Allow handshake with child process during startup

Allow the parent process to perform a bi-directional handshake
with the child process during fork/exec. The child process
will fork and do its initial setup. Immediately prior to the
exec(), it will stop & wait for a handshake from the parent
process. The parent process will spawn the child and wait
until the child reaches the handshake point. It will do
whatever extra setup work is required, before signalling the
child to continue.

The implementation of this is done using two pairs of blocking
pipes. The first pair is used to block the parent, until the
child writes a single byte. Then the second pair pair is used
to block the child, until the parent confirms with another
single byte.

* src/util/command.c, src/util/command.h,
  src/libvirt_private.syms: Add APIs to perform a handshake

src/libvirt_private.syms

@@ -109,11 +109,14 @@ virCommandClearCaps;
 virCommandDaemonize;
 virCommandExec;
 virCommandFree;
+virCommandHandshakeNotify;
+virCommandHandshakeWait;
 virCommandNew;
 virCommandNewArgList;
 virCommandNewArgs;
 virCommandNonblockingFDs;
 virCommandPreserveFD;
+virCommandRequireHandshake;
 virCommandRun;
 virCommandRunAsync;
 virCommandSetErrorBuffer;

src/util/command.c

@@ -36,6 +36,8 @@
 #include "files.h"
 #include "buf.h"
 
+#include <stdlib.h>
+
 #define VIR_FROM_THIS VIR_FROM_NONE
 
 #define virCommandError(code, ...)                                      \
@@ -77,6 +79,10 @@ struct _virCommand {
     int *outfdptr;
     int *errfdptr;
 
+    bool handshake;
+    int handshakeWait[2];
+    int handshakeNotify[2];
+
     virExecHook hook;
     void *opaque;
 
@@ -108,6 +114,11 @@ virCommandNewArgs(const char *const*args)
     if (VIR_ALLOC(cmd) < 0)
         return NULL;
 
+    cmd->handshakeWait[0] = -1;
+    cmd->handshakeWait[1] = -1;
+    cmd->handshakeNotify[0] = -1;
+    cmd->handshakeNotify[1] = -1;
+
     FD_ZERO(&cmd->preserve);
     FD_ZERO(&cmd->transfer);
     cmd->infd = cmd->outfd = cmd->errfd = -1;
@@ -1174,12 +1185,61 @@ virCommandHook(void *data)
     virCommandPtr cmd = data;
     int res = 0;
 
-    if (cmd->hook)
+    if (cmd->hook) {
+        VIR_DEBUG("Run hook %p %p", cmd->hook, cmd->opaque);
         res = cmd->hook(cmd->opaque);
+        VIR_DEBUG("Done hook %d", res);
+    }
     if (res == 0 && cmd->pwd) {
         VIR_DEBUG("Running child in %s", cmd->pwd);
         res = chdir(cmd->pwd);
+        if (res < 0) {
+            virReportSystemError(errno,
+                                 _("Unable to change to %s"), cmd->pwd);
         }
+    }
+    if (cmd->handshake) {
+        char c = res < 0 ? '0' : '1';
+        int rv;
+        VIR_DEBUG("Notifying parent for handshake start on %d", cmd->handshakeWait[1]);
+        if (safewrite(cmd->handshakeWait[1], &c, sizeof(c)) != sizeof(c)) {
+            virReportSystemError(errno, "%s", _("Unable to notify parent process"));
+            return -1;
+        }
+
+        /* On failure we pass the error message back to parent,
+         * so they don't have to dig through stderr logs
+         */
+        if (res < 0) {
+            virErrorPtr err = virGetLastError();
+            const char *msg = err ? err->message :
+                _("Unknown failure during hook execution");
+            size_t len = strlen(msg) + 1;
+            if (safewrite(cmd->handshakeWait[1], msg, len) != len) {
+                virReportSystemError(errno, "%s", _("Unable to send error to parent process"));
+                return -1;
+            }
+            return -1;
+        }
+
+        VIR_DEBUG("Waiting on parent for handshake complete on %d", cmd->handshakeNotify[0]);
+        if ((rv = saferead(cmd->handshakeNotify[0], &c, sizeof(c))) != sizeof(c)) {
+            if (rv < 0)
+                virReportSystemError(errno, "%s", _("Unable to wait on parent process"));
+            else
+                virReportSystemError(EIO, "%s", _("libvirtd quit during handshake"));
+            return -1;
+        }
+        if (c != '1') {
+            virReportSystemError(EINVAL, _("Unexpected confirm code '%c' from parent process"), c);
+            return -1;
+        }
+        VIR_FORCE_CLOSE(cmd->handshakeWait[1]);
+        VIR_FORCE_CLOSE(cmd->handshakeNotify[0]);
+    }
+
+    VIR_DEBUG("Hook is done %d", res);
+
     return res;
 }
 
@@ -1409,6 +1469,119 @@ virCommandAbort(virCommandPtr cmd ATTRIBUTE_UNUSED)
 }
 #endif
 
