Add "nix mount-store" command

This mounts an arbitrary Nix store on the specified mount point.

Typical usage:

  $ /nix/store/d0am5d8gwh2kfdcgyxh4y684mb5b2v54-blender-2.79/bin/blender --version
  bash: /nix/store/d0am5d8gwh2kfdcgyxh4y684mb5b2v54-blender-2.79/bin/blender: No such file or directory

  $ nix mount-store /tmp/mp --store https://cache.nixos.org?local-nar-cache=/tmp/nars

  $ unshare -m -r

  $ mount -o bind /tmp/mp /nix/store

  $ /nix/store/d0am5d8gwh2kfdcgyxh4y684mb5b2v54-blender-2.79/bin/blender --version
  [after a lot of downloading...]
  Blender 2.79 (sub 0)

One application is to replace the current remote store file access in
hydra-server implemented via "nix {cat,ls}-store", which doesn't work
all that well (e.g. it doesn't resolve symlinks properly).

Another application would be on-demand fetching of build inputs on
Hydra build slaves (to speed up builds that don't access their entire
closure). However, that will require a lot more machinery.

local.mk

@@ -7,7 +7,7 @@ dist-files += configure config.h.in nix.spec perl/configure
 
 clean-files += Makefile.config
 
-GLOBAL_CXXFLAGS += -I . -I src -I src/libutil -I src/libstore -I src/libmain -I src/libexpr
+GLOBAL_CXXFLAGS += -I . -I src -I src/libutil -I src/libstore -I src/libmain -I src/libexpr -D_FILE_OFFSET_BITS=64
 
 $(foreach i, config.h $(call rwildcard, src/lib*, *.hh), \
   $(eval $(call install-file-in, $(i), $(includedir)/nix, 0644)))

release.nix

@@ -79,7 +79,7 @@ let
             git
             mercurial
           ]
-          ++ lib.optional stdenv.isLinux libseccomp
+          ++ lib.optionals stdenv.isLinux [ libseccomp fuse ]
           ++ lib.optional (stdenv.isLinux || stdenv.isDarwin) libsodium
           ++ lib.optional (stdenv.isLinux || stdenv.isDarwin)
             (aws-sdk-cpp.override {

shell.nix

@@ -27,7 +27,7 @@ with import ./release-common.nix { inherit pkgs; };
       git
       mercurial
     ]
-    ++ lib.optional stdenv.isLinux libseccomp;
+    ++ lib.optionals stdenv.isLinux [ libseccomp fuse ];
 
   inherit configureFlags;
 

src/nix/local.mk

@@ -6,4 +6,6 @@ nix_SOURCES := $(wildcard $(d)/*.cc) $(wildcard src/linenoise/*.cpp)
 
 nix_LIBS = libexpr libmain libstore libutil libformat
 
+nix_LDFLAGS = -lfuse
+
 $(eval $(call install-symlink, nix, $(bindir)/nix-hash))

