Remove setting of the immutable bit

Using the immutable bit is problematic, especially in conjunction with
store optimisation.  For instance, if the garbage collector deletes a
file, it has to clear its immutable bit, but if the file has
additional hard links, we can't set the bit afterwards because we
don't know the remaining paths.

So now that we support having the entire Nix store as a read-only
mount, we may as well drop the immutable bit.  Unfortunately, we have
to keep the code to clear the immutable bit for backwards
compatibility.

src/libstore/build.cc

@@ -1355,11 +1355,6 @@ void DerivationGoal::buildDone()
         /* Delete the chroot (if we were using one). */
         autoDelChroot.reset(); /* this runs the destructor */
 
-        /* Deleting the chroot will have caused the immutable bits on
-           hard-linked inputs to be cleared.  So set them again. */
-        foreach (PathSet::iterator, i, regularInputPaths)
-            makeImmutable(*i);
-
         /* Delete redirected outputs (when doing hash rewriting). */
         foreach (PathSet::iterator, i, redirectedOutputs)
             deletePath(*i);
@@ -1766,11 +1761,8 @@ void DerivationGoal::startBuilder()
                     /* Hard-linking fails if we exceed the maximum
                        link count on a file (e.g. 32000 of ext3),
                        which is quite possible after a `nix-store
-                       --optimise'.  It can also fail if another
+                       --optimise'. */
-                       process called makeImmutable() on *i after we
+                    if (errno != EMLINK)
-                       did makeMutable().  In those cases, make a copy
-                       instead. */
-                    if (errno != EMLINK && errno != EPERM)
                         throw SysError(format("linking `%1%' to `%2%'") % p % *i);
                     StringSink sink;
                     dumpPath(*i, sink);
@@ -1778,7 +1770,6 @@ void DerivationGoal::startBuilder()
                     restorePath(p, source);
                 }
 
-                makeImmutable(*i);
                 regularInputPaths.insert(*i);
             }
         }

src/libstore/local-store.cc

@@ -517,8 +517,6 @@ void canonicalisePathMetaData(const Path & path, bool recurse)
         foreach (Strings::iterator, i, names)
             canonicalisePathMetaData(path + "/" + *i, true);
     }
-
-    makeImmutable(path);
 }
 
 

src/libstore/optimise-store.cc

@@ -33,8 +33,7 @@ struct MakeReadOnly
     ~MakeReadOnly()
     {
         try {
-            /* This will make the path read-only (and restore the
+            /* This will make the path read-only. */
-               immutable bit on platforms that support it). */
             if (path != "") canonicalisePathMetaData(path, false);
         } catch (...) {
             ignoreException();
@@ -43,14 +42,6 @@ struct MakeReadOnly
 };
 
 
-struct MakeImmutable
-{
-    Path path;
-    MakeImmutable(const Path & path) : path(path) { }
-    ~MakeImmutable() { makeImmutable(path); }
-};
-
-
 void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
 {
     checkInterrupt();
@@ -101,7 +92,6 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
     if (!pathExists(linkPath)) {
         /* Nope, create a hard link in the links directory. */
         makeMutable(path);
-        MakeImmutable mk1(path);
         if (link(path.c_str(), linkPath.c_str()) == 0) return;
         if (errno != EEXIST)
             throw SysError(format("cannot link `%1%' to `%2%'") % linkPath % path);
@@ -134,33 +124,23 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
     MakeReadOnly makeReadOnly(mustToggle ? dirOf(path) : "");
 
     /* If ‘linkPath’ is immutable, we can't create hard links to it,
-       so make it mutable first (and make it immutable again when
+       so make it mutable first.  We also have to make ‘path’ mutable,
-       we're done).  We also have to make ‘path’ mutable, otherwise
+       otherwise rename() will fail to delete it. */
-       rename() will fail to delete it. */
     makeMutable(path);
-    MakeImmutable mk2(path);
-
-    /* Another process might be doing the same thing (creating a new
-       link to ‘linkPath’) and make ‘linkPath’ immutable before we're
-       done.  In that case, just retry. */
-    unsigned int retries = 1024;
-    while (--retries > 0) {
     makeMutable(linkPath);
-        MakeImmutable mk1(linkPath);
 
     Path tempLink = (format("%1%/.tmp-link-%2%-%3%")
         % settings.nixStore % getpid() % rand()).str();
 
     if (link(linkPath.c_str(), tempLink.c_str()) == -1) {
         if (errno == EMLINK) {
-                /* Too many links to the same file (>= 32000 on most
+            /* Too many links to the same file (>= 32000 on most file
-                   file systems).  This is likely to happen with empty
+               systems).  This is likely to happen with empty files.
-                   files.  Just shrug and ignore. */
+               Just shrug and ignore. */
             if (st.st_size)
                 printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
             return;
         }
-            if (errno == EPERM) continue;
         throw SysError(format("cannot link `%1%' to `%2%'") % tempLink % linkPath);
     }
 
@@ -169,23 +149,17 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
         if (unlink(tempLink.c_str()) == -1)
             printMsg(lvlError, format("unable to unlink `%1%'") % tempLink);
         if (errno == EMLINK) {
-                /* Some filesystems generate too many links on the
+            /* Some filesystems generate too many links on the rename,
-                   rename, rather than on the original link.
+               rather than on the original link.  (Probably it
-                   (Probably it temporarily increases the st_nlink
+               temporarily increases the st_nlink field before
-                   field before decreasing it again.) */
+               decreasing it again.) */
             if (st.st_size)
                 printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
             return;
         }
-            if (errno == EPERM) continue;
         throw SysError(format("cannot rename `%1%' to `%2%'") % tempLink % path);
     }
 
-        break;
-    }
-
-    if (retries == 0) throw Error(format("cannot link `%1%' to `%2%'") % path % linkPath);
-
     stats.filesLinked++;
     stats.bytesFreed += st.st_size;
     stats.blocksFreed += st.st_blocks;

src/libutil/immutable.cc

@@ -16,7 +16,7 @@
 namespace nix {
 
 
-void changeMutable(const Path & path, bool mut)
+void makeMutable(const Path & path)
 {
 #if defined(FS_IOC_SETFLAGS) && defined(FS_IOC_GETFLAGS) && defined(FS_IMMUTABLE_FL)
 
@@ -38,30 +38,12 @@ void changeMutable(const Path & path, bool mut)
     /* Silently ignore errors getting/setting the immutable flag so
        that we work correctly on filesystems that don't support it. */
     if (ioctl(fd, FS_IOC_GETFLAGS, &flags)) return;
-
     old = flags;
-    
+    flags &= ~FS_IMMUTABLE_FL;
-    if (mut) flags &= ~FS_IMMUTABLE_FL;
-    else flags |= FS_IMMUTABLE_FL;
-
     if (old == flags) return;
-
     if (ioctl(fd, FS_IOC_SETFLAGS, &flags)) return;
-    
 #endif
 }
 
 
-void makeImmutable(const Path & path)
-{
-    changeMutable(path, false);
-}
-
-
-void makeMutable(const Path & path)
-{
-    changeMutable(path, true);
-}
-
-
 }

src/libutil/immutable.hh

@@ -4,12 +4,6 @@
 
 namespace nix {
 
-/* Make the given path immutable, i.e., prevent it from being modified
-   in any way, even by root.  This is a no-op on platforms that do not
-   support this, or if the calling user is not privileged.  On Linux,
-   this is implemented by doing the equivalent of ‘chattr +i path’. */
-void makeImmutable(const Path & path);
-
 /* Make the given path mutable. */
 void makeMutable(const Path & path);