handle broken evaluation worker pipes more gracefully

writeLine will throw a SysError exception, which obfuscates out-of-memory events where the eval worker is killed by the OS.
readLine is suffering from the same problem and will be handled in a subsequent commit.

src/nix-eval-jobs.cc

@@ -2,7 +2,6 @@
 #include <iostream>
 #include <thread>
 #include <filesystem>
-
 #include <nix/eval-settings.hh>
 #include <nix/config.h>
 #include <nix/shared.hh>
@@ -283,6 +282,22 @@ std::string attrPathJoin(json input) {
                            });
 }
 
+[[nodiscard]] static int tryWriteLine(int fd, std::string s) {
+    s += "\n";
+    std::string_view sv{s};
+    while (!sv.empty()) {
+        checkInterrupt();
+        ssize_t res = write(fd, sv.data(), sv.size());
+        if (res == -1 && errno != EINTR) {
+            return -errno;
+        }
+        if (res > 0) {
+            sv.remove_prefix(res);
+        }
+    }
+    return 0;
+}
+
 static void worker(ref<EvalState> state, Bindings &autoArgs, AutoCloseFD &to,
                    AutoCloseFD &from) {
 
@@ -303,7 +318,9 @@ static void worker(ref<EvalState> state, Bindings &autoArgs, AutoCloseFD &to,
 
     while (true) {
         /* Wait for the collector to send us a job name. */
-        writeLine(to.get(), "next");
+        if (tryWriteLine(to.get(), "next") < 0) {
+            return; // main process died
+        }
 
         auto s = readLine(from.get());
         if (s == "exit")
@@ -388,7 +405,9 @@ static void worker(ref<EvalState> state, Bindings &autoArgs, AutoCloseFD &to,
             printError(e.msg());
         }
 
-        writeLine(to.get(), reply.dump());
+        if (tryWriteLine(to.get(), reply.dump()) < 0) {
+            return; // main process died
+        }
 
         /* If our RSS exceeds the maximum, exit. The collector will
            start a new process. */
@@ -398,7 +417,9 @@ static void worker(ref<EvalState> state, Bindings &autoArgs, AutoCloseFD &to,
             break;
     }
 
-    writeLine(to.get(), "restart");
+    if (tryWriteLine(to.get(), "restart") < 0) {
+        return; // main process died
+    };
 }
 
 typedef std::function<void(ref<EvalState> state, Bindings &autoArgs,
@@ -430,10 +451,14 @@ struct Proc {
                     auto msg = e.msg();
                     err["error"] = filterANSIEscapes(msg, true);
                     printError(msg);
-                    writeLine(to->get(), err.dump());
+                    if (tryWriteLine(to->get(), err.dump()) < 0) {
+                        return; // main process died
+                    };
                     // Don't forget to print it into the STDERR log, this is
                     // what's shown in the Hydra UI.
-                    writeLine(to->get(), "restart");
+                    if (tryWriteLine(to->get(), "restart") < 0) {
+                        return; // main process died
+                    }
                 }
             },
             ProcessOptions{.allowVfork = false});
@@ -452,6 +477,25 @@ struct State {
     std::exception_ptr exc;
 };
 
+void handleBrokenWorkerPipe(pid_t child) {
+    while (1) {
+        int rc = waitpid(child, nullptr, WNOHANG);
+        if (rc == 0) {
+            throw Error("BUG: worker pipe closed but worker still running?");
+        } else if (rc == -1) {
+            throw Error("BUG: waitpid waiting for worker failed: %s", strerror(errno));
+        } else {
+            if (WIFEXITED(rc)) {
+                throw Error("evaluation worker exited with %d",
+                            WEXITSTATUS(rc));
+            } else if (WIFSIGNALED(rc)) {
+                throw Error("evaluation worker killed by signal %d",
+                            WTERMSIG(rc));
+            } // else ignore WIFSTOPPED and WIFCONTINUED
+        }
+    }
+}
+
 std::function<void()> collector(Sync<State> &state_,
                                 std::condition_variable &wakeup) {
     return [&]() {
@@ -481,7 +525,9 @@ std::function<void()> collector(Sync<State> &state_,
                     auto state(state_.lock());
                     if ((state->todo.empty() && state->active.empty()) ||
                         state->exc) {
-                        writeLine(proc->to.get(), "exit");
+                        if (tryWriteLine(proc->to.get(), "exit") < 0) {
+                            handleBrokenWorkerPipe(proc->pid);
+                        }
                         return;
                     }
                     if (!state->todo.empty()) {
@@ -494,7 +540,9 @@ std::function<void()> collector(Sync<State> &state_,
                 }
 
                 /* Tell the worker to evaluate it. */
-                writeLine(proc->to.get(), "do " + attrPath.dump());
+                if (tryWriteLine(proc->to.get(), "do " + attrPath.dump()) < 0) {
+                    handleBrokenWorkerPipe(proc->pid);
+                }
 
                 /* Wait for the response. */
                 auto respString = readLine(proc->from.get());