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libstore: Improve SSH connection sharing #311

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4e4775a
libstore: Improve SSH connection sharing
pjungkamp Jan 6, 2026
e5f6bb9
libstore/ssh: Move -S into addCommonSSHOpts
pjungkamp Jan 7, 2026
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4 changes: 2 additions & 2 deletions src/libstore/include/nix/store/ssh.hh
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Original file line number Diff line number Diff line change
Expand Up @@ -39,8 +39,8 @@ private:

Sync<State> state_;

void addCommonSSHOpts(Strings & args);
bool isMasterRunning();
void addCommonSSHOpts(Strings & args, Path socketPath);
bool isMasterRunning(Path socketPath);

#ifndef _WIN32 // TODO re-enable on Windows, once we can start processes.
Path startMaster();
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23 changes: 12 additions & 11 deletions src/libstore/ssh.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -89,7 +89,7 @@ SSHMaster::SSHMaster(
checkValidAuthority(authority);
}

void SSHMaster::addCommonSSHOpts(Strings & args)
void SSHMaster::addCommonSSHOpts(Strings & args, Path socketPath)
{
auto sshArgs = getNixSshOpts();
args.insert(args.end(), sshArgs.begin(), sshArgs.end());
Expand All @@ -113,12 +113,15 @@ void SSHMaster::addCommonSSHOpts(Strings & args)
// the remote session won't be garbled if the local command is slow.
args.push_back("-oPermitLocalCommand=yes");
args.push_back("-oLocalCommand=echo started");
args.insert(args.end(), {"-S", socketPath});
}

bool SSHMaster::isMasterRunning()
bool SSHMaster::isMasterRunning(Path socketPath)
{
assert(useMaster);

Strings args = {"-O", "check", hostnameAndUser};
addCommonSSHOpts(args);
addCommonSSHOpts(args, socketPath);

auto res = runProgram(RunOptions{.program = "ssh", .args = args, .mergeStderrToStdout = true});
return res.first == 0;
Expand Down Expand Up @@ -183,9 +186,7 @@ std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(Strings && comman

if (!fakeSSH) {
args = {"ssh", hostnameAndUser.c_str(), "-x"};
addCommonSSHOpts(args);
if (socketPath != "")
args.insert(args.end(), {"-S", socketPath});
addCommonSSHOpts(args, socketPath);
if (verbosity >= lvlChatty)
args.push_back("-v");
args.splice(args.end(), std::move(extraSshArgs));
Expand All @@ -206,7 +207,7 @@ std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(Strings && comman

// Wait for the SSH connection to be established,
// So that we don't overwrite the password prompt with our progress bar.
if (!fakeSSH && !useMaster && !isMasterRunning()) {
if (!fakeSSH && !(useMaster && isMasterRunning(socketPath))) {
std::string reply;
try {
reply = readLine(out.readSide.get());
Expand All @@ -231,7 +232,7 @@ std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(Strings && comman
Path SSHMaster::startMaster()
{
if (!useMaster)
return "";
return "none";

auto state(state_.lock());

Expand All @@ -248,7 +249,7 @@ Path SSHMaster::startMaster()

auto suspension = logger->suspend();

if (isMasterRunning())
if (isMasterRunning(state->socketPath))
return state->socketPath;

state->sshMaster = startProcess(
Expand All @@ -260,10 +261,10 @@ Path SSHMaster::startMaster()
if (dup2(out.writeSide.get(), STDOUT_FILENO) == -1)
throw SysError("duping over stdout");

Strings args = {"ssh", hostnameAndUser.c_str(), "-M", "-N", "-S", state->socketPath};
Strings args = {"ssh", hostnameAndUser.c_str(), "-M", "-N"};
if (verbosity >= lvlChatty)
args.push_back("-v");
addCommonSSHOpts(args);
addCommonSSHOpts(args, state->socketPath);
auto env = createSSHEnv();
nix::execvpe(args.begin()->c_str(), stringsToCharPtrs(args).data(), stringsToCharPtrs(env).data());

Expand Down