* Support for doing builds in a chroot under Linux. The builder is

executed in a chroot that contains just the Nix store, the temporary
  build directory, and a configurable set of additional directories
  (/dev and /proc by default).  This allows a bit more purity
  enforcement: hidden build-time dependencies on directories such as
  /usr or /nix/var/nix/profiles are no longer possible.  As an added
  benefit, accidental network downloads (cf. NIXPKGS-52) are prevented
  as well (because files such as /etc/resolv.conf are not available in
  the chroot).

  However the usefulness of chroots is diminished by the fact that
  many builders depend on /bin/sh, so you need /bin in the list of
  additional directories.  (And then on non-NixOS you need /lib as
  well...)

src/libstore/build.cc

@@ -583,6 +583,84 @@ void deletePathWrapped(const Path & path)
 //////////////////////////////////////////////////////////////////////
 
 
+#include <sys/mount.h>
+
+
+/* Helper class for automatically unmounting bind-mounts in chroots. */
+struct BindMount
+{
+    Path source, target;
+    Paths created;
+
+    BindMount()
+    {
+    }
+
+    BindMount(const Path & source, const Path & target)
+    {
+        bind(source, target);
+    }
+
+    ~BindMount()
+    {
+        try {
+            unbind();
+        } catch (...) {
+            ignoreException();
+        }
+    }
+    
+    void bind(const Path & source, const Path & target)
+    {
+        printMsg(lvlError, format("bind mounting `%1%' to `%2%'") % source % target);
+
+        this->source = source;
+        this->target = target;
+        
+        created = createDirs(target);
+        
+        if (mount(source.c_str(), target.c_str(), "", MS_BIND, 0) == -1)
+            throw SysError(format("bind mount from `%1%' to `%2%' failed") % source % target);
+    }
+
+    void unbind()
+    {
+        if (source == "") return;
+        printMsg(lvlError, format("umount `%1%'") % target);
+
+        /* Urgh.  Unmount sometimes doesn't succeed right away because
+           the mount point is still busy.  It shouldn't be, because
+           we've killed all the build processes by now (at least when
+           using a build user; see the check in killUser()).  But
+           maybe this is because those processes are still zombies and
+           are keeping some kernel structures busy (open files,
+           current directories, etc.).  So retry a few times
+           (actually, a 1 second sleep is almost certainly enough for
+           the zombies to be cleaned up). */
+        unsigned int tries = 0;
+        while (umount(target.c_str()) == -1) {
+            if (errno == EBUSY && ++tries < 10) {
+                printMsg(lvlError, format("unmounting `%1%' failed, retrying after 1 second...") % target);
+                sleep(1);
+            }
+            else
+                throw SysError(format("unmounting `%1%' failed") % target);
+        }
+
+        /* Get rid of the directories for the mount point created in
+           bind(). */
+        for (Paths::reverse_iterator i = created.rbegin(); i != created.rend(); ++i) {
+            printMsg(lvlError, format("delete `%1%'") % *i);
+            if (remove(i->c_str()) == -1)
+                throw SysError(format("cannot unlink `%1%'") % *i);
+        }
+    }
+};
+
+
+//////////////////////////////////////////////////////////////////////
+
+
 class DerivationGoal : public Goal
 {
 private:
@@ -623,6 +701,14 @@ private:
     Pipe toHook;
     Pipe fromHook;
 
+    /* Whether we're currently doing a chroot build. */
+    bool useChroot;
+
+    /* In chroot builds, the list of bind mounts currently active.
+       The destructor of BindMount will cause the binds to be
+       unmounted. */
+    list<boost::shared_ptr<BindMount> > bindMounts;
+    
     typedef void (DerivationGoal::*GoalState)();
     GoalState state;
     
@@ -678,7 +764,7 @@ private:
     void openLogFile();
 
     /* Common initialisation to be performed in child processes (i.e.,
-       both in builders and in build hooks. */
+       both in builders and in build hooks). */
     void initChild();
     
