bitcoind: Add -daemonwait option to wait for initialization

This adds a `-daemonwait` flag that does the same as `-daemon` except
it, from a user perspective, backgrounds the process only after
initialization is complete.

This can be useful when the process launching bitcoind wants to
guarantee that either the RPC server is running, or that initialization
failed, before continuing. The exit code indicates the initialization
result.

This replaces the use of the libc function `daemon()` by a custom
implementation which is inspired by the glibc implementation, but also
creates a pipe from the child to the parent process for communication.

An additional advantage of having our own `daemon()` implementation is
that no MACOS-specific pragmas are needed anymore to silence a
deprecation warning.

1 files changed, 111 insertions, 12 deletions

diff --git a/src/bitcoind.cpp b/src/bitcoind.cpp
index b7bcb534ef..32f06aec2c 100644
--- a/src/bitcoind.cpp
+++ b/src/bitcoind.cpp
@@ -20,6 +20,7 @@
 #include <util/strencodings.h>
 #include <util/system.h>
 #include <util/threadnames.h>
+#include <util/tokenpipe.h>
 #include <util/translation.h>
 #include <util/url.h>
 
@@ -28,6 +29,79 @@
 const std::function<std::string(const char*)> G_TRANSLATION_FUN = nullptr;
 UrlDecodeFn* const URL_DECODE = urlDecode;
 
+#if HAVE_DECL_FORK
+
+/** Custom implementation of daemon(). This implements the same order of operations as glibc.
+ * Opens a pipe to the child process to be able to wait for an event to occur.
+ *
+ * @returns 0 if successful, and in child process.
+ *          >0 if successful, and in parent process.
+ *          -1 in case of error (in parent process).
+ *
+ *          In case of success, endpoint will be one end of a pipe from the child to parent process,
+ *          which can be used with TokenWrite (in the child) or TokenRead (in the parent).
+ */
+int fork_daemon(bool nochdir, bool noclose, TokenPipeEnd& endpoint)
+{
+    // communication pipe with child process
+    std::optional<TokenPipe> umbilical = TokenPipe::Make();
+    if (!umbilical) {
+        return -1; // pipe or pipe2 failed.
+    }
+
+    int pid = fork();
+    if (pid < 0) {
+        return -1; // fork failed.
+    }
+    if (pid != 0) {
+        // Parent process gets read end, closes write end.
+        endpoint = umbilical->TakeReadEnd();
+        umbilical->TakeWriteEnd().Close();
+
+        int status = endpoint.TokenRead();
+        if (status != 0) { // Something went wrong while setting up child process.
+            endpoint.Close();
+            return -1;
+        }
+
+        return pid;
+    }
+    // Child process gets write end, closes read end.
+    endpoint = umbilical->TakeWriteEnd();
+    umbilical->TakeReadEnd().Close();
+
+#if HAVE_DECL_SETSID
+    if (setsid() < 0) {
+        exit(1); // setsid failed.
+    }
+#endif
+
+    if (!nochdir) {
+        if (chdir("/") != 0) {
+            exit(1); // chdir failed.
+        }
+    }
+    if (!noclose) {
+        // Open /dev/null, and clone it into STDIN, STDOUT and STDERR to detach
+        // from terminal.
+        int fd = open("/dev/null", O_RDWR);
+        if (fd >= 0) {
+            bool err = dup2(fd, STDIN_FILENO) < 0 || dup2(fd, STDOUT_FILENO) < 0 || dup2(fd, STDERR_FILENO) < 0;
+            // Don't close if fd<=2 to try to handle the case where the program was invoked without any file descriptors open.
+            if (fd > 2) close(fd);
+            if (err) {
+                exit(1); // dup2 failed.
+            }
+        } else {
+            exit(1); // open /dev/null failed.
+        }
+    }
+    endpoint.TokenWrite(0); // Success
+    return 0;
+}
+
+#endif
+
 static bool AppInit(int argc, char* argv[])
 {
     NodeContext node;
@@ -59,6 +133,14 @@ static bool AppInit(int argc, char* argv[])
         return true;
     }
 
+#if HAVE_DECL_FORK
+    // Communication with parent after daemonizing. This is used for signalling in the following ways:
+    // - a boolean token is sent when the initialization process (all the Init* functions) have finished to indicate
+    // that the parent process can quit, and whether it was successful/unsuccessful.
+    // - an unexpected shutdown of the child process creates an unexpected end of stream at the parent
+    // end, which is interpreted as failure to start.
+    TokenPipeEnd daemon_ep;
+#endif
     util::Ref context{node};
     try
     {
@@ -105,24 +187,34 @@ static bool AppInit(int argc, char* argv[])
             // InitError will have been called with detailed error, which ends up on console
             return false;
         }
-        if (args.GetBoolArg("-daemon", false)) {
-#if HAVE_DECL_DAEMON
-#if defined(MAC_OSX)
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
-#endif
+        if (args.GetBoolArg("-daemon", DEFAULT_DAEMON) || args.GetBoolArg("-daemonwait", DEFAULT_DAEMONWAIT)) {
+#if HAVE_DECL_FORK
             tfm::format(std::cout, PACKAGE_NAME " starting\n");
 
             // Daemonize
-            if (daemon(1, 0)) { // don't chdir (1), do close FDs (0)
-                return InitError(Untranslated(strprintf("daemon() failed: %s\n", strerror(errno))));
+            switch (fork_daemon(1, 0, daemon_ep)) { // don't chdir (1), do close FDs (0)
+            case 0: // Child: continue.
+                // If -daemonwait is not enabled, immediately send a success token the parent.
+                if (!args.GetBoolArg("-daemonwait", DEFAULT_DAEMONWAIT)) {
+                    daemon_ep.TokenWrite(1);
+                    daemon_ep.Close();
+                }
+                break;
+            case -1: // Error happened.
+                return InitError(Untranslated(strprintf("fork_daemon() failed: %s\n", strerror(errno))));
+            default: { // Parent: wait and exit.
+                int token = daemon_ep.TokenRead();
+                if (token) { // Success
+                    exit(EXIT_SUCCESS);
+                } else { // fRet = false or token read error (premature exit).
+                    tfm::format(std::cerr, "Error during initializaton - check debug.log for details\n");
+                    exit(EXIT_FAILURE);
+                }
+            }
             }
-#if defined(MAC_OSX)
-#pragma GCC diagnostic pop
-#endif
 #else
             return InitError(Untranslated("-daemon is not supported on this operating system\n"));
-#endif // HAVE_DECL_DAEMON
+#endif // HAVE_DECL_FORK
         }
         // Lock data directory after daemonization
         if (!AppInitLockDataDirectory())
@@ -138,6 +230,13 @@ static bool AppInit(int argc, char* argv[])
         PrintExceptionContinue(nullptr, "AppInit()");
     }
 
+#if HAVE_DECL_FORK
+    if (daemon_ep.IsOpen()) {
+        // Signal initialization status to parent, then close pipe.
+        daemon_ep.TokenWrite(fRet);
+        daemon_ep.Close();
+    }
+#endif
     if (fRet) {
         WaitForShutdown();
     }