Add nix processes command

Fixes NixOS#3751

src/nix/processes.cc

@@ -0,0 +1,131 @@
+#include "command.hh"
+#include "store-api.hh"
+#include "pathlocks.hh"
+
+#include <fcntl.h>
+
+using namespace nix;
+
+struct CmdProcesses : StoreCommand
+{
+    CmdProcesses()
+    {
+    }
+
+    std::string description() override
+    {
+        return "show processes";
+    }
+
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To show what processes are currently building:",
+                "nix processes"
+            },
+        };
+    }
+
+    Category category() override { return catSecondary; }
+
+    static std::optional<std::string> getCmdline(int pid)
+    {
+        auto cmdlinePath = fmt("/proc/%d/cmdline", pid);
+        if (pathExists(cmdlinePath)) {
+            string cmdline = readFile(cmdlinePath);
+            string cmdline_;
+            for (auto & i : cmdline) {
+                if (i == 0) cmdline_ += ' ';
+                else cmdline_ += i;
+            }
+            return cmdline_;
+        }
+        return std::nullopt;
+    }
+
+    // fuser just checks /proc on Linux, so we could just do that instead of calling an external program
+    // TODO: do this in C++ on Linux
+    static int fuser(Path path)
+    {
+        int fds[2];
+        if (pipe(fds) == -1)
+            throw Error("failed to make fuser pipe");
+
+        pid_t pid = fork();
+        if (pid < 0)
+            throw Error("failed to fork for fuser");
+
+        if (pid == 0) {
+            dup2(fds[1], fileno(stdout));
+            dup2(open("/dev/null", 0), fileno(stderr));
+            close(fds[0]); close(fds[1]);
+            if (!execlp("fuser", "fuser", path.c_str(), NULL))
+                throw Error("failed to execute program fuser");
+        }
+
+        int status;
+        waitpid(pid, &status, 0);
+        if (!WIFEXITED(status) || WEXITSTATUS(status) != 0)
+            throw Error("failed to execute fuser with status '%d'", status);
+        char buffer[4096];
+        ssize_t size = read(fds[0], &buffer, sizeof(buffer));
+        return std::stoi(std::string(buffer, size));
+    }
+
+    void run(ref<Store> store) override
+    {
+        if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>()) {
+            auto userPoolDir = store2->stateDir + "/userpool";
+
+            struct stat st;
+            stat(userPoolDir.c_str(), &st);
+            if (st.st_uid != geteuid())
+                throw Error("you don't have permissions to see the userpool locks");
+
+            auto dirs = readDirectory(userPoolDir);
+            for (auto i = dirs.begin(); i != dirs.end(); i++) {
+                auto uid = i->name;
+                auto uidPath = userPoolDir + "/" + uid;
+
+                // try to lock it ourselves
+                int fd = open(uidPath.c_str(), O_CLOEXEC | O_RDWR, 0600);
+                if (lockFile(fd, ltWrite, false)) {
+                    close(fd);
+                    continue;
+                }
+                close(fd);
+
+                int pid = fuser(uidPath);
+
+                if (i != dirs.begin())
+                    std::cout << std::endl;
+
+                struct passwd * pw = getpwuid(std::stoi(uid));
+                if (!pw)
+                    throw Error("can't find uid for '%s'", uid);
+                std::cout << fmt("Build User: %s (%d)", pw->pw_name, uid) << std::endl;
+
+                if (auto cmdline = getCmdline(pid))
+                    std::cout << fmt("Build Process: %s (%d)", *cmdline, pid) << std::endl;
+                else
+                    std::cout << fmt("Build Process: %d", pid) << std::endl;
+
+                // TODO: print leaves of child process by searching for ppid in /proc
+                // if (pathExists(fmt("/proc/%d/task", pid)))
+                //     printLeafProcesses(pid);
+
+                auto openFds = fmt("/proc/%d/fd", pid);
+                if (pathExists(openFds))
+                    for (auto & entry : readDirectory(openFds)) {
+                        auto path = readLink(fmt("/proc/%d/fd/%s", pid, entry.name));
+                        if (hasSuffix(path, ".lock"))
+                            std::cout << fmt("File Lock: %s", path) << std::endl;
+                    }
+            }
+        } else
+            throw Error("must provide local store for nix process, found '%s'", store->getUri());
+    }
+};
+
+static auto r1 = registerCommand<CmdProcesses>("processes");