Check for and repair bad .links entries

A corrupt entry in .links prevents adding a fixed version of that file
to the store in any path.  The user experience is that corruption
present in the store 'spreads' to new paths added to the store:

(With store optimisation enabled)

1. A file in the store gets corrupted somehow (eg: filesystem bug).
2. The user tries to add a thing to the store which contains a good copy
   of the corrupted file.
3. The file being added to the store is hashed, found to match the bad
   .links entry, and is replaced by a link to the bad .links entry.
   (The .links entry's hash is not verified during add -- this would
   impose a substantial performance burden.)
4. The user observes that the thing in the store that is supposed to be
   a copy of what they were trying to add is not a correct copy -- some
   files have different contents!  Running "nix-store --verify
   --check-contents --repair" does not fix the problem.

This change makes "nix-store --verify --check-contents --repair" fix
this problem.  Bad .links entries are simply removed, allowing future
attempts to insert a good copy of the file to succeed.

1 files changed, 29 insertions, 1 deletions

diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index b5b0f1ba7..2a21f816c 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1238,7 +1238,35 @@ bool LocalStore::verifyStore(bool checkContents, RepairFlag repair)
 
     /* Optionally, check the content hashes (slow). */
     if (checkContents) {
-        printInfo("checking hashes...");
+
+        printInfo("checking link hashes...");
+
+        AutoCloseDir dir(opendir(linksDir.c_str()));
+        if (!dir) throw SysError(format("opening directory '%1%'") % linksDir);
+
+        struct dirent * dirent;
+        while (errno = 0, dirent = readdir(dir.get())) { /* sic */
+            checkInterrupt();
+            if (strcmp(dirent->d_name, ".") == 0 || strcmp(dirent->d_name, "..") == 0) continue;
+            Path linkPath = linksDir + "/" + dirent->d_name;
+            string hash = hashPath(htSHA256, linkPath).first.to_string(Base32, false);
+            if (hash != dirent->d_name) {
+                printError(format("link '%1%' was modified! "
+                                  "expected hash '%2%', got '%3%'")
+                                  % linkPath % dirent->d_name % hash);
+                if (repair) {
+                    if (unlink(linkPath.c_str()) == 0)
+                        printError(format("Removed link '%1%'") % linkPath);
+                    else
+                        throw SysError(format("removing corrupt link '%1%'") % linkPath);
+                } else {
+                    errors = true;
+                }
+            }
+        }
+        if (errno) throw SysError(format("reading directory '%1%'") % linksDir);
+
+        printInfo("checking store hashes...");
 
         Hash nullHash(htSHA256);