WIP Bindmount files instead of hardlinking

16591eb#diff-19f999107b609d37cfb22c58e7f0bc1cf76edf1180e238dd6389e03cc279b604 (2013) added support for files to doBind

This is work towards allowing users to change the location of chrootRootDir, to, for example, a tmpfs.

inspired by trofi on matrix

> It looks like build sandbox created by nix-daemon runs on the same filesystem, as /nix/store including things like /tmp which makes all small temporary files hit the disk. Is it intentional? If it is is there an easy way to redirect chroot's root to be tmpfs?

src/libstore/build/local-derivation-goal.cc

@@ -714,7 +714,7 @@ void LocalDerivationGoal::startBuilder()
             if (S_ISDIR(lstat(r).st_mode))
                 dirsInChroot.insert_or_assign(p, r);
             else
-                linkOrCopy(r, chrootRootDir + p);
+                filesInChroot.insert_or_assign(p, r);
         }
 
         /* If we're repairing, checking or rebuilding part of a
@@ -1858,6 +1858,10 @@ void LocalDerivationGoal::runChild()
                     doBind(i.second.source, chrootRootDir + i.first, i.second.optional);
             }
 
+            for (auto & i : filesInChroot) {
+                doBind(i.second.source, chrootRootDir + i.first, i.second.optional);
+            }
+
             /* Bind a new instance of procfs on /proc. */
             createDirs(chrootRootDir + "/proc");
             if (mount("none", (chrootRootDir + "/proc").c_str(), "proc", 0, 0) == -1)

src/libstore/build/local-derivation-goal.hh

@@ -89,6 +89,9 @@ struct LocalDerivationGoal : public DerivationGoal
     typedef map<Path, ChrootPath> DirsInChroot; // maps target path to source path
     DirsInChroot dirsInChroot;
 
+    typedef map<Path, ChrootPath> FilesInChroot; // maps target path to source path
+    FilesInChroot filesInChroot;
+
     typedef map<std::string, std::string> Environment;
     Environment env;