+
+void virCommandRequireHandshake(virCommandPtr cmd)
+{
+    if (!cmd || cmd->has_error)
+        return;
+
+    if (cmd->handshake) {
+        cmd->has_error = -1;
+        VIR_DEBUG("Cannot require handshake twice");
+        return;
+    }
+
+    if (pipe(cmd->handshakeWait) < 0) {
+        cmd->has_error = errno;
+        return;
+    }
+    if (pipe(cmd->handshakeNotify) < 0) {
+        VIR_FORCE_CLOSE(cmd->handshakeWait[0]);
+        VIR_FORCE_CLOSE(cmd->handshakeWait[1]);
+        cmd->has_error = errno;
+        return;
+    }
+
+    VIR_DEBUG("Transfer handshake wait=%d notify=%d",
+              cmd->handshakeWait[1], cmd->handshakeNotify[0]);
+    virCommandTransferFD(cmd, cmd->handshakeWait[1]);
+    virCommandTransferFD(cmd, cmd->handshakeNotify[0]);
+    cmd->handshake = true;
+}
+
+int virCommandHandshakeWait(virCommandPtr cmd)
+{
+    char c;
+    int rv;
+    if (!cmd ||cmd->has_error == ENOMEM) {
+        virReportOOMError();
+        return -1;
+    }
+    if (cmd->has_error || !cmd->handshake) {
+        virCommandError(VIR_ERR_INTERNAL_ERROR, "%s",
+                        _("invalid use of command API"));
+        return -1;
+    }
+
+    if (cmd->handshakeWait[0] == -1) {
+        virCommandError(VIR_ERR_INTERNAL_ERROR, "%s",
+                        _("Handshake is already complete"));
+        return -1;
+    }
+
+    VIR_DEBUG("Wait for handshake on %d", cmd->handshakeWait[0]);
+    if ((rv = saferead(cmd->handshakeWait[0], &c, sizeof(c))) != sizeof(c)) {
+        if (rv < 0)
+            virReportSystemError(errno, "%s", _("Unable to wait for child process"));
+        else
+            virReportSystemError(EIO, "%s", _("Child process quit during startup handshake"));
+        VIR_FORCE_CLOSE(cmd->handshakeWait[0]);
+        return -1;
+    }
+    if (c != '1') {
+        char *msg;
+        ssize_t len;
+        if (VIR_ALLOC_N(msg, 1024) < 0) {
+            virReportOOMError();
+            VIR_FORCE_CLOSE(cmd->handshakeWait[0]);
+            return -1;
+        }
+        if ((len = saferead(cmd->handshakeWait[0], msg, 1024)) < 0) {
+            VIR_FORCE_CLOSE(cmd->handshakeWait[0]);
+            VIR_FREE(msg);
+            virReportSystemError(errno, "%s", _("No error message from child failure"));
+            return -1;
+        }
+        VIR_FORCE_CLOSE(cmd->handshakeWait[0]);
+        msg[len-1] = '\0';
+        virCommandError(VIR_ERR_INTERNAL_ERROR, "%s", msg);
+        VIR_FREE(msg);
+        return -1;
+    }
+    VIR_FORCE_CLOSE(cmd->handshakeWait[0]);
+    return 0;
+}
+
+int virCommandHandshakeNotify(virCommandPtr cmd)
+{
+    char c = '1';
+    if (!cmd ||cmd->has_error == ENOMEM) {
+        virReportOOMError();
+        return -1;
+    }
+    if (cmd->has_error || !cmd->handshake) {
+        virCommandError(VIR_ERR_INTERNAL_ERROR, "%s",
+                        _("invalid use of command API"));
+        return -1;
+    }
+
+    if (cmd->handshakeNotify[1] == -1) {
+        virCommandError(VIR_ERR_INTERNAL_ERROR, "%s",
+                        _("Handshake is already complete"));
+        return -1;
+    }
+
+    VIR_DEBUG("Notify handshake on %d", cmd->handshakeWait[0]);
+    if (safewrite(cmd->handshakeNotify[1], &c, sizeof(c)) != sizeof(c)) {
+        virReportSystemError(errno, "%s", _("Unable to notify child process"));
+        VIR_FORCE_CLOSE(cmd->handshakeNotify[1]);
+        return -1;
+    }
+    VIR_FORCE_CLOSE(cmd->handshakeNotify[1]);
+    return 0;
+}
+
+
 /*
  * Release all resources
  */
@@ -1440,6 +1613,14 @@ virCommandFree(virCommandPtr cmd)
 
     VIR_FREE(cmd->pwd);
 
+    if (cmd->handshake) {
+        /* The other 2 fds in these arrays are closed
+         * due to use with virCommandTransferFD
+         */
+        VIR_FORCE_CLOSE(cmd->handshakeWait[0]);
+        VIR_FORCE_CLOSE(cmd->handshakeNotify[1]);
+    }
+
     VIR_FREE(cmd->pidfile);
 
     if (cmd->reap)

src/util/command.h

@@ -291,6 +291,28 @@ int virCommandRunAsync(virCommandPtr cmd,
 int virCommandWait(virCommandPtr cmd,
                    int *exitstatus) ATTRIBUTE_RETURN_CHECK;
 
+/*
+ * Request that the child perform a handshake with
+ * the parent when the hook function has completed
+ * execution. The child will not exec() until the
+ * parent has notified
+ */
+void virCommandRequireHandshake(virCommandPtr cmd);
+
+/*
+ * Wait for the child to complete execution of its
+ * hook function
+ */
+int virCommandHandshakeWait(virCommandPtr cmd)
+    ATTRIBUTE_RETURN_CHECK;
+
+/*
+ * Notify the child that it is OK to exec() the
+ * real binary now
+ */
+int virCommandHandshakeNotify(virCommandPtr cmd)
+    ATTRIBUTE_RETURN_CHECK;
+
 /*
  * Abort an async command if it is running, without issuing
  * any errors or affecting errno.  Designed for error paths