Add 'nix dev-shell' and 'nix print-dev-env' command

'nix dev-shell' is intended to replace nix-shell. It supports flakes,
e.g.

  $ nix dev-shell nixpkgs:hello

starts a bash shell providing an environment for building 'hello'.

Like Lorri (and unlike nix-shell), it computes the build environment
by building a modified top-level derivation that writes the
environment after running $stdenv/setup to $out and exits. This
provides some caching, so it's faster than nix-shell in some cases
(especially for packages with lots of dependencies, where the setup
script takes a long time).

There also is a command 'nix print-dev-env' that prints out shell code
for setting up the build environment in an existing shell, e.g.

  $ . <(nix print-dev-env nixpkgs:hello)

https://github.com/tweag/nix/issues/21

src/nix/shell.cc

@@ -0,0 +1,276 @@
+#include "eval.hh"
+#include "command.hh"
+#include "common-args.hh"
+#include "shared.hh"
+#include "store-api.hh"
+#include "derivations.hh"
+#include "affinity.hh"
+#include "progress-bar.hh"
+
+using namespace nix;
+
+struct BuildEnvironment
+{
+    // FIXME: figure out which vars should be exported.
+    std::map<std::string, std::string> env;
+    std::map<std::string, std::string> functions;
+};
+
+BuildEnvironment readEnvironment(const Path & path)
+{
+    BuildEnvironment res;
+
+    auto lines = tokenizeString<Strings>(readFile(path), "\n");
+
+    auto getLine =
+        [&]() {
+            if (lines.empty())
+                throw Error("shell environment '%s' ends unexpectedly", path);
+            auto line = lines.front();
+            lines.pop_front();
+            return line;
+        };
+
+    while (!lines.empty()) {
+        auto line = getLine();
+
+        auto eq = line.find('=');
+        if (eq != std::string::npos) {
+            std::string name(line, 0, eq);
+            std::string value(line, eq + 1);
+            // FIXME: parse arrays
+            res.env.insert({name, value});
+        }
+
+        else if (hasSuffix(line, " () ")) {
+            std::string name(line, 0, line.size() - 4);
+            // FIXME: validate name
+            auto l = getLine();
+            if (l != "{ ") throw Error("shell environment '%s' has unexpected line '%s'", path, l);
+            std::string body;
+            while ((l = getLine()) != "}") {
+                body += l;
+                body += '\n';
+            }
+            res.functions.insert({name, body});
+        }
+
+        else throw Error("shell environment '%s' has unexpected line '%s'", path, line);
+    }
+
+    return res;
+}
+
+/* Given an existing derivation, return the shell environment as
+   initialised by stdenv's setup script. We do this by building a
+   modified derivation with the same dependencies and nearly the same
+   initial environment variables, that just writes the resulting
+   environment to a file and exits. */
+BuildEnvironment getDerivationEnvironment(ref<Store> store, Derivation drv)
+{
+    auto builder = baseNameOf(drv.builder);
+    if (builder != "bash")
+        throw Error("'nix shell' only works on derivations that use 'bash' as their builder");
+
+    drv.args = {"-c", "set -e; if [[ -n $stdenv ]]; then source $stdenv/setup; fi; set > $out"};
+
+    /* Remove derivation checks. */
+    drv.env.erase("allowedReferences");
+    drv.env.erase("allowedRequisites");
+    drv.env.erase("disallowedReferences");
+    drv.env.erase("disallowedRequisites");
+
+    // FIXME: handle structured attrs
+
+    /* Rehash and write the derivation. FIXME: would be nice to use
+       'buildDerivation', but that's privileged. */
+    auto drvName = drv.env["name"] + "-env";
+    for (auto & output : drv.outputs)
+        drv.env.erase(output.first);
+    drv.env["out"] = "";
+    drv.env["outputs"] = "out";
+    drv.outputs["out"] = DerivationOutput("", "", "");
+    Hash h = hashDerivationModulo(*store, drv);
+    Path shellOutPath = store->makeOutputPath("out", h, drvName);
+    drv.outputs["out"].path = shellOutPath;
+    drv.env["out"] = shellOutPath;
+    Path shellDrvPath2 = writeDerivation(store, drv, drvName);
+
+    /* Build the derivation. */
+    store->buildPaths({shellDrvPath2});
+
+    assert(store->isValidPath(shellOutPath));
+
+    return readEnvironment(shellOutPath);
+}
+
+struct Common : InstallableCommand
+{
+    /*
+    std::set<string> keepVars{
+        "DISPLAY",
+        "HOME",
