make nix store sign use a thread pool like nix store copy-sigs does

Fixes #399.

Change-Id: I0571036362eaf52a2efad99ba07788d7e3fd7f35

doc/manual/rl-next/parallelise-signs.md

@@ -0,0 +1,11 @@
+---
+synopsis: "Paralellise `nix store sign` using a thread pool"
+issues: [399]
+cls: [2606]
+category: Fixes
+credits: [Lunaphied]
+---
+
+`nix store sign` with a large collection of provided paths (such as when using with `--all`) has historically
+signed these paths serially. Taking extreme amounts of time when preforming operations such as fixing binary
+caches. This has been changed. Now these signatures are performed using a thread pool like `nix store copy-sigs`.

lix/nix/sigs.cc

@@ -115,9 +115,16 @@ struct CmdSign : StorePathsCommand
 
         SecretKey secretKey(readFile(secretKeyFile));
 
-        size_t added{0};
+        ThreadPool pool{"Sign pool"};
+
+        std::atomic<size_t> added{0};
+
+        auto doPath = [&](const Path & storePathS) {
+
+            checkInterrupt();
+
+            auto storePath = store->parseStorePath(storePathS);
 
-        for (auto & storePath : storePaths) {
             auto info = store->queryPathInfo(storePath);
 
             auto info2(*info);
@@ -129,7 +136,12 @@ struct CmdSign : StorePathsCommand
                 store->addSignatures(storePath, info2.sigs);
                 added++;
             }
-        }
+        };
+
+        for (auto & storePath : storePaths)
+            pool.enqueue(std::bind(doPath, store->printStorePath(storePath)));
+
+        pool.process();
 
         printInfo("added %d signatures", added);
     }