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.

1 files changed, 31 insertions, 57 deletions

diff --git a/src/libstore/optimise-store.cc b/src/libstore/optimise-store.cc
index 9d0242bbc8..43b3c9b4fb 100644
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@@ -33,8 +33,7 @@ struct MakeReadOnly
     ~MakeReadOnly()
     {
         try {
-            /* This will make the path read-only (and restore the
-               immutable bit on platforms that support it). */
+            /* This will make the path read-only. */
             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);
@@ -134,58 +124,42 @@ 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
-       we're done).  We also have to make ‘path’ mutable, otherwise
-       rename() will fail to delete it. */
+       so make it mutable first.  We also have to make ‘path’ mutable,
+       otherwise 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
-                   file systems).  This is likely to happen with empty
-                   files.  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);
+    makeMutable(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 file
+               systems).  This is likely to happen with empty files.
+               Just shrug and ignore. */
+            if (st.st_size)
+                printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
+            return;
         }
+        throw SysError(format("cannot link `%1%' to `%2%'") % tempLink % linkPath);
+    }
 
-        /* Atomically replace the old file with the new hard link. */
-        if (rename(tempLink.c_str(), path.c_str()) == -1) {
-            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
-                   rename, rather than on the original link.
-                   (Probably it temporarily increases the st_nlink
-                   field before 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);
+    /* Atomically replace the old file with the new hard link. */
+    if (rename(tempLink.c_str(), path.c_str()) == -1) {
+        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 rename,
+               rather than on the original link.  (Probably it
+               temporarily increases the st_nlink field before
+               decreasing it again.) */
+            if (st.st_size)
+                printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
+            return;
         }
-
-        break;
+        throw SysError(format("cannot rename `%1%' to `%2%'") % tempLink % path);
     }
 
-    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;