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.

1 files changed, 4 insertions, 3 deletions

diff --git a/scripts/download-from-binary-cache.pl.in b/scripts/download-from-binary-cache.pl.in
index 751623eebf2e..317989e401df 100644
--- a/scripts/download-from-binary-cache.pl.in
+++ b/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 {