daemon: Move 'Agent' to libutil.

* nix/libstore/build.cc (DerivationGoal::tryBuildHook): Add "offload" to
'args' and pass settings.guixProgram as the first argument to
Agent::Agent.
(pathNullDevice, commonChildInit, Agent, Agent::Agent)
(Agent::~Agent): Move to...
* nix/libutil/util.cc: ... here.
* nix/libutil/util.hh (struct Agent, commonChildInit): New
declarations.

2 files changed, 109 insertions, 0 deletions

diff --git a/nix/libutil/util.cc b/nix/libutil/util.cc
index 17d145b4c64..59a29813593 100644
--- a/nix/libutil/util.cc
+++ b/nix/libutil/util.cc
@@ -1142,5 +1142,89 @@ void ignoreException()
     }
 }
 
+static const string pathNullDevice = "/dev/null";
+
+/* Common initialisation performed in child processes. */
+void commonChildInit(Pipe & logPipe)
+{
+    /* Put the child in a separate session (and thus a separate
+       process group) so that it has no controlling terminal (meaning
+       that e.g. ssh cannot open /dev/tty) and it doesn't receive
+       terminal signals. */
+    if (setsid() == -1)
+        throw SysError(format("creating a new session"));
+
+    /* Dup the write side of the logger pipe into stderr. */
+    if (dup2(logPipe.writeSide, STDERR_FILENO) == -1)
+        throw SysError("cannot pipe standard error into log file");
+
+    /* Dup stderr to stdout. */
+    if (dup2(STDERR_FILENO, STDOUT_FILENO) == -1)
+        throw SysError("cannot dup stderr into stdout");
+
+    /* Reroute stdin to /dev/null. */
+    int fdDevNull = open(pathNullDevice.c_str(), O_RDWR);
+    if (fdDevNull == -1)
+        throw SysError(format("cannot open `%1%'") % pathNullDevice);
+    if (dup2(fdDevNull, STDIN_FILENO) == -1)
+        throw SysError("cannot dup null device into stdin");
+    close(fdDevNull);
+}
+
+//////////////////////////////////////////////////////////////////////
+
+Agent::Agent(const string &command, const Strings &args)
+{
+    debug(format("starting agent '%1%'") % command);
+
+    /* Create a pipe to get the output of the child. */
+    fromAgent.create();
+
+    /* Create the communication pipes. */
+    toAgent.create();
+
+    /* Create a pipe to get the output of the builder. */
+    builderOut.create();
+
+    /* Fork the hook. */
+    pid = startProcess([&]() {
+
+        commonChildInit(fromAgent);
+
+        if (chdir("/") == -1) throw SysError("changing into `/");
+
+        /* Dup the communication pipes. */
+        if (dup2(toAgent.readSide, STDIN_FILENO) == -1)
+            throw SysError("dupping to-hook read side");
+
+        /* Use fd 4 for the builder's stdout/stderr. */
+        if (dup2(builderOut.writeSide, 4) == -1)
+            throw SysError("dupping builder's stdout/stderr");
+
+        Strings allArgs;
+        allArgs.push_back(command);
+        allArgs.insert(allArgs.end(), args.begin(), args.end()); // append
+
+        execv(command.c_str(), stringsToCharPtrs(allArgs).data());
+
+        throw SysError(format("executing `%1%'") % command);
+    });
+
+    pid.setSeparatePG(true);
+    fromAgent.writeSide.close();
+    toAgent.readSide.close();
+}
+
+
+Agent::~Agent()
+{
+    try {
+        toAgent.writeSide.close();
+        pid.kill(true);
+    } catch (...) {
+        ignoreException();
+    }
+}
+
 
 }
diff --git a/nix/libutil/util.hh b/nix/libutil/util.hh
index 9e3c14bdd48..13cff44316d 100644
--- a/nix/libutil/util.hh
+++ b/nix/libutil/util.hh
@@ -264,6 +264,29 @@ public:
     void setKillSignal(int signal);
 };
 
+/* An "agent" is a helper program that runs in the background and that we talk
+   to over pipes, such as the "guix offload" program.  */
+struct Agent
+{
+    /* Pipes for talking to the agent. */
+    Pipe toAgent;
+
+    /* Pipe for the agent's standard output/error. */
+    Pipe fromAgent;
+
+    /* Pipe for build standard output/error--e.g., for build processes started
+       by "guix offload".  */
+    Pipe builderOut;
+
+    /* The process ID of the agent. */
+    Pid pid;
+
+    /* The command and arguments passed to the agent.  */
+    Agent(const string &command, const Strings &args);
+
+    ~Agent();
+};
+
 
 /* Kill all processes running under the specified uid by sending them
    a SIGKILL. */
@@ -295,6 +318,8 @@ void closeMostFDs(const set<int> & exceptions);
 /* Set the close-on-exec flag for the given file descriptor. */
 void closeOnExec(int fd);
 
+/* Common initialisation performed in child processes. */
+void commonChildInit(Pipe & logPipe);
 
 /* User interruption. */