Add operation ‘nix-store --repair-path’

This operation allows fixing corrupted or accidentally deleted store
paths by redownloading them using substituters, if available.

Since the corrupted path cannot be replaced atomically, there is a
very small time window (one system call) during which neither the old
(corrupted) nor the new (repaired) contents are available.  So
repairing should be used with some care on critical packages like
Glibc.

3 files changed, 88 insertions, 28 deletions

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index fecee04d5..3097b55ef 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -242,7 +242,7 @@ public:
 
     /* Make a goal (with caching). */
     GoalPtr makeDerivationGoal(const Path & drvPath);
-    GoalPtr makeSubstitutionGoal(const Path & storePath);
+    GoalPtr makeSubstitutionGoal(const Path & storePath, bool repair = false);
 
     /* Remove a dead goal. */
     void removeGoal(GoalPtr goal);
@@ -2344,11 +2344,18 @@ private:
     /* Lock on the store path. */
     boost::shared_ptr<PathLocks> outputLock;
 
+    /* Whether to try to repair a valid path. */
+    bool repair;
+
+    /* Location where we're downloading the substitute.  Differs from
+       storePath when doing a repair. */
+    Path destPath;
+
     typedef void (SubstitutionGoal::*GoalState)();
     GoalState state;
 
 public:
-    SubstitutionGoal(const Path & storePath, Worker & worker);
+    SubstitutionGoal(const Path & storePath, Worker & worker, bool repair = false);
     ~SubstitutionGoal();
 
     void cancel();
@@ -2371,9 +2378,10 @@ public:
 };
 
 
-SubstitutionGoal::SubstitutionGoal(const Path & storePath, Worker & worker)
+SubstitutionGoal::SubstitutionGoal(const Path & storePath, Worker & worker, bool repair)
     : Goal(worker)
     , hasSubstitute(false)
+    , repair(repair)
 {
     this->storePath = storePath;
     state = &SubstitutionGoal::init;
@@ -2415,7 +2423,7 @@ void SubstitutionGoal::init()
     worker.store.addTempRoot(storePath);
 
     /* If the path already exists we're done. */
-    if (worker.store.isValidPath(storePath)) {
+    if (!repair && worker.store.isValidPath(storePath)) {
         amDone(ecSuccess);
         return;
     }
@@ -2519,7 +2527,7 @@ void SubstitutionGoal::tryToRun()
     }
 
     /* Check again whether the path is invalid. */
-    if (worker.store.isValidPath(storePath)) {
+    if (!repair && worker.store.isValidPath(storePath)) {
         debug(format("store path `%1%' has become valid") % storePath);
         outputLock->setDeletion(true);
         amDone(ecSuccess);
@@ -2531,9 +2539,11 @@ void SubstitutionGoal::tryToRun()
     outPipe.create();
     logPipe.create();
 
+    destPath = repair ? storePath + ".tmp" : storePath;
+
     /* Remove the (stale) output path if it exists. */
-    if (pathExists(storePath))
-        deletePathWrapped(storePath);
+    if (pathExists(destPath))
+        deletePathWrapped(destPath);
 
     /* Fork the substitute program. */
     pid = fork();
@@ -2561,6 +2571,7 @@ void SubstitutionGoal::tryToRun()
             args.push_back(baseNameOf(sub));
             args.push_back("--substitute");
             args.push_back(storePath);
+            args.push_back(destPath);
             const char * * argArr = strings2CharPtrs(args);
 
             execv(sub.c_str(), (char * *) argArr);
@@ -2617,10 +2628,10 @@ void SubstitutionGoal::finished()
             throw SubstError(format("fetching path `%1%' %2%")
                 % storePath % statusToString(status));
 
-        if (!pathExists(storePath))
-            throw SubstError(format("substitute did not produce path `%1%'") % storePath);
+        if (!pathExists(destPath))
+            throw SubstError(format("substitute did not produce path `%1%'") % destPath);
 
-        hash = hashPath(htSHA256, storePath);
+        hash = hashPath(htSHA256, destPath);
 
         /* Verify the expected hash we got from the substituer. */
         if (expectedHashStr != "") {
@@ -2631,7 +2642,7 @@ void SubstitutionGoal::finished()
             if (hashType == htUnknown)
                 throw Error(format("unknown hash algorithm in `%1%'") % expectedHashStr);
             Hash expectedHash = parseHash16or32(hashType, string(expectedHashStr, n + 1));
-            Hash actualHash = hashType == htSHA256 ? hash.first : hashPath(hashType, storePath).first;
+            Hash actualHash = hashType == htSHA256 ? hash.first : hashPath(hashType, destPath).first;
             if (expectedHash != actualHash)
                 throw SubstError(format("hash mismatch in downloaded path `%1%': expected %2%, got %3%")
                     % storePath % printHash(expectedHash) % printHash(actualHash));
@@ -2652,9 +2663,26 @@ void SubstitutionGoal::finished()
         return;
     }
 
