Make SSHMaster::startCommand work on an args list

This avoids split-on-whitespace errors:

- No more `bash -c` needed

- No more `shellEscape` needed

- `remote-program` ssh store setting also cleanly supports args (e.g.
  `nix daemon`)

- `ssh` uses `--` to separate args for SSH from args for the command to
  run.

and will help with Hydra dedup.

Some code taken from #6628.

Co-Authored-By: Alexander Bantyev <balsoft@balsoft.ru>

src/libstore/legacy-ssh-store.cc

@@ -55,9 +55,14 @@ LegacySSHStore::LegacySSHStore(const std::string & scheme, const std::string & h
 ref<LegacySSHStore::Connection> LegacySSHStore::openConnection()
 {
     auto conn = make_ref<Connection>();
-    conn->sshConn = master.startCommand(
+    Strings command = remoteProgram.get();
-        fmt("%s --serve --write", remoteProgram)
+    command.push_back("--serve");
-        + (remoteStore.get() == "" ? "" : " --store " + shellEscape(remoteStore.get())));
+    command.push_back("--write");
+    if (remoteStore.get() != "") {
+        command.push_back("--store");
+        command.push_back(remoteStore.get());
+    }
+    conn->sshConn = master.startCommand(std::move(command));
     conn->to = FdSink(conn->sshConn->in.get());
     conn->from = FdSource(conn->sshConn->out.get());
 

src/libstore/legacy-ssh-store.hh

@@ -13,7 +13,7 @@ struct LegacySSHStoreConfig : virtual CommonSSHStoreConfig
 {
     using CommonSSHStoreConfig::CommonSSHStoreConfig;
 
-    const Setting<Path> remoteProgram{this, "nix-store", "remote-program",
+    const Setting<Strings> remoteProgram{this, {"nix-store"}, "remote-program",
         "Path to the `nix-store` executable on the remote machine."};
 
     const Setting<int> maxConnections{this, 1, "max-connections",

src/libstore/ssh-store.cc

@@ -17,7 +17,7 @@ struct SSHStoreConfig : virtual RemoteStoreConfig, virtual CommonSSHStoreConfig
     using RemoteStoreConfig::RemoteStoreConfig;
     using CommonSSHStoreConfig::CommonSSHStoreConfig;
 
-    const Setting<Path> remoteProgram{this, "nix-daemon", "remote-program",
+    const Setting<Strings> remoteProgram{this, {"nix-daemon"}, "remote-program",
         "Path to the `nix-daemon` executable on the remote machine."};
 
     const std::string name() override { return "Experimental SSH Store"; }
@@ -212,14 +212,15 @@ public:
 ref<RemoteStore::Connection> SSHStore::openConnection()
 {
     auto conn = make_ref<Connection>();
-
+    Strings command = remoteProgram.get();
-    std::string command = remoteProgram + " --stdio";
+    command.push_back("--stdio");
-    if (remoteStore.get() != "")
+    if (remoteStore.get() != "") {
-        command += " --store " + shellEscape(remoteStore.get());
+        command.push_back("--store");
-    for (auto & arg : extraRemoteProgramArgs)
+        command.push_back(remoteStore.get());
-        command += " " + shellEscape(arg);
+    }
-
+    command.insert(command.end(),
-    conn->sshConn = master.startCommand(command);
+        extraRemoteProgramArgs.begin(), extraRemoteProgramArgs.end());
+    conn->sshConn = master.startCommand(std::move(command));
     conn->to = FdSink(conn->sshConn->in.get());
     conn->from = FdSource(conn->sshConn->out.get());
     return conn;

src/libstore/ssh.cc

@@ -52,7 +52,8 @@ bool SSHMaster::isMasterRunning() {
     return res.first == 0;
 }
 
-std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(const std::string & command)
+std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(
+    Strings && command, Strings && extraSshArgs)
 {
     Path socketPath = startMaster();
 
@@ -84,18 +85,19 @@ std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(const std::string
 
         Strings args;
 
-        if (fakeSSH) {
+        if (!fakeSSH) {
-            args = { "bash", "-c" };
-        } else {
             args = { "ssh", host.c_str(), "-x" };
             addCommonSSHOpts(args);
             if (socketPath != "")
                 args.insert(args.end(), {"-S", socketPath});
             if (verbosity >= lvlChatty)
                 args.push_back("-v");
+            args.splice(args.end(), std::move(extraSshArgs));
+            args.push_back("--");
         }
 
-        args.push_back(command);
+        args.splice(args.end(), std::move(command));
+
         execvp(args.begin()->c_str(), stringsToCharPtrs(args).data());
 
         // could not exec ssh/bash

src/libstore/ssh.hh

@@ -41,7 +41,16 @@ public:
         AutoCloseFD out, in;
     };
 
-    std::unique_ptr<Connection> startCommand(const std::string & command);
+    /**
+     * @param command The command (arg vector) to execute.
+     *
+     * @param extraSShArgs Extra args to pass to SSH (not the command to
+     * execute). Will not be used when "fake SSHing" to the local
+     * machine.
+     */
+    std::unique_ptr<Connection> startCommand(
+        Strings && command,
+        Strings && extraSshArgs = {});
 
     Path startMaster();
 };