+        "IN_NIX_SHELL",
+        "LOGNAME",
+        "NIX_BUILD_SHELL",
+        "PAGER",
+        "PATH",
+        "TERM",
+        "TZ",
+        "USER",
+    };
+    */
+
+    std::set<string> ignoreVars{
+        "BASHOPTS",
+        "EUID",
+        "NIX_BUILD_TOP",
+        "PPID",
+        "PWD",
+        "SHELLOPTS",
+        "SHLVL",
+        "TEMP",
+        "TEMPDIR",
+        "TMP",
+        "TMPDIR",
+        "UID",
+    };
+
+    void makeRcScript(const BuildEnvironment & buildEnvironment, std::ostream & out)
+    {
+        for (auto & i : buildEnvironment.env) {
+            // FIXME: shellEscape
+            // FIXME: figure out what to export
+            // FIXME: handle arrays
+            if (!ignoreVars.count(i.first) && !hasPrefix(i.first, "BASH_"))
+                out << fmt("export %s=%s\n", i.first, i.second);
+        }
+
+        for (auto & i : buildEnvironment.functions) {
+            out << fmt("%s () {\n%s\n}\n", i.first, i.second);
+        }
+
+        // FIXME: set outputs
+
+        out << "export NIX_BUILD_TOP=\"$(mktemp -d --tmpdir nix-shell.XXXXXX)\"\n";
+        for (auto & i : {"TMP", "TMPDIR", "TEMP", "TEMPDIR"})
+            out << fmt("export %s=\"$NIX_BUILD_TOP\"\n", i);
+    }
+};
+
+std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix = "nix")
+{
+    Path tmpl(getEnv("TMPDIR", "/tmp") + "/" + prefix + ".XXXXXX");
+    // Strictly speaking, this is UB, but who cares...
+    AutoCloseFD fd(mkstemp((char *) tmpl.c_str()));
+    if (!fd)
+        throw SysError("creating temporary file '%s'", tmpl);
+    return {std::move(fd), tmpl};
+}
+
+struct CmdDevShell : Common
+{
+
+    std::string name() override
+    {
+        return "dev-shell";
+    }
+
+    std::string description() override
+    {
+        return "run a bash shell that provides the build environment of a derivation";
+    }
+
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To get the build environment of GNU hello:",
+                "nix dev-shell nixpkgs:hello"
+            },
+            Example{
+                "To get the build environment of the default package of flake in the current directory:",
+                "nix dev-shell"
+            },
+        };
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto drvs = toDerivations(store, {installable});
+
+        if (drvs.size() != 1)
+            throw Error("'%s' needs to evaluate to a single derivation, but it evaluated to %d derivations",
+                installable->what(), drvs.size());
+
+        auto & drvPath = *drvs.begin();
+
+        auto buildEnvironment = getDerivationEnvironment(store, store->derivationFromPath(drvPath));
+
+        auto [rcFileFd, rcFilePath] = createTempFile("nix-shell");
+
+        std::ostringstream ss;
+        makeRcScript(buildEnvironment, ss);
+
+        ss << fmt("rm -f '%s'\n", rcFilePath);
+
+        writeFull(rcFileFd.get(), ss.str());
+
+        stopProgressBar();
+
+        auto shell = getEnv("SHELL", "bash");
+
+        auto args = Strings{baseNameOf(shell), "--rcfile", rcFilePath};
+
+        restoreAffinity();
+        restoreSignals();
+
+        execvp(shell.c_str(), stringsToCharPtrs(args).data());
+
+        throw SysError("executing shell '%s'", shell);
+    }
+};
+
+struct CmdPrintDevEnv : Common
+{
+
+    std::string name() override
+    {
+        return "print-dev-env";
+    }
+
+    std::string description() override
+    {
+        return "print shell code that can be sourced by bash to reproduce the build environment of a derivation";
+    }
+
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To apply the build environment of GNU hello to the current shell:",
+                ". <(nix print-dev-env nixpkgs:hello)"
+            },
+        };
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto drvs = toDerivations(store, {installable});
+
+        if (drvs.size() != 1)
+            throw Error("'%s' needs to evaluate to a single derivation, but it evaluated to %d derivations",
+                installable->what(), drvs.size());
+
+        auto & drvPath = *drvs.begin();
+
+        auto buildEnvironment = getDerivationEnvironment(store, store->derivationFromPath(drvPath));
+
+        stopProgressBar();
+
+        makeRcScript(buildEnvironment, std::cout);
+    }
+};
+
+static RegisterCommand r1(make_ref<CmdPrintDevEnv>());
+static RegisterCommand r2(make_ref<CmdDevShell>());