src/nix/mount.cc

@@ -0,0 +1,207 @@
+#include "command.hh"
+#include "store-api.hh"
+#include "fs-accessor.hh"
+#include "nar-accessor.hh"
+
+#define FUSE_USE_VERSION 30
+#include <fuse.h>
+
+#include <cstring>
+
+using namespace nix;
+
+std::shared_ptr<Store> store;
+std::shared_ptr<FSAccessor> accessor;
+
+static int op_getattr(const char * path_, struct stat * stbuf)
+{
+    try {
+
+        Path path(path_);
+
+        memset(stbuf, 0, sizeof(struct stat));
+        stbuf->st_uid = 0;
+        stbuf->st_gid = 0;
+        stbuf->st_nlink = 1;
+
+        if (path == "/") {
+            stbuf->st_mode = S_IFDIR | 0111;
+        } else {
+            auto st = accessor->stat(store->storeDir + path);
+
+            switch (st.type) {
+            case FSAccessor::tRegular:
+                stbuf->st_mode = S_IFREG | (st.isExecutable ? 0555 : 0444);
+                stbuf->st_size = st.fileSize;
+                break;
+            case FSAccessor::tSymlink:
+                stbuf->st_mode = S_IFLNK | 0777;
+                break;
+            case FSAccessor::tDirectory:
+                stbuf->st_mode = S_IFDIR | 0555;
+                break;
+            default:
+                return -ENOENT;
+            }
+        }
+
+        return 0;
+
+    } catch (...) {
+        ignoreException();
+        return -EIO;
+    }
+}
+
+static int op_readdir(const char * path_, void * buf, fuse_fill_dir_t filler,
+    off_t offset, struct fuse_file_info * fi)
+{
+    try {
+
+        Path path(path_);
+
+        if (path == "/")
+            /* FIXME: could use queryAllValidPaths(), but it will be
+               superslow for binary caches, and won't include name
+               parts. */
+            return 0;
+
+        auto st = accessor->stat(store->storeDir + path);
+        if (st.type == FSAccessor::tMissing) return -ENOENT;
+        if (st.type != FSAccessor::tDirectory) return -ENOTDIR;
+
+        for (auto & entry : accessor->readDirectory(store->storeDir + path))
+            filler(buf, entry.c_str(), nullptr, 0);
+
+        return 0;
+
+    } catch (...) {
+        ignoreException();
+        return -EIO;
+    }
+}
+
+static int op_open(const char * path_, struct fuse_file_info * fi)
+{
+    try {
+
+        Path path(path_);
+
+        auto st = accessor->stat(store->storeDir + path);
+        if (st.type == FSAccessor::tMissing) return -ENOENT;
+        if (st.type == FSAccessor::tDirectory) return -EISDIR;
+        if (st.type != FSAccessor::tRegular) return -EINVAL;
+
+        return 0;
+
+    } catch (...) {
+        ignoreException();
+        return -EIO;
+    }
+}
+
+static int op_read(const char * path_, char * buf, size_t size, off_t offset,
+    struct fuse_file_info * fi)
+{
+    try {
+
+        Path path(path_);
+
+        // FIXME: absolutely need to cache this and/or provide random
+        // access.
+
+        auto s = accessor->readFile(store->storeDir + path);
+
+        if (offset >= (off_t) s.size()) return 0;
+
+        if (offset + size > s.size())
+            size = s.size() - offset;
+
+        memcpy(buf, s.data() + offset, size);
+
+        return size;
+
+    } catch (...) {
+        ignoreException();
+        return -EIO;
+    }
+}
+
+static int op_readlink(const char * path_, char * buf, size_t size)
+{
+    try {
+
+        Path path(path_);
+
+        auto st = accessor->stat(store->storeDir + path);
+        if (st.type == FSAccessor::tMissing) return -ENOENT;
+        if (st.type != FSAccessor::tSymlink) return -EINVAL;
+
+        auto s = accessor->readLink(store->storeDir + path);
+
+        if (s.size() >= size) return ENAMETOOLONG; // FIXME
+
+        strncpy(buf, s.c_str(), size);
+
+        return 0;
+
+    } catch (...) {
+        ignoreException();
+        return -EIO;
+    }
+}
+
+struct CmdMountStore : StoreCommand
+{
+    Path mountPoint;
+
+    CmdMountStore()
+    {
+        expectArg("mount-point", &mountPoint);
+    }
+
+    std::string name() override
+    {
+        return "mount-store";
+    }
+
+    std::string description() override
+    {
+        return "mount a Nix store as a FUSE file system";
+    }
+
+    void run(ref<Store> store) override
+    {
+        ::store = store;
+        accessor = store->getFSAccessor();
+
+        Strings fuseArgs = { "nix", mountPoint, "-o", "debug" };
+        auto fuseArgs2 = stringsToCharPtrs(fuseArgs);
+
+        struct fuse * fuse;
+        char * mountpoint;
+        int multithreaded;
+
+        fuse_operations oper;
+        memset(&oper, 0, sizeof(oper));
+        oper.getattr = op_getattr;
+        oper.readdir = op_readdir;
+        oper.open = op_open;
+        oper.read = op_read;
+        oper.readlink = op_readlink;
+
+        fuse = fuse_setup(fuseArgs2.size() - 1, fuseArgs2.data(),
+            &oper, sizeof(oper),
+            &mountpoint, &multithreaded, nullptr);
+        if (!fuse) throw Error("FUSE setup failed");
+
+        if (multithreaded)
+            fuse_loop_mt(fuse);
+        else
+            fuse_loop(fuse);
+
+        fuse_teardown(fuse, mountpoint);
+    }
+};
+
+static RegisterCommand r(make_ref<CmdMountStore>());