Handle error return from zygote recvmsg call.

This trips a check far away from the actual issue. The CHECK is too
strong and prevents executing the error handling path beneath it.

Shuffle a few blocks and comments around so that the flow makes
more sense and is easier to read, and initial fork failures are
caught earlier.

Bug: 835778
Change-Id: I46b960f9fd1095f5fc5224bf9cc80703e8fc325f
Reviewed-on: https://chromium-review.googlesource.com/1030876
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554949}

services/service_manager/zygote/zygote_linux.cc

@@ -438,6 +438,8 @@ int Zygote::ForkWithRealPid(const std::string& process_type,
   }
 
   if (pid == 0) {
+    // In the child process.
+
     // If the process is the init process inside a PID namespace, it must have
     // explicit signal handlers.
     if (getpid() == 1) {
@@ -449,7 +451,6 @@ int Zygote::ForkWithRealPid(const std::string& process_type,
       }
     }
 
-    // In the child process.
     write_pipe.reset();
 
     // Ping the PID oracle socket so the browser can find our PID.
@@ -476,32 +477,34 @@ int Zygote::ForkWithRealPid(const std::string& process_type,
   }
 
   // In the parent process.
+  if (pid < 0) {
+    // Fork failed.
+    return -1;
+  }
+
   read_pipe.reset();
   pid_oracle.reset();
 
   // Always receive a real PID from the zygote host, though it might
   // be invalid (see below).
-  base::ProcessId real_pid;
+  base::ProcessId real_pid = -1;
   {
     std::vector<base::ScopedFD> recv_fds;
     char buf[kZygoteMaxMessageLength];
     const ssize_t len = base::UnixDomainSocket::RecvMsg(
         kZygoteSocketPairFd, buf, sizeof(buf), &recv_fds);
-    CHECK_GT(len, 0);
-    CHECK(recv_fds.empty());
 
-    base::Pickle pickle(buf, len);
-    base::PickleIterator iter(pickle);
+    if (len > 0) {
+      CHECK(recv_fds.empty());
 
-    int kind;
-    CHECK(iter.ReadInt(&kind));
-    CHECK(kind == kZygoteCommandForkRealPID);
-    CHECK(iter.ReadInt(&real_pid));
-  }
+      base::Pickle pickle(buf, len);
+      base::PickleIterator iter(pickle);
 
-  // Fork failed.
-  if (pid < 0) {
-    return -1;
+      int kind;
+      CHECK(iter.ReadInt(&kind));
+      CHECK(kind == kZygoteCommandForkRealPID);
+      CHECK(iter.ReadInt(&real_pid));
+    }
   }
 
   // If we successfully forked a child, but it crashed without sending