virprocess: Introduce virProcessRunInFork

This new helper can be used to spawn a child process and run
passed callback from it. This will come handy esp. if the
callback is not thread safe.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>

src/libvirt_private.syms

@@ -2633,6 +2633,7 @@ virProcessKill;
 virProcessKillPainfully;
 virProcessKillPainfullyDelay;
 virProcessNamespaceAvailable;
+virProcessRunInFork;
 virProcessRunInMountNamespace;
 virProcessSchedPolicyTypeFromString;
 virProcessSchedPolicyTypeToString;

src/util/virprocess.c

@@ -1165,6 +1165,92 @@ virProcessRunInMountNamespace(pid_t pid,
 }
 
 
+static int
+virProcessRunInForkHelper(int errfd,
+                          pid_t ppid,
+                          virProcessForkCallback cb,
+                          void *opaque)
+{
+    if (cb(ppid, opaque) < 0) {
+        virErrorPtr err = virGetLastError();
+        if (err) {
+            size_t len = strlen(err->message) + 1;
+            ignore_value(safewrite(errfd, err->message, len));
+        }
+
+        return -1;
+    }
+
+    return 0;
+}
+
+
+/**
+ * virProcessRunInFork:
+ * @cb: callback to run
+ * @opaque: opaque data to @cb
+ *
+ * Do the fork and run @cb in the child. This can be used when
+ * @cb does something thread unsafe, for instance.  All signals
+ * will be reset to have their platform default handlers and
+ * unmasked. @cb must only use async signal safe functions. In
+ * particular no mutexes should be used in @cb, unless steps were
+ * taken before forking to guarantee a predictable state. @cb
+ * must not exec any external binaries, instead
+ * virCommand/virExec should be used for that purpose.
+ *
+ * On return, the returned value is either -1 with error message
+ * reported if something went bad in the parent, if child has
+ * died from a signal or if the child returned EXIT_CANCELED.
+ * Otherwise the returned value is the exit status of the child.
+ */
+int
+virProcessRunInFork(virProcessForkCallback cb,
+                    void *opaque)
+{
+    int ret = -1;
+    pid_t child = -1;
+    pid_t parent = getpid();
+    int errfd[2] = { -1, -1 };
+
+    if (pipe2(errfd, O_CLOEXEC) < 0) {
+        virReportSystemError(errno, "%s",
+                             _("Cannot create pipe for child"));
+        return -1;
+    }
+
+    if ((child = virFork()) < 0)
+        goto cleanup;
+
+    if (child == 0) {
+        VIR_FORCE_CLOSE(errfd[0]);
+        ret = virProcessRunInForkHelper(errfd[1], parent, cb, opaque);
+        VIR_FORCE_CLOSE(errfd[1]);
+        _exit(ret < 0 ? EXIT_CANCELED : ret);
+    } else {
+        int status;
+        VIR_AUTOFREE(char *) buf = NULL;
+
+        VIR_FORCE_CLOSE(errfd[1]);
+        ignore_value(virFileReadHeaderFD(errfd[0], 1024, &buf));
+        ret = virProcessWait(child, &status, false);
+        if (ret == 0) {
+            ret = status == EXIT_CANCELED ? -1 : status;
+            if (ret) {
+                virReportError(VIR_ERR_INTERNAL_ERROR,
+                               _("child reported (status=%d): %s"),
+                               status, NULLSTR(buf));
+            }
+        }
+    }
+
+ cleanup:
+    VIR_FORCE_CLOSE(errfd[0]);
+    VIR_FORCE_CLOSE(errfd[1]);
+    return ret;
+}
+
+
 #if defined(HAVE_SYS_MOUNT_H) && defined(HAVE_UNSHARE)
 int
 virProcessSetupPrivateMountNS(void)

src/util/virprocess.h

@@ -93,6 +93,22 @@ int virProcessRunInMountNamespace(pid_t pid,
                                   virProcessNamespaceCallback cb,
                                   void *opaque);
 
+/**
+ * virProcessForkCallback:
+ * @ppid: parent's pid
+ * @opaque: opaque data
+ *
+ * Callback to run in fork()-ed process.
+ *
+ * Returns: 0 on success,
+ *         -1 on error (treated as EXIT_CANCELED)
+ */
+typedef int (*virProcessForkCallback)(pid_t ppid,
+                                      void *opaque);
+
+int virProcessRunInFork(virProcessForkCallback cb,
+                        void *opaque);
+
 int virProcessSetupPrivateMountNS(void);
 
 int virProcessSetScheduler(pid_t pid,