@ -101,6 +103,16 @@ All options not listed here are passed to `nix-store
     When a `--pure` shell is started, keep the listed environment
     variables.
 
+  - [`--no-out-link`]{#opt-no-out-link}\
+    Do not create a symlink to the output path. This is the default.
+    Note that as a result the output does not become a root of the
+    garbage collector, and so might be deleted by `nix-store --gc`.
+
+  - [`--out-link`]{#opt-out-link} / `-o` *outlink*\
+    Create a symlink to the output path of the shell environment. The
+    output will then become a root of the garbage collector and will
+    not be deleted by `nix-store --gc`.
+
 The following common options are supported:
 
 # Environment variables

doc/manual/src/release-notes/rl-next.md

@@ -5,3 +5,4 @@
   arguments will be ignored and the resulting derivation will have
   `__impure` set to `true`, making it an impure derivation.
 
+* `nix-shell` now accepts `--out-link` and `--no-out-link` flags.

src/nix-build/nix-build.cc

@@ -99,7 +99,7 @@ static void main_nix_build(int argc, char * * argv)
 
     AutoDelete tmpDir(createTempDir("", myName));
 
-    std::string outLink = "./result";
+    std::optional<std::string> outLink = runEnv ? std::nullopt : std::optional{"./result"};
 
     // List of environment variables kept for --pure
     std::set<std::string> keepVars{
@@ -155,7 +155,7 @@ static void main_nix_build(int argc, char * * argv)
             ; // obsolete
 
         else if (*arg == "--no-out-link" || *arg == "--no-link")
-            outLink = (Path) tmpDir + "/result";
+            outLink = std::nullopt;
 
         else if (*arg == "--attr" || *arg == "-A")
             attrPaths.push_back(getArg(*arg, arg, end));
@@ -266,7 +266,8 @@ static void main_nix_build(int argc, char * * argv)
         joined << "{...}@args: with import <nixpkgs> args; (pkgs.runCommandCC or pkgs.runCommand) \"shell\" { buildInputs = [ ";
         for (const auto & i : left)
             joined << '(' << i << ") ";
-        joined << "]; } \"\"";
+        joined << "]; } ";
+        joined << (outLink ? "\"touch $out\"" : "\"\"");
         fromArgs = true;
         left = {joined.str()};
     } else if (!fromArgs) {
@@ -366,6 +367,14 @@ static void main_nix_build(int argc, char * * argv)
             store->buildPaths(paths, buildMode, evalStore);
     };
 
+    auto writeOutLink = [&](std::string_view prefix, const StorePath & outputPath, std::string_view outputName) {
+        if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>()) {
+            std::string symlink(prefix);
+            if (outputName != "out") symlink += "-" + outputName;
+            store2->addPermRoot(outputPath, absPath(symlink));
+        }
+    };
+
     if (runEnv) {
         if (drvs.size() != 1)
             throw UsageError("nix-shell requires a single derivation");
@@ -431,6 +440,13 @@ static void main_nix_build(int argc, char * * argv)
             pathsToCopy.insert(src);
         }
 
+        if (outLink) {
+            pathsToBuild.push_back(DerivedPath::Built {
+                .drvPath = drvInfo.requireDrvPath(),
+                .outputs = drv.outputNames()
+            });
+        }
+
         buildPaths(pathsToBuild);
 
         if (dryRun) return;
@@ -446,6 +462,11 @@ static void main_nix_build(int argc, char * * argv)
             drv = *resolvedDrv;
         }
 
+        if (outLink) {
+            for (auto & [outputName, output] : drv.outputsAndOptPaths(*store))
+                if (output.second) writeOutLink(*outLink, *output.second, outputName);
+        }
+
         // Set the environment.
         auto env = getEnv();
 
@@ -610,7 +631,7 @@ static void main_nix_build(int argc, char * * argv)
 
         for (auto & [drvPath, outputName] : pathsToBuildOrdered) {
             auto & [counter, _wantedOutputs] = drvMap.at({drvPath});
-            std::string drvPrefix = outLink;
+            std::string drvPrefix = outLink.value_or((Path) tmpDir + "/result");
             if (counter)
                 drvPrefix += fmt("-%d", counter + 1);
 
@@ -620,11 +641,7 @@ static void main_nix_build(int argc, char * * argv)
             assert(maybeOutputPath);
             auto outputPath = *maybeOutputPath;
 
-            if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>()) {
-                std::string symlink = drvPrefix;
-                if (outputName != "out") symlink += "-" + outputName;
-                store2->addPermRoot(outputPath, absPath(symlink));
-            }
+            writeOutLink(drvPrefix, outputPath, outputName);
 
             outPaths.push_back(outputPath);
         }

tests/nix-shell.sh

@@ -84,6 +84,14 @@ chmod a+rx $TEST_ROOT/spaced\ \\\'\"shell.shebang.rb
 output=$($TEST_ROOT/spaced\ \\\'\"shell.shebang.rb abc ruby)
 [ "$output" = '-e load(ARGV.shift) -- '"$TEST_ROOT"'/spaced \'\''"shell.shebang.rb abc ruby' ]
 
+# Test nix-shell --out-link -p creates gcroot
+nix-shell --pure -p foo --out-link foo-root
+[ -L foo-root ] && rm foo-root
+
+# Test nix-shell --out-link -A creates gcroot
+nix-shell --pure -A foo --out-link foo-root
+[ -L foo-root ] && rm foo-root
+
 # Test 'nix develop'.
 nix develop -f "$shellDotNix" shellDrv -c bash -c '[[ -n $stdenv ]]'