     /* Delete the temporary directory, if we have one. */
@@ -711,11 +797,18 @@ DerivationGoal::~DerivationGoal()
     /* Careful: we should never ever throw an exception from a
        destructor. */
     try {
+        printMsg(lvlError, "DESTROY");
         killChild();
         deleteTmpDir(false);
     } catch (...) {
         ignoreException();
     }
+    try {
+        //sleep(1);
+        bindMounts.clear();
+    } catch (...) {
+        ignoreException();
+    }
 }
 
 
@@ -1024,6 +1117,9 @@ void DerivationGoal::buildDone()
     
         deleteTmpDir(true);
 
+        /* In chroot builds, unmount the bind mounts ASAP. */
+        bindMounts.clear(); /* the destructors will do the rest */
+
         /* Compute the FS closure of the outputs and register them as
            being valid. */
         computeClosure();
@@ -1173,7 +1269,7 @@ DerivationGoal::HookReply DerivationGoal::tryBuildHook()
             throw SysError(format("executing `%1%'") % buildHook);
             
         } catch (std::exception & e) {
-            std::cerr << format("build hook error: %1%\n") % e.what();
+            std::cerr << format("build hook error: %1%") % e.what() << std::endl;
         }
         quickExit(1);
     }
@@ -1544,6 +1640,31 @@ void DerivationGoal::startBuilder()
                 % buildUser.getGID() % nixStore);
     }
 
+
+    /* Are we doing a chroot build? */
+    useChroot = queryBoolSetting("build-use-chroot", false);
+    Path tmpRootDir;
+    
+    if (useChroot) {
+        tmpRootDir = createTempDir();
+        
+        printMsg(lvlChatty, format("setting up chroot environment in `%1%'") % tmpRootDir);
+
+        Paths defaultDirs;
+        defaultDirs.push_back("/dev");
+        defaultDirs.push_back("/proc");
+        Paths dirsInChroot = querySetting("build-chroot-dirs", defaultDirs);
+
+        dirsInChroot.push_front(nixStore);
+        dirsInChroot.push_front(tmpDir);
+
+        /* Push BindMounts at the front of the list so that they get
+           unmounted in LIFO order.  (!!! Does the C++ standard
+           guarantee that list elements are destroyed in order?) */
+        for (Paths::iterator i = dirsInChroot.begin(); i != dirsInChroot.end(); ++i)
+            bindMounts.push_front(boost::shared_ptr<BindMount>(new BindMount(*i, tmpRootDir + *i)));
+    }
+    
     
     /* Run the builder. */
     printMsg(lvlChatty, format("executing builder `%1%'") %
@@ -1569,6 +1690,15 @@ void DerivationGoal::startBuilder()
 
         try { /* child */
 
+            /* If building in a chroot, do the chroot right away.
+               initChild() will do a chdir() to the temporary build
+               directory to make sure the current directory is in the
+               chroot.  (Actually the order doesn't matter, since due
+               to the bind mount tmpDir and tmpRootDit/tmpDir are the
+               same directories.) */
+            if (useChroot && chroot(tmpRootDir.c_str()) == -1)
+                throw SysError(format("cannot change root directory to `%1%'") % tmpRootDir);
+            
             initChild();
 
             /* Fill in the environment. */
@@ -1632,7 +1762,7 @@ void DerivationGoal::startBuilder()
                 % drv.builder);
             
         } catch (std::exception & e) {
-            std::cerr << format("build error: %1%\n") % e.what();
+            std::cerr << format("build error: %1%") % e.what() << std::endl;
         }
         quickExit(1);
     }
@@ -2143,7 +2273,7 @@ void SubstitutionGoal::tryToRun()
             throw SysError(format("executing `%1%'") % sub);
             
         } catch (std::exception & e) {
-            std::cerr << format("substitute error: %1%\n") % e.what();
+            std::cerr << format("substitute error: %1%") % e.what() << std::endl;
         }
         quickExit(1);
     }