debuggerd: diagnostics for dumps that are guaranteed to fail.

Print diagnostics when the user requests a dump that is guaranteed to
fail, such as trying to dump a process you can't send a signal to.

Bug: http://b/63008395
Change-Id: I5c6bf2a5751f858e0534990b8d2ab6932eb9f11d
Test: manually tested

debuggerd/Android.bp

@@ -274,6 +274,7 @@ cc_binary {
         "libbase",
         "libdebuggerd_client",
         "liblog",
+        "libprocinfo",
         "libselinux",
     ],
 

debuggerd/debuggerd.cpp

@@ -27,6 +27,7 @@
 #include <android-base/parseint.h>
 #include <android-base/unique_fd.h>
 #include <debuggerd/client.h>
+#include <procinfo/process.h>
 #include <selinux/selinux.h>
 #include "util.h"
 
@@ -66,6 +67,24 @@ int main(int argc, char* argv[]) {
     usage(1);
   }
 
+  if (getuid() != 0) {
+    errx(1, "root is required");
+  }
+
+  // Check to see if the process exists and that we can actually send a signal to it.
+  android::procinfo::ProcessInfo proc_info;
+  if (!android::procinfo::GetProcessInfo(pid, &proc_info)) {
+    err(1, "failed to fetch info for process %d", pid);
+  }
+
+  if (proc_info.state == android::procinfo::kProcessStateZombie) {
+    errx(1, "process %d is a zombie", pid);
+  }
+
+  if (kill(pid, 0) != 0) {
+    err(1, "cannot send signal to process %d", pid);
+  }
+
   unique_fd piperead, pipewrite;
   if (!Pipe(&piperead, &pipewrite)) {
     err(1, "failed to create pipe");