Add command 'nix store path-from-hash-part'

This exposes the Store::queryPathFromHashPart() interface in the CLI.

src/nix/path-from-hash-part.cc

@@ -0,0 +1,39 @@
+#include "command.hh"
+#include "store-api.hh"
+
+using namespace nix;
+
+struct CmdPathFromHashPart : StoreCommand
+{
+    std::string hashPart;
+
+    CmdPathFromHashPart()
+    {
+        expectArgs({
+            .label = "hash-part",
+            .handler = {&hashPart},
+        });
+    }
+
+    std::string description() override
+    {
+        return "get a store path from its hash part";
+    }
+
+    std::string doc() override
+    {
+        return
+          #include "path-from-hash-part.md"
+          ;
+    }
+
+    void run(ref<Store> store) override
+    {
+        if (auto storePath = store->queryPathFromHashPart(hashPart))
+            logger->cout(store->printStorePath(*storePath));
+        else
+            throw Error("there is no store path corresponding to '%s'", hashPart);
+    }
+};
+
+static auto rCmdPathFromHashPart = registerCommand2<CmdPathFromHashPart>({"store", "path-from-hash-part"});

src/nix/path-from-hash-part.md

@@ -0,0 +1,20 @@
+R""(
+
+# Examples
+
+* Return the full store path with the given hash part:
+
+  ```console
+  # nix store path-from-hash-part --store https://cache.nixos.org/ 0i2jd68mp5g6h2sa5k9c85rb80sn8hi9
+  /nix/store/0i2jd68mp5g6h2sa5k9c85rb80sn8hi9-hello-2.10
+  ```
+
+# Description
+
+Given the hash part of a store path (that is, the 32 characters
+following `/nix/store/`), return the full store path. This is
+primarily useful in the implementation of binary caches, where a
+request for a `.narinfo` file only supplies the hash part
+(e.g. `https://cache.nixos.org/0i2jd68mp5g6h2sa5k9c85rb80sn8hi9.narinfo`).
+
+)""