Support arbitrary store URIs in nix.machines

For backwards compatibility, if the URI is just a hostname, ssh://
(i.e. LegacySSHStore) is prepended automatically.

Also, all fields except the URI are now optional. For example, this is
a valid nix.machines file:

  local?root=/tmp/nix

This is useful for testing the remote build machinery since you don't
have to mess around with ssh.

src/libstore/build.cc

@@ -1862,6 +1862,7 @@ void DerivationGoal::startBuilder()
                 dirsInChroot[i] = r;
             else {
                 Path p = chrootRootDir + i;
+                debug("linking ‘%1%’ to ‘%2%’", p, r);
                 if (link(r.c_str(), p.c_str()) == -1) {
                     /* Hard-linking fails if we exceed the maximum
                        link count on a file (e.g. 32000 of ext3),

src/libstore/store-api.cc

@@ -709,10 +709,11 @@ namespace nix {
 RegisterStoreImplementation::Implementations * RegisterStoreImplementation::implementations = 0;
 
 
-ref<Store> openStore(const std::string & uri_)
+ref<Store> openStore(const std::string & uri_,
+    const Store::Params & extraParams)
 {
     auto uri(uri_);
-    Store::Params params;
+    Store::Params params(extraParams);
     auto q = uri.find('?');
     if (q != std::string::npos) {
         for (auto s : tokenizeString<Strings>(uri.substr(q + 1), "&")) {
@@ -722,11 +723,7 @@ ref<Store> openStore(const std::string & uri_)
         }
         uri = uri_.substr(0, q);
     }
-    return openStore(uri, params);
-}
 
-ref<Store> openStore(const std::string & uri, const Store::Params & params)
-{
     for (auto fun : *RegisterStoreImplementation::implementations) {
         auto store = fun(uri, params);
         if (store) {
@@ -735,7 +732,7 @@ ref<Store> openStore(const std::string & uri, const Store::Params & params)
         }
     }
 
-    throw Error(format("don't know how to open Nix store ‘%s’") % uri);
+    throw Error("don't know how to open Nix store ‘%s’", uri);
 }
 
 

src/libstore/store-api.hh

@@ -668,20 +668,31 @@ void removeTempRoots();
 /* Return a Store object to access the Nix store denoted by
    ‘uri’ (slight misnomer...). Supported values are:
 
-   * ‘direct’: The Nix store in /nix/store and database in
+   * ‘local’: The Nix store in /nix/store and database in
      /nix/var/nix/db, accessed directly.
 
    * ‘daemon’: The Nix store accessed via a Unix domain socket
      connection to nix-daemon.
 
+   * ‘auto’ or ‘’: Equivalent to ‘local’ or ‘daemon’ depending on
+     whether the user has write access to the local Nix
+     store/database.
+
    * ‘file://<path>’: A binary cache stored in <path>.
 
-   If ‘uri’ is empty, it defaults to ‘direct’ or ‘daemon’ depending on
-   whether the user has write access to the local Nix store/database.
-   set to true *unless* you're going to collect garbage. */
-ref<Store> openStore(const std::string & uri = getEnv("NIX_REMOTE"));
+   * ‘https://<path>’: A binary cache accessed via HTTP.
 
-ref<Store> openStore(const std::string & uri, const Store::Params & params);
+   * ‘s3://<path>’: A writable binary cache stored on Amazon's Simple
+     Storage Service.
+
+   * ‘ssh://[user@]<host>’: A remote Nix store accessed by running
+     ‘nix-store --serve’ via SSH.
+
+   You can pass parameters to the store implementation by appending
+   ‘?key=value&key=value&...’ to the URI.
+*/
+ref<Store> openStore(const std::string & uri = getEnv("NIX_REMOTE"),
+    const Store::Params & extraParams = Store::Params());
 
 
 void copyPaths(ref<Store> from, ref<Store> to, const PathSet & storePaths, bool substitute = false);