-    canonicalisePathMetaData(storePath);
+    canonicalisePathMetaData(destPath);
 
-    worker.store.optimisePath(storePath); // FIXME: combine with hashPath()
+    worker.store.optimisePath(destPath); // FIXME: combine with hashPath()
+
+    if (repair) {
+        /* We can't atomically replace storePath (the original) with
+           destPath (the replacement), so we have to move it out of
+           the way first.  We'd better not be interrupted here,
+           because if we're repairing (say) Glibc, we end up with a
+           broken system. */
+        Path oldPath = (format("%1%.old-%2%-%3%") % storePath % getpid() % rand()).str();
+        if (pathExists(storePath)) {
+            makeMutable(storePath);
+            rename(storePath.c_str(), oldPath.c_str());
+        }
+        if (rename(destPath.c_str(), storePath.c_str()) == -1)
+            throw SysError(format("moving `%1%' to `%2%'") % destPath % storePath);
+        if (pathExists(oldPath))
+            deletePathWrapped(oldPath);
+    }
 
     ValidPathInfo info2;
     info2.path = storePath;
@@ -2724,29 +2752,27 @@ Worker::~Worker()
 }
 
 
-template<class T>
-static GoalPtr addGoal(const Path & path,
-    Worker & worker, WeakGoalMap & goalMap)
+GoalPtr Worker::makeDerivationGoal(const Path & path)
 {
-    GoalPtr goal = goalMap[path].lock();
+    GoalPtr goal = derivationGoals[path].lock();
     if (!goal) {
-        goal = GoalPtr(new T(path, worker));
-        goalMap[path] = goal;
-        worker.wakeUp(goal);
+        goal = GoalPtr(new DerivationGoal(path, *this));
+        derivationGoals[path] = goal;
+        wakeUp(goal);
     }
     return goal;
 }
 
 
-GoalPtr Worker::makeDerivationGoal(const Path & nePath)
+GoalPtr Worker::makeSubstitutionGoal(const Path & path, bool repair)
 {
-    return addGoal<DerivationGoal>(nePath, *this, derivationGoals);
-}
-
-
-GoalPtr Worker::makeSubstitutionGoal(const Path & storePath)
-{
-    return addGoal<SubstitutionGoal>(storePath, *this, substitutionGoals);
+    GoalPtr goal = substitutionGoals[path].lock();
+    if (!goal) {
+        goal = GoalPtr(new SubstitutionGoal(path, *this, repair));
+        substitutionGoals[path] = goal;
+        wakeUp(goal);
+    }
+    return goal;
 }
 
 
@@ -3109,4 +3135,17 @@ void LocalStore::ensurePath(const Path & path)
 }
 
 
+void LocalStore::repairPath(const Path & path)
+{
+    Worker worker(*this);
+    GoalPtr goal = worker.makeSubstitutionGoal(path, true);
+    Goals goals = singleton<Goals>(goal);
+
+    worker.run(goals);
+
+    if (goal->getExitCode() != Goal::ecSuccess)
+        throw Error(format("cannot repair path `%1%'") % path, worker.exitStatus());
+}
+
+
 }
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 8899873a7..80db10de1 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -197,6 +197,10 @@ public:
 
     void vacuumDB();
 
+    /* Repair the contents of the given path by redownloading it using
+       a substituter (if available). */
+    void repairPath(const Path & path);
+
 private:
 
     Path schemaPath;
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index d3a707f0d..ce415ce4a 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -734,6 +734,21 @@ static void opVerifyPath(Strings opFlags, Strings opArgs)
 }
 
 
+/* Repair the contents of the given path by redownloading it using a
+   substituter (if available). */
+static void opRepairPath(Strings opFlags, Strings opArgs)
+{
+    if (!opFlags.empty())
+        throw UsageError("no flags expected");
+
+    foreach (Strings::iterator, i, opArgs) {
+        Path path = followLinksToStorePath(*i);
+        printMsg(lvlTalkative, format("repairing path `%1%'...") % path);
+        ensureLocalStore().repairPath(path);
+    }
+}
+
+
 static void showOptimiseStats(OptimiseStats & stats)
 {
     printMsg(lvlError,
@@ -834,6 +849,8 @@ void run(Strings args)
             op = opVerify;
         else if (arg == "--verify-path")
             op = opVerifyPath;
+        else if (arg == "--repair-path")
+            op = opRepairPath;
         else if (arg == "--optimise")
             op = opOptimise;
         else if (arg == "--query-failed-paths")