Keep created temp dirs inside store, but protect from GC

Implements the approach suggested by feedback on PR #6994, where
tempdir paths are created in the store (now with an exclusive lock).

As part of this work, the currently-broken and unused
`createTempDirInStore` function is updated to create an exclusive lock
on the temp directory in the store.

The GC now makes a non-blocking attempt to lock any store directories
that "look like" the temp directories created by this function, and if
it can't acquire one, ignores the directory.

1 files changed, 12 insertions, 0 deletions

diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index 4c1a82279..6cd7efbc9 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -619,6 +619,18 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
         Path path = storeDir + "/" + std::string(baseName);
         Path realPath = realStoreDir + "/" + std::string(baseName);
 
+        /* There may be temp directories in the store that are still in use
+           by another process. We need to be sure that we can acquire an
+           exclusive lock before deleting them. */
+        AutoCloseFD tmpDirFd;
+        if (baseName.rfind("add-", 0) == 0) {
+            tmpDirFd = open(realPath.c_str(), O_RDONLY | O_DIRECTORY);
+            if (tmpDirFd.get() == -1 || !lockFile(tmpDirFd.get(), ltWrite, false)) {
+                debug("skipping locked tempdir '%s'", realPath);
+                return;
+            }
+        }
+
         printInfo("deleting '%1%'", path);
 
         results.paths.insert(path);