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.

doc/manual/nix-store.xml

@@ -848,6 +848,52 @@ $ nix-store --verify-path $(nix-store -qR $(which svn))
 </refsection>
 
 
+<!--######################################################################-->
+
+<refsection><title>Operation <option>--repair-path</option></title>
+
+<refsection>
+  <title>Synopsis</title>
+  <cmdsynopsis>
+    <command>nix-store</command>
+    <arg choice='plain'><option>--repair-path</option></arg>
+    <arg choice='plain' rep='repeat'><replaceable>paths</replaceable></arg>
+  </cmdsynopsis>
+</refsection>
+
+<refsection><title>Description</title>
+            
+<para>The operation <option>--repair-path</option> attempts to
+“repair” the specified paths by redownloading them using the available
+substituters.  If no substitutes are available, then repair is not
+possible.</para>
+
+<warning><para>During repair, there is a very small time window during
+which the old path (if it exists) is moved out of the way and replaced
+with the new path.  If repair is interrupted in between, then the
+system may be left in a broken state (e.g., if the path contains a
+critical system component like the GNU C Library).</para></warning>
+
+</refsection>
+            
+<refsection><title>Example</title>
+
+<screen>
+$ nix-store --verify-path /nix/store/cj7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13
+path `/nix/store/cj7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13' was modified!
+  expected hash `2db57715ae90b7e31ff1f2ecb8c12ec1cc43da920efcbe3b22763f36a1861588',
+  got `481c5aa5483ebc97c20457bb8bca24deea56550d3985cda0027f67fe54b808e4'
+
+$ nix-store --repair-path /nix/store/cj7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13
+fetching path `/nix/store/cj7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13'...
+…
+</screen>
+
+</refsection>
+            
+</refsection>
+
+
 <!--######################################################################-->
 
 <refsection xml:id='refsec-nix-store-dump'><title>Operation <option>--dump</option></title>

doc/manual/release-notes.xml

@@ -19,6 +19,12 @@
     names.</para>
   </listitem>
 
+  <listitem>
+    <para>The new operation <command>nix-store --repair-path</command>
+    allows corrupted or deleted store paths to be repaired by
+    redownloading them.</para>
+  </listitem>
+
   <listitem>
     <para>Nix no longer sets the immutable bit on files in the Nix
     store.  Instead, the recommended way to guard the Nix store

scripts/copy-from-other-stores.pl.in

@@ -83,12 +83,13 @@ if ($ARGV[0] eq "--query") {
 
 
 elsif ($ARGV[0] eq "--substitute") {
-    die unless scalar @ARGV == 2;
+    die unless scalar @ARGV == 3;
     my $storePath = $ARGV[1];
+    my $destPath = $ARGV[2];
     my ($store, $sourcePath) = findStorePath $storePath;
     die unless $store;
     print STDERR "\n*** Copying `$storePath' from `$sourcePath'\n\n";
-    system("$binDir/nix-store --dump $sourcePath | $binDir/nix-store --restore $storePath") == 0
+    system("$binDir/nix-store --dump $sourcePath | $binDir/nix-store --restore $destPath") == 0
         or die "cannot copy `$sourcePath' to `$storePath'";
     print "\n"; # no hash to verify
 }

scripts/download-from-binary-cache.pl.in

@@ -486,7 +486,7 @@ sub printSubstitutablePaths {
 
 
 sub downloadBinary {
-    my ($storePath) = @_;
+    my ($storePath, $destPath) = @_;
 
     foreach my $cache (@caches) {
         my $info = getCachedInfoFrom($storePath, $cache);
@@ -510,7 +510,7 @@ sub downloadBinary {
         my $url = "$cache->{url}/$info->{url}"; # FIXME: handle non-relative URLs
         print STDERR "\n*** Downloading ‘$url’ to ‘$storePath’...\n";
         Nix::Utils::checkURL $url;
-        if (system("$Nix::Config::curl --fail --location --insecure '$url' | $decompressor | $Nix::Config::binDir/nix-store --restore $storePath") != 0) {
+        if (system("$Nix::Config::curl --fail --location --insecure '$url' | $decompressor | $Nix::Config::binDir/nix-store --restore $destPath") != 0) {
             die "download of `$info->{url}' failed" . ($! ? ": $!" : "") . "\n" unless $? == 0;
             next;
         }
@@ -557,8 +557,9 @@ if ($ARGV[0] eq "--query") {
 
 elsif ($ARGV[0] eq "--substitute") {
     my $storePath = $ARGV[1] or die;
+    my $destPath = $ARGV[2] or die;
     getAvailableCaches;
-    downloadBinary($storePath);
+    downloadBinary($storePath, $destPath);
 }
 
 else {

scripts/download-using-manifests.pl.in

@@ -238,8 +238,9 @@ elsif ($ARGV[0] ne "--substitute") {
 }
 
 
-die unless scalar @ARGV == 2;
+die unless scalar @ARGV == 3;
 my $targetPath = $ARGV[1];
+my $destPath = $ARGV[2];
 $fast = 0;
 
 
@@ -324,7 +325,7 @@ while (scalar @path > 0) {
             # This was the last patch.  Unpack the final NAR archive
             # into the target path.
             print STDERR "  unpacking patched archive...\n";
-            system("$Nix::Config::binDir/nix-store --restore $v < $tmpNar2") == 0
+            system("$Nix::Config::binDir/nix-store --restore $destPath < $tmpNar2") == 0
                 or die "cannot unpack $tmpNar2 to `$v'\n";
         }
 
@@ -351,7 +352,7 @@ while (scalar @path > 0) {
                 or die "cannot download and unpack `$narFile->{url}' to `$v'\n";
         } else {
             # Unpack the archive to the target path.
-            system("$curl '$narFile->{url}' | $decompressor | $Nix::Config::binDir/nix-store --restore '$v'") == 0
+            system("$curl '$narFile->{url}' | $decompressor | $Nix::Config::binDir/nix-store --restore '$destPath'") == 0
                 or die "cannot download and unpack `$narFile->{url}' to `$v'\n";
         }
 

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());
+}
+
+
 }

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;

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")