Fix deadlock in SSHMaster::addCommonSSHOpts()

When useMaster is true, startMaster() acquires the state lock, then
calls isMasterRunning(), which calls addCommonSSHOpts(), which tries
to acquire the state lock again, causing a deadlock.

The solution is to move tmpDir out of the state. It doesn't need to be
there in the first place because it never changes.

src/libstore/include/nix/store/ssh.hh

@@ -1,6 +1,7 @@
 #pragma once
 ///@file
 
+#include "nix/util/ref.hh"
 #include "nix/util/sync.hh"
 #include "nix/util/url.hh"
 #include "nix/util/processes.hh"
@@ -26,12 +27,13 @@ private:
     const bool compress;
     const Descriptor logFD;
 
+    ref<AutoDelete> tmpDir;
+
     struct State
     {
 #ifndef _WIN32 // TODO re-enable on Windows, once we can start processes.
         Pid sshMaster;
 #endif
-        std::unique_ptr<AutoDelete> tmpDir;
         Path socketPath;
     };
 

src/libstore/ssh.cc

@@ -84,23 +84,20 @@ SSHMaster::SSHMaster(
     , useMaster(useMaster && !fakeSSH)
     , compress(compress)
     , logFD(logFD)
+    , tmpDir(make_ref<AutoDelete>(createTempDir("", "nix", 0700)))
 {
     checkValidAuthority(authority);
-    auto state(state_.lock());
-    state->tmpDir = std::make_unique<AutoDelete>(createTempDir("", "nix", 0700));
 }
 
 void SSHMaster::addCommonSSHOpts(Strings & args)
 {
-    auto state(state_.lock());
-
     auto sshArgs = getNixSshOpts();
     args.insert(args.end(), sshArgs.begin(), sshArgs.end());
 
     if (!keyFile.empty())
         args.insert(args.end(), {"-i", keyFile});
     if (!sshPublicHostKey.empty()) {
-        std::filesystem::path fileName = state->tmpDir->path() / "host-key";
+        std::filesystem::path fileName = tmpDir->path() / "host-key";
         writeFile(fileName.string(), authority.host + " " + sshPublicHostKey + "\n");
         args.insert(args.end(), {"-oUserKnownHostsFile=" + fileName.string()});
     }
@@ -241,7 +238,7 @@ Path SSHMaster::startMaster()
     if (state->sshMaster != INVALID_DESCRIPTOR)
         return state->socketPath;
 
-    state->socketPath = (Path) *state->tmpDir + "/ssh.sock";
+    state->socketPath = (Path) *tmpDir + "/ssh.sock";
 
     Pipe out;
     out.create();