nix-shell: Use bashInteractive from <nixpkgs>

This adds about 0.1s to nix-shell runtime in the case where
bashInteractive already exists.

See discussion at https://github.com/NixOS/nixpkgs/issues/27493.

src/libexpr/get-drvs.cc

@@ -267,15 +267,14 @@ static bool getDerivation(EvalState & state, Value & v,
 }
 
 
-bool getDerivation(EvalState & state, Value & v, DrvInfo & drv,
+std::experimental::optional<DrvInfo> getDerivation(EvalState & state, Value & v,
     bool ignoreAssertionFailures)
 {
     Done done;
     DrvInfos drvs;
     getDerivation(state, v, "", drvs, done, ignoreAssertionFailures);
-    if (drvs.size() != 1) return false;
-    drv = drvs.front();
-    return true;
+    if (drvs.size() != 1) return {};
+    return std::move(drvs.front());
 }
 
 

src/libexpr/get-drvs.hh

@@ -75,10 +75,10 @@ typedef list<DrvInfo> DrvInfos;
 #endif
 
 
-/* If value `v' denotes a derivation, store information about the
-   derivation in `drv' and return true.  Otherwise, return false. */
-bool getDerivation(EvalState & state, Value & v, DrvInfo & drv,
-    bool ignoreAssertionFailures);
+/* If value `v' denotes a derivation, return a DrvInfo object
+   describing it. Otherwise return nothing. */
+std::experimental::optional<DrvInfo> getDerivation(EvalState & state,
+    Value & v, bool ignoreAssertionFailures);
 
 void getDerivations(EvalState & state, Value & v, const string & pathPrefix,
     Bindings & autoArgs, DrvInfos & drvs,

src/nix-build/local.mk

@@ -4,6 +4,6 @@ nix-build_DIR := $(d)
 
 nix-build_SOURCES := $(d)/nix-build.cc
 
-nix-build_LIBS = libmain libstore libutil libformat
+nix-build_LIBS = libmain libexpr libstore libutil libformat
 
 $(eval $(call install-symlink, nix-build, $(bindir)/nix-shell))

src/nix-build/nix-build.cc

@@ -13,6 +13,8 @@
 #include "affinity.hh"
 #include "util.hh"
 #include "shared.hh"
+#include "eval.hh"
+#include "get-drvs.hh"
 
 using namespace nix;
 using namespace std::string_literals;
@@ -75,6 +77,8 @@ int main(int argc, char ** argv)
 {
     return handleExceptions(argv[0], [&]() {
         initNix();
+        initGC();
+
         auto store = openStore();
         auto dryRun = false;
         auto verbose = false;
@@ -88,6 +92,7 @@ int main(int argc, char ** argv)
         Strings instArgs;
         Strings buildArgs;
         Strings exprs;
+        Strings searchPath;
 
         auto shell = getEnv("SHELL", "/bin/sh");
         std::string envCommand; // interactive shell
@@ -320,6 +325,8 @@ int main(int argc, char ** argv)
             }
         }
 
+        EvalState state(searchPath, store);
+
         if (packages && fromArgs) {
             throw UsageError("‘-p’ and ‘-E’ are mutually exclusive");
         }
@@ -465,7 +472,42 @@ int main(int argc, char ** argv)
 
                 auto envPtrs = stringsToCharPtrs(envStrs);
 
-                auto shell = getEnv("NIX_BUILD_SHELL", "bash");
+                auto shell = getEnv("NIX_BUILD_SHELL", "");
+
+                if (shell == "") {
+
+                    try {
+
+                        auto expr = state.parseExprFromString("(import <nixpkgs> {}).bashInteractive", absPath("."));
+
+                        Value v;
+                        state.eval(expr, v);
+
+                        auto drv = getDerivation(state, v, false);
+                        if (!drv)
+                            throw Error("the ‘bashInteractive’ attribute in <nixpkgs> did not evaluate to a derivation");
+
+                        auto drvPath = drv->queryDrvPath();
+
+                        unsigned long long downloadSize, narSize;
+                        PathSet willBuild, willSubstitute, unknown;
+                        store->queryMissing({drvPath},
+                            willBuild, willSubstitute, unknown, downloadSize, narSize);
+
+                        if (settings.printMissing)
+                            printMissing(ref<Store>(store), willBuild, willSubstitute, unknown, downloadSize, narSize);
+
+                        store->buildPaths({drvPath});
+
+                        shell = drv->queryOutPath() + "/bin/bash";
+                        if (!pathExists(shell))
+                            throw Error("expected shell ‘%s’ to exist, but it doesn't", shell);
+
+                    } catch (Error & e) {
+                        printError("warning: %s; will use bash from your environment", e.what());
+                        shell = "bash";
+                    }
+                }
 
                 environ = envPtrs.data();
 

src/nix/repl.cc

@@ -292,10 +292,10 @@ bool isVarName(const string & s)
 
 
 Path NixRepl::getDerivationPath(Value & v) {
-    DrvInfo drvInfo(state);
-    if (!getDerivation(state, v, drvInfo, false))
+    auto drvInfo = getDerivation(state, v, false);
+    if (!drvInfo)
         throw Error("expression does not evaluate to a derivation, so I can't build it");
-    Path drvPath = drvInfo.queryDrvPath();
+    Path drvPath = drvInfo->queryDrvPath();
     if (drvPath == "" || !state.store->isValidPath(drvPath))
         throw Error("expression did not evaluate to a valid derivation");
     return drvPath;