Reserve some disk space for the garbage collector

We can't open a SQLite database if the disk is full.  Since this
prevents the garbage collector from running when it's most needed, we
reserve some dummy space that we can free just before doing a garbage
collection.  This actually revives some old code from the Berkeley DB
days.

Fixes #27.

1 files changed, 19 insertions, 1 deletions

diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index f04436b7f6..1ce62aeafc 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -195,7 +195,7 @@ void checkStoreNotSymlink()
 }
 
 
-LocalStore::LocalStore()
+LocalStore::LocalStore(bool reserveSpace)
 {
     substitutablePathsLoaded = false;
     
@@ -221,6 +221,24 @@ LocalStore::LocalStore()
   
     checkStoreNotSymlink();
 
+    /* We can't open a SQLite database if the disk is full.  Since
+       this prevents the garbage collector from running when it's most
+       needed, we reserve some dummy space that we can free just
+       before doing a garbage collection. */
+    try {
+        Path reservedPath = nixDBPath + "/reserved";
+        if (reserveSpace) {
+            int reservedSize = queryIntSetting("gc-reserved-space", 1024 * 1024);
+            struct stat st;
+            if (stat(reservedPath.c_str(), &st) == -1 ||
+                st.st_size != reservedSize)
+                writeFile(reservedPath, string(reservedSize, 'X'));
+        }
+        else
+            deletePath(reservedPath);
+    } catch (SysError & e) { /* don't care about errors */
+    }
+
     /* Acquire the big fat lock in shared mode to make sure that no
        schema upgrade is in progress. */
     try {